Kepler-101 is a Planetary System in Reverse

Kepler-101 is a planetary system with a hot super-Neptune (Kepler-101b) and an Earth-sized planet (Kepler-101c) in orbit around a metal-rich G-type star. Kepler-101b and Kpler-101c were first discovered by NASA’s Kepler space telescope thanks to the transit method. Both planets orbit a host star with ~1.17 times the mass and ~1.56 times the radius of the Sun. The star’s effective surface temperature is ~5667 K and it has approximately twice the metallicity of the Sun.

Figure 1: Artist’s impression of an exoplanet.

The radius of Kepler-101b is ~5.77 Rᴇ, where Rᴇ denotes the Earth’s radius. Subsequent radial velocity measurements obtained with the HARPS-N spectrograph indicate that the mass of Kepler-101b is ~51.1 Mᴇ, where Mᴇ denotes the Earth’s mass. With the planet’s size and mass known, its density is found to be ~1.45 g/cm³. Interior models of Kepler-101b suggest that a significant fraction of its interior is made up of heavy elements; more than 60 percent of the planet’s total mass. Kepler-101b is 0.047 AU from its host star and its orbital period is 3.488 days. As a result of its close-in orbit, the planet is hot, and its estimated equilibrium temperature is ~1500 K.

Kepler-101c is an Earth-sized planet whose radius is ~1.25 Rᴇ. Although its mass could not be determined, a 1σ upper limit of less than 3.8 Mᴇ can be imposed. Interior models of Kepler-101c exclude a pure iron composition with a 68.3 percent probability. Kepler-101c orbits its host star at 0.068 AU, taking 6.030 days to complete an orbit. It is further from its host star than Kepler-101b. The estimated equilibrium temperature on Kepler-101c is ~1400 K, not as hot as Kepler-101b.

Kepler-101 is an interesting planetary system because it does not follow the trend whereby in compact two-planet systems with at least one Neptune-sized or larger planet, the larger of the two planets usually has the longer orbital period. Kepler-101b is a relatively uncommon planet because in the mass-radius diagram of known transiting planets with radius less than 12 Rᴇ and mass less than 500 Mᴇ, the position of the planet is in the underpopulated transition region between Neptune-like and Saturn-like planets.

Figure 2: Transit light curve indicating the presence of the hot super-Neptune Kepler-101b. Bonomo et al. (2014)

Figure 3: Radial velocity curve indicating the presence of the hot super-Neptune Kepler-101b. Bonomo et al. (2014)

Figure 4: Transit light curve indicating the presence of the Earth-sized planet Kepler-101c. Bonomo et al. (2014)

Figure 5: The locations of Kepler-101b and Kepler-101c on the mass-radius diagram of known transiting planets with radius less than 12 Rᴇ and mass less than 500 Mᴇ. Bonomo et al. (2014)

Reference:
Bonomo et al. (2014), “Characterization of the Kepler-101 planetary system with HARPS-N. A hot super-Neptune with an Earth-sized low-mass companion”, arXiv:1409.4592 [astro-ph.EP]

The Hottest White Dwarfs in the Galaxy

White dwarfs are the dense leftover cores of stars that were not massive enough to end their lives in supernova explosions. After a white dwarf forms, it can be extremely hot, with surface temperatures exceeding ~100,000 K. H1504+65 and RXJ0439.8-6809 are two of the hottest white dwarfs known in the galaxy. Both white dwarfs are estimated to have ~0.015 ± 0.01 times the radius of the Sun, which translates to a radius of roughly 10,000 km.

Figure 1: Artist’s impression of a white dwarf.

Observations of H1504+65 and RXJ0439.8-6809 show that both white dwarfs have extreme surface compositions comprised of carbon-oxygen dominated atmospheres that are devoid of hydrogen and helium. It remains unknown as to how the hydrogen-helium envelopes of both white dwarfs can be eroded away to expose their hot carbon-oxygen interiors.

H1504+65 is estimated to have 0.68 to 1.02 times the mass of the Sun and its progenitor was probably a massive main-sequence star with 8 to 10 times the mass of the Sun. The surface temperature of H1504+65 is estimated to be 200,000 ± 20,000 K. H1504+65 is also relatively nearby, located ~2,000 light years away.

RXJ0439.8-6809 is record holder for the hottest white dwarf discovered to date and its surface temperature is estimated to be 250,000 ± 30,000 K. It has 0.73 to 1.02 times the mass of the Sun and it is the leftover core of a relatively massive star that contained several times the mass of the Sun. RXJ0439.8-6809 is located ~30,000 light years away, in the outskirts of the Milky Way galaxy.

The location of RXJ0439.8-6809 is puzzling because its progenitor star is probably too massive to have formed in the outskirts of the galaxy. As a result, the progenitor star of RXJ0439.8-6809 may once have been part of a binary system comprised of two massive stars located within the main disk of the galaxy. The progenitor star of RXJ0439.8-6809 was ejected into the outskirts of the galaxy when its companion star exploded in a supernova.

Figure 2: Positions of H1504+65 and RXJ0439.8-6809 in comparison with other white dwarfs. Werner & Rauch (2015)

Reference:
Werner & Rauch (2015), “Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo”, arXiv:1509.08942 [astro-ph.SR]

WASP-103b is a Hot-Jupiter Stretched by Tidal Forces

WASP-103b is a transiting hot-Jupiter in an ultra-short period orbit around a F8V star with 1.4 times the diameter and 1.2 times the mass of the Sun. Transit and radial velocity measurements show that WASP-103b has 1.6 times the diameter and 1.5 times the mass of Jupiter. Because WASP-103b orbits so close to its host star, it is expected to raise significant tides on its host star and experience tidally-induced orbital decay. Over a time interval of 10 years, the orbital period of WASP-103b could decrease by ~100 seconds.

Tidally-induced orbital decay may be detectable by precisely measuring when WASP-103b transits its host star to look for any slight deviations in periodicity. At present, this is not detectable as it requires many years of observations with high quality transit timing data. Nevertheless, more precise transit observations of WASP-103b have improved the time of mid-transit to an accuracy of 4.8 seconds. For comparison, the time of mid-transit was only accurate to 67.4 seconds at the time of the planet’s discovery. A more accurate time of mid-transit would help in future searches for tidally-induced orbital decay.

The extreme closeness of WASP-103b to its host star causes it to be tidally stretched into an ellipsoid with its longest axis oriented towards its host star. Assuming Rᴊᴜᴘ denotes the equatorial radius of Jupiter (i.e. 71,492 km); the dimensions of WASP-103b are 1.721 ± 0.075 Rᴊᴜᴘ at the substellar point, 1.710 ± 0.072 Rᴊᴜᴘ at the antistellar point, 1.537 ± 0.043 Rᴊᴜᴘ at its poles, and 1.571 ± 0.047 Rᴊᴜᴘ at its sides. WASP-103b is significantly distorted from a spherical shape, with its longest axis ~10 percent longer than its shortest axis.

Reference:
Southworth et al. (2014), “High-precision photometry by telescope defocussing. VII. The ultra-short period planet WASP-103”, arXiv:1411.2767 [astro-ph.EP]

Two Hot-Jupiters in a Twin Star System

WASP-94 is a wide binary system comprised of two stars with a projected separation of approximately 2700 AU. This system hosts two hot-Jupiters, one for each star. The primary and secondary stars in this system are identified as WASP-94A and WASP-94B, respectively. Observations have shown that hot-Jupiters are very rare objects. As a result, it is very unlikely to find a binary system with each star hosting a hot-Jupiter.

Figure 1: Artist’s impression of an exoplanet in a binary star system.

WASP-94A is a F8V star with 1.29 ± 0.10 times the mass and 1.36 ± 0.13 times the radius of the Sun, and its effective surface temperature is 6170 ± 80 K. It hosts a transiting hot-Jupiter, identified as WASP-94Ab. WASP-94Ab has 0.445 ± 0.026 times the mass and 1.72 ± 0.06 times the radius of Jupiter, and its orbital period is 3.95 days. The mass of WASP-94Ab was determined using the radial velocity method which measures how much the planet’s host star wobbles due to gravitational perturbations from the planet itself. Additionally, the Rossiter-McLaughlin effect is clearly observable each time WASP-94Ab transits its host star.

The Rossiter-McLaughlin effect occurs when a planet transits across the face of its host star. Since the star is rotating, half of its observable hemisphere will be rotating towards the observer (i.e. approaching quadrant) and the other half of its observable hemisphere will be rotating away from the observer (i.e. receding quadrant). Light from the star is blue-shifted on the approaching quadrant and red-shifted on the receding quadrant. Since the approaching and receding quadrants are symmetrical, a net redshift is generated when the planet is in front of the approaching quadrant and a net blueshift is generated when the planet is in front of the receding quadrant.

A net redshift to blueshift change indicates the planet is in a prograde orbit (i.e. planet orbits in the same direction as the star’s spin); while a net blueshift to redshift change indicates the planet is in a retrograde orbit (i.e. planet orbits in the opposite direction to the star’s spin). Measuring the Rossiter-McLaughlin effect allows the spin-orbit angle (i.e. angle of the planet’s orbital plane with respect to the spin axis of its host star) of WASP-94Ab to be determined and the measurements indicate that WASP-94Ab is in a retrograde orbit.

WASP-94B is a F9V star with 1.24 ± 0.09 times the mass and 1.35 ± 0.12 times the radius of the Sun, and its effective surface temperature is 6040 ± 90 K. It hosts a non-transiting hot-Jupiter identified as WASP-94Bb. WASP-94Bb was detected using the radial velocity method. The amplitude of the radial velocity measurements indicates that WASP-94Bb has a mass of at least 0.617 ± 0.028 times the mass of Jupiter, and the periodicity of the radial velocity measurements show that the orbital period of WASP-94Bb is 2.008 days.

Figure 2: Top: radial velocity measurements indicating the presence of WASP-94Ab. Middle: residuals of the best-fit curve to the radial velocity measurements. Bottom: zoom-in on the radial velocities measurements taken during the transit over-plotted with the best solution for the Rossiter-McLaughlin effect. M. Neveu-VanMalle et al. (2014)

Figure 3: Top: radial velocity measurements indicating the presence of WASP-94Bb. Bottom: residuals of the best-fit curve to the radial velocity measurements. M. Neveu-VanMalle et al. (2014)

Reference:
M. Neveu-VanMalle et al. (2014), “WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system”, arXiv:1409.7566 [astro-ph.EP]

OGLE-2011-BLG-0265Lb is a Cold Jupiter-Mass Planet

Figure 1: Artist’s impression of a gas giant planet.

OGLE-2011-BLG-0265Lb is a Jupiter-mass gas giant planet in orbit around a red dwarf star. It was detected using a technique known as gravitational microlensing. When a foreground star crosses the line of sight to a background star, the gravitational field of the foreground star can act as a lens and magnify the brightness of the background star. This phenomenon can be observed as a light curve where the brightness of the background star changes with time. If the foreground star has a planet around it, the presence of the planet can induce perturbations in the light curve.

The gravitational microlensing light curve indicating the presence of OGLE-2011-BLG-0265Lb yields two solutions. For the first solution, the planet has 1.0 ± 0.3 times the mass of Jupiter and it orbits a star with 0.23 ± 0.07 times the mass of the Sun. For the second solution, the planet has 0.6 ± 0.2 times the mass of Jupiter and it orbits a star with 0.15 ± 0.06 times the mass of the Sun. In both cases, the planet is ~2 AU from its host star - a red dwarf star. At that distance, the planet is well beyond the "snow line" of its host star and it can be considered a "cold Jupiter".

The discovery of OGLE-2011-BLG-0265Lb is an important one because gas giant planets are very rare around red dwarf stars. The core accretion mechanism of planet formation predicts that gas giant planets rarely form around red dwarf stars, while the disk instability mechanism of planet formation predicts that gas giant planets can form around red dwarf stars. Detecting more planets like OGLE-2011-BLG-0265Lb around red dwarf stars can provide more insight on the formation scenario of such planets.

Figure 2: Gravitational microlensing light curve indicating the presence of OGLE-2011-BLG-0265Lb. The planet’s perturbations to the light curve are marked with arrows. Skowron et al. (2014)

Reference:

Skowron et al. (2014), "OGLE-2011-BLG-0265Lb: a Jovian Microlensing Planet Orbiting an M Dwarf", arXiv:1410.8252 [astro-ph.EP]

Ongoing Star Formation in the Dwarf Galaxy Leo P

Leo P is a faint, gas-rich dwarf galaxy with ongoing star formation. It is located 5.28 ± 0.49 million light years away, at the edge of the Local Group, far from the influence of any massive galaxy. The total mass of stars in Leo P is estimated to be ~600,000 times the Sun’s mass. Leo P is also home to a massive star with at least 25 times the mass of the Sun. The presence of this star shows that massive stars can form even with star formation rates as low as ~0.00001 solar-mass per year. Observations of the different populations of stars in Leo P show a constant rate of star formation over the lifetime of the dwarf galaxy.

The luminosity of Leo P appears similar to the dwarf spheroidal (dSph) galaxies around the Milky Way. However, unlike Leo P, the dSph galaxies around the Milky Way contain little to no gas, and have no ongoing stars formation. This indicates that Leo P is what a dSph galaxy would look like if it evolved in an isolated environment and held on to its gas content. It also shows that the environment around the Milky Way has the effect of quenching star formation in its satellite dwarf galaxies.

Reference:
McQuinn et al. (2015), “Leo P: An Unquenched Very Low-Mass Galaxy”, arXiv:1506.05495 [astro-ph.GA]

2013 AZ60 is a Potential Super-Comet

2013 AZ60 is an extreme object in a highly eccentric orbit around the Sun. It comes as close as 7.9 AU from the Sun (i.e. between the orbits of Jupiter and Saturn) and swings out to a whopping ~1950 AU from the Sun. Spending most of its time far from the Sun, the orbital period of 2013 AZ60 is estimated to be ~30,000 years. 2013 AZ60 is a trans-Neptunian object (TNO), and it may be classified either as a Centaur based on its closest distance from the Sun or as a scattered disk object based on its large average distance from the Sun.

Figure 1: Artist’s impression of an icy object far from the Sun. Image credit: ESA.

Optical measurements of 2013 AZ60 show that it has a rotation period of roughly 9.4 hours and a change in brightness of only 4.5 percent during each rotation. Thermal measurements of 2013 AZ60 were also done to estimate the object’s size and reflectivity. The benefit of thermal measurements is that it can distinguish whether an object is “large but dim” or “small but bright”. From the thermal measurements, 2013 AZ60 is estimated to have a diameter of 62.3 ± 5.3 km and a remarkably low geometric albedo of only 2.9 percent. The low albedo indicates that the surface of 2013 AZ60 is extremely dark.

Simulations of the orbit of 2013 AZ60 show that its present orbit is highly unstable. There is a 50 percent chance 2013 AZ60 will be ejected from the Solar System within the next ~700,000 years and a ~4 percent chance it will be perturbed into an Earth-crossing orbit. Given its relatively large size, 2013 AZ60 will be a super-comet if it ever gets perturbed into the inner Solar System. The highly unstable orbit of 2013 AZ60 indicates that the object was only recently perturbed into its current orbit and it is likely a pristine object that came in from the Oort cloud.

Figure 2: Slope parameter versus albedo relations for 111 TNOs, including 2013 AZ60 and 2012 DR30 (an object with a similar orbit as 2013 AZ60). The purple square at the very left side of the diagram represents 2013 AZ60 and the other purple square represents 2012 DR30. A. Pál et al. (2015)

Reference:
A. Pál et al. (2015), “Physical properties of the extreme centaur and super-comet candidate 2013 AZ60”, arXiv:1507.05468 [astro-ph.EP]

Pluto with an Iron Core

Figure 1: Image of Pluto taken by NASA’s New Horizons spacecraft. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Observations of Pluto and Charon by NASA’s New Horizons spacecraft show the presence of numerous geologically young surface features which serve as evidence for recent geological activity. For example, Pluto has enormous ice mountains ~3 km in hight, and a large, craterless icy plain named Sputnik Planum. It is worth considering whether the presence of an iron core in Pluto and in Charon may have given rise to the geological features seen on their surfaces.

Since all inner planets in the Solar System, including the Moon, have iron-rich cores, it is not improbable for Pluto and Charon to also possess iron cores. Interior models of Pluto and Charon typically have two components - a rocky core and an overlying shell of ice. The presence of an iron core will mean a three component interior structure consisting of an iron core, a rocky mantle and an outer shell of ice. Just after formation, the interior of Pluto was supposedly much hotter than it currently is, and may have supported a molten iron core.

As Pluto cools, its iron core solidifies inwards, beginning from the core-rock boundary. Since molten iron is denser than solid iron, the iron core contracts and rock from the mantle above descends to occupy the space that becomes available. To fill the space created by the descending rock, some of the ice at the rock-ice boundary descends as well. Such a process occurring on Pluto can also occur, possibly to a lesser extend, on Charon. The descending ice at the rock-ice boundary can potentially create widespread surface deformations on Pluto and Charon, thereby explaining the many geologically young surface features observed by the New Horizons spacecraft.

Reference:
A. Aitta et al. (2015), “Internal structure of Pluto and Charon with an iron core”, arXiv:1510.06604 [astro-ph.EP]

Supernovae Enrichment in the Globular Cluster NGC 6273

Massive stars end their lives in powerful supernovae explosions. These explosions eject massive quantities of heavy elements into space and these heavy elements can get incorporated into subsequent generations of stars. As a result, these stars become more enriched with heavy elements. A number of massive globular clusters such as Omega Centauri and Terzan 5 show evidence for supernova enrichment. These massive globular clusters are believed to be the leftover nuclei of disrupted dwarf galaxies, and unlike typical globular clusters, they are massive enough to retain supernovae ejecta.

Observations of the globular cluster NGC 6273 show that it too has evidence for supernovae enrichment. The red giant stars in NGC 6273 appear to fall into two distinct populations with different calcium abundances. Other observations of the red giant stars in NGC 6237 also show two distinct populations - a metal-poor group with a lower abundance of iron and a metal-rich group with a higher abundance of iron. These observations suggest that NGC 6273 was massive enough to retain supernovae ejecta, allowing it to have subsequent generations of stars that are more enriched with heavy elements.

Figure 1: Artist’s impression of the view from a planet near a globular cluster.

References:
- Han et al. (2015), “Evidence for Enrichment by Supernovae in the Globular Cluster NGC 6273”, arXiv:1510.06044 [astro-ph.GA]
- Johnson et al. (2015), “A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)”, arXiv:1507.00756 [astro-ph.SR]

The Leftover Nucleus of a Dwarf Galaxy

NGC 3628-UCD1 is an ultra-compact dwarf that is embedded in a stream of stars around the spiral galaxy NGC 3628. UCD1 is made up of a compact cluster of stars with a total luminosity of approximately 1.4 million times the Sun’s luminosity and a half-light radius of roughly 30 light years. UCD1 is likely to be associated with the stream of stars it is embedded in. This is because UCD1 is located in the brightest region of the stream of stars and the spatial density of stars in the stream appears to fall off gradually in all directions away from UCD1.

The size and luminosity of UCD1 is remarkably similar to Omega Centauri, the most luminous Milky Way globular cluster. Omega Centauri is believed to be the leftover nucleus of a dwarf galaxy that was tidally stripped as it got accreted by the Milky Way. UCD1 and the stream of stars it is embedded in were probably once a dwarf galaxy before it got tidally stripped and accreted by the much larger spiral galaxy NGC 3628. Measurements of the total brightness of the stream of stars UCD1 is embedded in suggest it was once a dwarf galaxy with approximately 40 million times the Sun’s luminosity.

Reference:
Jennings et al. (2015), “NGC 3628-UCD1: A possible ω Cen Analog Embedded in a Stellar Stream”, arXiv:1509.04710 [astro-ph.GA]

Possible Planet Orbiting the White Dwarf PG 0010+280

PG 0010+280 is a young white dwarf with an estimated surface temperature of 27,220 K and a cooling age of approximately 16 million years. This object is the dense leftover core of what was once a star with roughly 1.8 times the mass of the Sun and with a main sequence lifetime of about 2.3 billion years. PG 0010+280 is estimated to have 57 percent the mass of the Sun and a diameter of roughly 20,000 km (~1.6 times the diameter of Earth).

Observations of PG 0010+280 reveal an infrared excess at the 3 to 8 μm waveband. This infrared excess can be attributed to the presence of either an opaque dusty disk orbiting the white dwarf within its tidal radius or a blackbody with a temperature of roughly 1,300 K. If the infrared excess is due to the presence of an opaque dusty disk, the inner edge of such a disk can be as close as ~380,000 km from the white dwarf. Any closer than that, the intense radiation from the white dwarf can sublimate the dust grains.

If the infrared excess is due to a ~1,300 K blackbody, such an object will have to be about 1.3 times the diameter of Jupiter. This is consistent with either an irradiated substellar object (i.e. brown dwarf) or a re-heated giant planet. Assuming the object is an irradiated substellar object and assuming that it is in thermal equilibrium with the incoming radiation from the white dwarf, such an object will need to be ~220,000 km from the white dwarf in order for its temperature to be ~1,300 K. At that distance, the irradiated substellar object will have an orbital period of 21 hours.

Before becoming a white dwarf, a star will swell into a red giant and eject its outer layers. Any giant planet around the star can be significantly re-heated from the accretion of material ejected from the star. For a giant planet around PG 0010+280, it only needs to accrete a tiny percentage of the ejected stellar material for it to be significantly re-heated to ~1,300 K. Such a process can involve the accretion of a substantial amount of carbon-rich material that can cause the planet’s atmosphere to be noticeably enriched in molecules such as methane (CH4) and carbon monoxide (CO).

Reference:

Siyi Xu t al. (2015), “A Young White Dwarf with an Infrared Excess”, arXiv:1505.02614 [astro-ph.SR]

Kepler’s Bonanza of Earth-Like Planet Candidates

Coughlin et al. (2015) present the seventh Kepler planet candidate catalogue. It contains data collected by NASA’s Kepler space telescope over 48 months. This catalogue brings the total number of planet candidates to 4696. It also contains several new planet candidates that are likely to be rocky and in the habitable zone of their host stars. A few noteworthy planet candidates are further discussed in this article.

Figure 1: Artist’s impression of a habitable planet.

Figure 2: A plot of planet radius versus insolation flux for all planet candidates known in this catalogue. (Note that some planet candidates, particularly those with large radii, lie outside the chosen axis limits for the plot, and thus are not shown.) The temperature of the host star is indicated via the colour of each point, and the signal-to-noise of the detection is indicated via the size of each point. The two vertical dashed lines indicate the insolation flux values of Mars and Venus as a broad guide to a potential habitable zone. The horizontal dotted line is set at 1.6 times the Earth’s radius as a suggested guide to where roughly half of the planets with this size are expected to be rocky. Coughlin et al. (2015)

KOI 1681.04 is a sub-Earth-sized planet candidate in a 21.9 day orbit around a red dwarf star with 0.35 times the Sun’s radius and effective temperature 3669 K. The planet receives an insolation flux 1.63 times what Earth gets from the Sun and the estimated equilibrium temperature on the planet is 288 K, or 15°C. The radius of KOI 1681.04 is 0.77 Rᴇ and it resides in a planetary system with three other planet candidates whose radii are 0.69, 0.71 and 0.99 Rᴇ, and whose orbital periods are 1.99, 3.53 and 6.94 days, respectively. The fact that KOI 1681.04 is in a multi-planet system gives it a high confidence that it is a real planet.

Figure 3: Artist’s impression of a potentially habitable planet.

Figure 4: Artist’s impression of a potentially habitable planet.

KOI 7179.01 is an Earth-sized planet candidate measuring 1.18 Rᴇ in radius. It orbits a Sun-like star with 1.2 times the Sun’s radius and effective temperature 5845 K. The planet’s orbital period is 407.1 days and it receives an insolation flux 1.29 times what Earth gets from the Sun. The equilibrium temperature on KOI 7179.01 is estimated to be 272 K, or -1°C. KOI 7179.01 appears to be a good Earth-like planet candidate given its Earth-like size and Earth-like insolation flux, and that it also orbits a Sun-like host star.

Measuring 1.53 Rᴇ in radius, KOI 7223.01 is somewhat larger than Earth. It orbits a star with 0.73 times the Sun’s radius and effective temperature 5370 K. The orbital period of KOI 7223.01 is 317.1 days and the planet receives an insolation flux 0.57 times what Earth gets from the Sun. The equilibrium temperature on KOI 7179.01 is estimated to be 221 K, or -52°C. KOI 7223.01 has a high probability of being a true rocky planet in orbit around a somewhat Sun-like star.

Reference:
Coughlin et al. (2015), “Planetary Candidates Observed by Kepler. VII. The First Fully Uniform Catalog Based on The Entire 48 Month Dataset (Q1-Q17 DR24)”, arXiv:1512.06149 [astro-ph.EP]

KIC 5621294 and its Circumbinary Substellar Companion

Figure 1: Artist’s impression of a brown dwarf as seen from a hypothetical planet in a close-in orbit around the brown dwarf.

KIC 5621294 is an eclipsing binary system consisting of two stars in a tight orbit that causes them to partially eclipse one another. The primary star has 1.95 times the mass and 0.99 times the radius of the Sun, and the secondary star has 0.43 times the mass and 1.39 times the radius of the Sun. Both stars orbit around each other every 0.9389 days. Given enough time, the eclipsing binary system of KIC 5621294 will evolve into a contact configuration as the system continues to lose angular momentum. A contact configuration is one where both stars are in direct contact with each other. Photometric observations by NASA’s Kepler space telescope reveal the presence of variations in the eclipse timings. These timing variations indicate the presence of a third-body in a circumbinary orbit around KIC 5621294. Gravitational effects from the third-body perturb the central binary, resulting in the eclipse timing variations.

The third-body is estimated to have at least 46.9 times the mass of Jupiter and the effect it has on the central binary indicates an orbital period of 961 days. However, the actual mass of the third-body will depend on its orbital inclination. A face-on orbit has an inclination of zero degrees and an edge-on orbit has an inclination of 90 degrees. As long as the orbital inclination of the third-body is more than about 40 degrees, it will be in the mass regime of brown dwarfs. However, if its orbital inclination is less than about 40 degrees, then its mass will be large enough for it to be a low-mass hydrogen-burning star. As seen from the third-body, the luminosity of the central binary will be dominated by the primary star which is over 30 times more luminous than the secondary star.

Reference:
Jae Woo Lee et al. (2014), “The Kepler Eclipsing System KIC 5621294 and its Substellar Companion”, arXiv:1412.7258 [astro-ph.SR]

Sun-Like Star Hosting a Jupiter-Like Planet

Figure 1: Artist’s impression of a gas giant planet similar to Jupiter.

As part of the Lick-Carnegie Exoplanet Survey (LCES), Meschiari et al. (2015) present the discovery of a Jupiter analogue around a Sun-like star after high precision radial velocity measurements of the star were made using the High Resolution Echelle Spectrometer (HIRES) on the Keck Observatory in Hawaii. This planet is identified as HD 32963b. It has at least 0.7 ± 0.03 times the mass of Jupiter and its orbital period is 6.49 ± 0.07 years. For comparison, the orbital period of Jupiter is 11.86 years.

A Jupiter analogue is defined as a planet with 0.3 to 3.0 times the mass of Jupiter and an orbital period between 5 to 15 years. Additionally, the planet’s orbital eccentricity has to be less than 0.3. Using this definition, only a handful of Jupiter analogues are known and the addition of HD 32963b is an important one. This discovery emphasises the importance of long-term radial velocity surveys since Jupiter analogues takes several years to orbit their host stars and long timescale observations are needed to detect them.

Figure 2: Period-eccentricity diagram of planets with 0.3 to 3.0 times the mass of Jupiter and orbital periods between 5 to 15 years. Only planets with orbital eccentricities less than 0.3 are Jupiter analogues. Meschiari et al. (2015)

Figure 3: Radial velocity curve indicating the presence of HD 32963b. Meschiari et al. (2015)

Reference:
Meschiari et al. (2015), “The Lick-Carnegie Exoplanet Survey: HD32963 -- A New Jupiter Analog Orbiting a Sun-like Star”, arXiv:1512.00417 [astro-ph.EP]

Grazing Transit by a Hot-Jupiter

Figure 1: Artist’s impression of a hot-Jupiter.

Grziwa et al. (2015) present the detection of EPIC 204129699b, the first hot-Jupiter found using data from NASA’s K2 mission. The orbit of EPIC 204129699b is such that it transits its host star in a grazing fashion, which means the planet does not entirely pass in front of its host star. EPIC 204129699b orbits around a G7V star with 91 percent the mass and 78 percent the radius of the Sun. The periodicity of the planet’s grazing transits indicates that the planet’s orbital period is 1.26 days. This makes EPIC 204129699b a short-period hot-Jupiter. The dayside of EPIC 204129699b is estimated to have a temperature of roughly 1750 K.

The size of EPIC 204129699b can only be weakly constrained due to the grazing nature of the planet’s transits and it lies somewhere between 0.7 to 1.4 times the size of Jupiter.  Follow-up radial velocity observations indicate that EPIC 204129699b has 1.774 ± 0.079 times the mass of Jupiter. A search for reflected light from EPIC 204129699b yielded no reliable detection, placing an upper limit on the planet-to-star surface-brightness ratio at around 0.01. This means the planet has an albedo of less than 0.4, indicating that it has a low-reflectivity that is consistent with most hot-Jupiters. EPIC 204129699b is an important addition to the small number of short-period hot-Jupiters detected to date.

Figure 2: Phase-folded transit light curve indicating the presence of EPIC 204129699b and the V-shaped curve is caused by a grazing transit. Grziwa et al. (2015)

Reference:
Grziwa et al. (2015), “EPIC 204129699b, a grazing transiting hot Jupiter on an 1.26-day orbit around a bright solar like star”, arXiv:1510.09149 [astro-ph.EP]

A Transiting Neptune-Sized Planet in the Hyades

Figure 1: Artist’s impression of a Neptune-sized planet.

The Hyades cluster is located ~150 light years away and the stars that make up the cluster are 650 to 800 million years old. Using data collected by NASA’s K2 mission over 71 days from 8 February to 20 April 2015, Mann et al. (2015) present the discovery of a Neptune-sized planet orbiting a red dwarf star in the Hyades cluster. This planet is identified as EPIC 210490365b and it is the first transiting planet discovered in the Hyades cluster. By measuring how much light the planet blocks when it transits its host star, the planet is found to have 3.43 [-0.31, +0.95] times the radius of Earth.

The host star of EPIC 210490365b has 29 percent the mass of the Sun and the planet has an orbital period of 3.484 days. For planets around red dwarf stars, there is a general trend whereby planets in longer period orbits around more massive red dwarf stars tend to be larger in size than planets in shorter period orbits around less massive red dwarf stars. Compared to other transiting planets with orbital periods less than 100 days and host star masses less than 50 percent the mass of the Sun, EPIC 210490365b appears unusually large for its orbital period and host star mass (Figure 3).

Figure 2: Phase-folded transit light curve indicating the presence of EPIC 210490365b. The red line shows the best-fit model and the bottom panel shows the residuals. Mann et al. (2015)

Figure 3: Planet size as a function of host star mass (left) and planet irradiance (right) for EPIC 210490365b (red) compared to transiting planets discovered by Kepler (black) and from the ground by MEarth (blue). Only transiting planets with orbital periods less than 100 days and host star masses less than 50 percent the mass of the Sun are included. Mann et al. (2015)

Red dwarf stars undergo a juvenile phase of elevated activity that can last for a few billion years. Such a phase of elevated activity can potentially strip away the primordial hydrogen-helium envelopes of mini-Neptunes and Neptune-like planets that are in close-in orbits around red dwarf stars. The host star of EPIC 210490365b is a member of the Hyades cluster and this means that the star is only a few hundred million years old - a relatively youthful age when compared with stars such as the Sun. As a result, the large size of EPIC 210490365b could indicate that it is still in an early phase of its evolution whereby the loss of its primordial hydrogen-helium envelope has only just started.

The deep transit depth of EPIC 210490365b makes it a good target for atmospheric characterisation. However, it is not known if a young Neptune-sized planet like EPIC 210490365b will exhibit features in its atmosphere that are no longer present in older planets of similar nature. Future measurements of the mass of EPIC 210490365b will be useful as it can provide constraints on the planet’s bulk density, hopefully giving more insights on the unusually large size of the planet.

Reference:
Mann et al. (2015), “Zodiacal Exoplanets In Time (ZEIT) I: A Neptune-sized planet orbiting an M4.5 dwarf in the Hyades Star Cluster”, arXiv:1512.00483 [astro-ph.EP]

Two Massive Planets Orbiting Old Sun-Like Stars

Figure 1: Artist’s impression of a gas giant planet.

Ciceri et al. (2015) present the discovery of two massive hot-Jupiters in close-in orbits around old Sun-like stars. The two planets are identified as HATS-15b and HATS-16b. HATS-15b has 2.17 ± 0.15 times the mass of Jupiter, 1.105 ± 0.040 times the radius of Jupiter and it orbits a G9V star in 1.75 days. HATS-16b has 3.27 ± 0.19 times the mass of Jupiter, 1.30 ± 0.15 times the radius of Jupiter and it orbits a G3V star in 2.69 days. For comparison, the Sun is a G2V star.

With the masses and radii of both planets known, the density of HATS-15b is 1.97 g/cm³ and the density of HATS-16b is 1.86 g/cm³. Interior models suggest that HATS-15b does not possess a core while HATS-16b probably has a low-mass core. Being so close to their parent stars, both planets have very high equilibrium temperatures and are classified as hot-Jupiters. HATS-15b has a temperature of 1500 K and HATS-16b has a temperature of 1600 K.

The parent star of HATS-16b has a relatively short rotation period of just 12 days. Such a short rotation period might be the result of tidal interactions between HATS-16b and its parent star, whereby the star has been tidally spun up by the planet. The parent star of HATS-15b is estimated to be ~11 billion years old and the parent star of HATS-16b is estimated to be ~9.5 billion years old. Both stars are relatively old. For comparison, the Sun is only 4.6 billion years old.

Figure 2: Transit light curves indicating the presence of HATS-15b (left) and HATS-16b (right). The transit depths indicate that HATS-15b and HATS-16b have 1.105 ± 0.040 and 1.30 ± 0.15 times the radius of Jupiter, respectively. Ciceri et al. (2015)

Figure 3: Radial velocity measurements for HATS-15b (left) and HATS-16b (right). The radial velocity amplitudes indicate that HATS-15b and HATS-16b have 2.17 ± 0.15 and 3.27 ± 0.19 times the radius of Jupiter, respectively. Ciceri et al. (2015)

Figure 4: HATS-15b and HATS-16b (highlighted in orange) are presented in the mass-density diagram showing the known transiting planets for which the mass is measured. The superimposed lines represent the expected density of planets having an inner core of 0, 25 and 50 Earth masses, and calculated for 10 bullion year old planets at 0.02 AU (solid lines), and 0.045 AU (dashed lines) from their parent stars. It can be seen in the diagram that planets with ~2 to 3 times the mass of Jupiter are much rarer than planets with ~0.1 to 1 times the mass of Jupiter. Ciceri et al. (2015)

Reference:
Ciceri et al. (2015), “HATS-15 b and HATS-16 b: Two massive planets transiting old G dwarf stars”, arXiv:1511.06305 [astro-ph.EP]

Giant Cluster of Galaxies 8.5 Billion Light Years Away

Gonzalez et al. (2015) present the discovery of a giant cluster of galaxies located 8.5 billion light years away. This cluster of galaxies is called Massive Overdense Object (MOO) J1142+1527 and it was detected using NASA’s Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE). Additionally, using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) to observe a phenomenon known as the Sunyaev-Zel'dovich effect, MOO J1142+1527 is estimated to have 1.1 ± 0.2 quadrillion times the mass of the Sun. One quadrillion is a thousand trillion. MOO J1142+1527 is the most massive known cluster of galaxies existing that far back in space and time.

Reference:
Gonzalez et al. (2015), “The Massive and Distant Clusters of WISE Survey: MOO J1142+1527”, arXiv:1509.01989 [astro-ph.CO]

Icy World in the Shape of an Elongated Triaxial Ellipsoid

Trans-Neptunian Objects (TNOs) are far-flung objects that orbit the Sun beyond the orbit of Neptune. E. Fernández-Valenzuela et al. (2015) present observations of 2008 OG19, revealing it to be a highly elongated TNO. 2008 OG19 orbits the Sun in an eccentric orbit; 38.6 AU from the Sun at its closest and 93.8 AU from the Sun at its furthest. As 2008 OG19 rotates, its brightness changes with time, creating an almost symmetric double-peaked light curve with two minima and two maxima.

The light curve’s peak to valley amplitude is 0.437 ± 0.011 mag (i.e. a brightness variation factor of roughly 1.5). The periodicity of the light curve indicates that 2008 OG19 has a rotational period of 8.727 ± 0.003 hours. Such a brightness variation is consistant with 2008 OG19 having the shape of a highly elongated triaxial ellipsoid. As different cross-sections of the triaxial ellipsoid rotate in and out of view, the observed brightness of 2008 OG19 changes, creating a double-peaked light curve each rotation. 2008 OG19 is estimated to have an equivalent diameter of approximately 620 km, making it similar in size to Varuna, another highly elongated TNO.

The presence of light and dark features on the surface of 2008 OG19 cannot be the cause of the brightness variation because the magnitude of the brightness variation is simply too large. Furthermore, the symmetrical double-peaked light curve is consistent with a rotating triaxial ellipsoid because it is unlikely for an object to have surface features that are symmetrical on both hemispheres.

Reference:
E. Fernández-Valenzuela et al. (2015), “2008 OG19: A highly elongated Trans-Neptunian Object”, arXiv:1511.06584 [astro-ph.EP]

Nearby Star Hosting a Potentially Habitable Super-Earth

Red dwarf stars are the most common stars in the Universe. With low masses and low luminosities, red dwarf stars are excellent targets to search for potentially habitable rocky planets. Due to their diminutive nature, the habitable zones around red dwarf stars are much closer-in. As a result, a planet in the habitable zone of a red dwarf star will exert a more detectable gravitational tug on its host star than if the planet were in the habitable zone of a more luminous star. Additionally, the low-mass nature of a red dwarf star only makes such a signal more detectable.

Wolf 1061 is a relatively nearby red dwarf star located only 14 light years away. It has 25 percent the Sun’s mass, 0.787 percent the Sun’s luminosity, and its estimated surface temperature is 3393 K. Using archival radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory’s 3.6 m telescope in La Silla in Chile, Wright et al. (2015) present the detection of three planets around Wolf 1061. The three planets were discovered due to the gravitational tug they exert on their host star, which causes their host star to exhibit a detectable wobbly motion.

The three planets are identified as Wolf 1061b, Wolf 1061c and Wolf 1061d. Wolf 1061b has at least 1.36 times the mass of Earth and an orbital period of 4.888 days (0.0355 AU); Wolf 1061c has at least 4.25 times the mass of Earth and an orbital period of 17.867 days (0.0843 AU); and Wolf 1061d has at least 5.21 times the mass of Earth and an orbital period of 67.274 days (0.2039 AU). The values in parentheses indicate the distance of each planet from Wolf 1061 in astronomical units (AU), whereby one AU is the average Earth-Sun distance. All three planets are small enough to potentially be rocky worlds.

The conservative habitable zone around Wolf 1061 is predicted to lie between 0.092 to 0.18 AU, while the optimistic habitable zone around Wolf 1061 is predicted to lie between 0.073 to 0.19 AU. The habitable zone around a star is where temperatures are just right for liquid water, and possibly even life, to exist on the surface of a planet. Wolf 1061b (inner planet) is well inside the inner boundary of the optimistic habitable zone, making it too hot to be habitable; Wolf 1061c (middle planet) is well inside the optimistic habitable zone and just outside the inner boundary of the conservative habitable zone, making it potentially habitable; and Wolf 1061d (outer planet) is outside the outer boundary of the optimistic habitable zone, making it a little too cold for habitability.

From the mass-radius relation for planets, the radii of the three planets are estimated to be 1.44 (inner planet), 1.64 (middle planet) and 2.04 (outer planet) times the radius of Earth. The middle planet, Wolf 1061c, is of particular interest because it is likely a potentially habitable rocky planet and it joins a small but growing group of potentially habitable rocky planets orbiting nearby stars cooler than the Sun. At present, Wolf 1061c is one of the closest known potentially habitable rocky planets.

Reference:
Wright et al. (2015), “Three planets orbiting Wolf 1061”, arXiv:1512.05154 [astro-ph.EP]

A Small Planet with Giant Companions

Figure 1: Artist’s impression of a Jupiter-like planet with a moon in orbit around it.

Kepler-454b is an exoplanet that orbits a Sun-like star. It was previously detected by NASA’s Kepler space telescope which hunts for planets by looking for the dip in a star’s brightness when a planet passes in front of it. Kepler-454b is estimated to have 2.37 ± 0.13 times the diameter of Earth based on how much light the planet blocks when it transits its host star. Additionally, the frequency of the transits indicates that Kepler-454b has an orbital period of 10.6 days.

Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 radial velocity observations obtained with the HIRES spectrograph at Keck Observatory, Gettel et al. (2015) show that Kepler-454b has 6.8 ± 1.4 times the mass of Earth. Furthermore, the radial velocity measurements also indicate the presence of two additional non-transiting companions.

Figure 2: Transit light curve indicating the presence of Kepler-454b. Gettel et al. (2015)

 Figure 3: Radial velocity measurements from HARPS-N (blue circles) and HIRES (red squares) indicating the presence of Kepler-454b. Gettel et al. (2015)

One companion, identified as Kepler-454c, is a Jupiter-like planet with at least 4.46 ± 0.12 times the mass of Jupiter in a nearly circular 524 day orbit. The other companion, identified as Kepler-454d, is a massive object with at least 12.1 times the mass of Jupiter and an orbital period of over 10 years. Determining the properties of Kepler-454d is difficult as its orbital period is much longer than the timescale over which the radial velocity measurements were performed. Kepler-454d is quite likely a brown dwarf.

Mass measurements of exoplanets that are smaller than 2.7 times the diameter of Earth show that they appear to fall into two populations. Those with less than 1.6 times the Earth’s diameter follow an Earth-like composition and those with more than 1.6 times Earth’s diameter contain a significant fraction of volatiles. With a density of 2.76 ± 0.73 g/cm³, Kepler-454b is intermediate between a rocky Earth-like world and a Neptune-like world. It falls into the category of planets that are expected to contain a significant of volatiles and/or hydrogen and helium gas.

Figure 4: Mass-radius diagram for planets with less than 2.7 times the Earth’s diameter and with masses measured to better than 20 percent precision. Gettel et al. (2015)

Reference:

Gettel et al. (2015), “The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion”, arXiv:1511.09097 [astro-ph.EP]

Detection of a Dense Super-Earth and a Mini-Neptune

Sinukoff et al. (2015) present a catalogue of 10 multi-planet systems from Campaigns 1 and 2 of the K2 mission - a mission using NASA’s repurposed Kepler space telescope. The catalogue shows the sizes and orbits of 24 planets in six 2-planet systems and four 3-planet systems. The K2 mission hunts for planets by looking for the telltale signature when a planet passes in front of its parent star. Measuring how much starlight the planet blocks and how frequent the planet passes in front of its host star allows the size and orbit of the planet to be determined.

For a particular 2-planet system in this catalogue of 10 multi-planet systems, radial velocity measurements were made using the High Resolution Echelle Spectrometer (HIRES) on the Keck Observatory in Hawaii to determine the masses of the two planets in the planetary system. This 2-planet system orbits EPIC 204221263, a Sun-like star with an effective temperature of 5757 ± 60 K, and 1.07 ± 0.05 times the mass and 1.10 ± 0.09 times the radius of the Sun. The two planets are identified as EPIC 204221263b and EPIC 204221263c, hereafter referred to as planet “b” and planet “c”, respectively.

Figure 1: Artist’s impression of a rocky planet. Image credit: Pauline Moss.

 Figure 2: Transit light curves of EPIC 204221263b and EPIC 204221263c. Sinukoff et al. (2015)

 Figure 3: Radial velocity curves indicating the presence of EPIC 204221263b and EPIC 204221263c. Sinukoff et al. (2015)

Planet “b” is a short-period super-Earth with 1.55 ± 0.16 times the radius and 12.0 ± 2.9 times the mass of Earth. The planet has an orbital period of only 4.02 days and an estimated equilibrium temperature of 1184 ± 51 K. The size and mass of planet “b” are consistent with the planet having a rocky or iron-rich composition. With a remarkably high bulk density of 17.5 [-6.2, +8.5] g/cm³, planet “b” could be the densest planet discovered to date. However, addition radial velocity measurements are needed to better constrain the planet’s mass to confirm this. Furthermore, with its high equilibrium temperature, planet “b” may be the dense remnant core of a gas giant planet whose atmosphere was stripped away by photoevaporation. Such a planet is known as a chthonian planet.

Planet “c” has 2.42 ± 0.29 times the radius and 9.9 ± 4.6 times the mass of Earth. The planet has an orbital period of 10.56 days and an estimated equilibrium temperature of 858 ± 37 K. With its low bulk density of 3.6 [-1.9, +2.7] g/cm³, planet “c” is more akin to a mini-Neptune. The planet probably possesses a substantial amount of low-density volatiles and/or a thick hydrogen-helium envelope. Planet “c” could also be a “water world”, with a core rich in water-ice and surrounded by a thick atmosphere of steam. More precise mass measurements and characterisation of its atmosphere are needed to determine the true bulk composition of planet “c”.

Figure 4: Radii and masses of all confirmed planets whose mass and radius are measured to better than 50 percent (2σ) precision (blue triangles). Solar System planets are represented as black squares. Red circles indicate measurements of EPIC 204221263b and EPIC 204221263c (bottom and top points, respectively). Green curves show the expected planet mass-radius curves for pure iron, rock, and water compositions according to models by Zeng & Sasselov (2013). Sinukoff et al. (2015)

References:

- Sinukoff et al. (2015), “Ten Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths”, arXiv:1511.09213 [astro-ph.EP]

- Zeng & Sasselov (2013), “A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses”, arXiv:1301.0818 [astro-ph.EP]

Observations of the Coldest Directly Imaged Exoplanet

GJ 504 b is a gas giant planet orbiting at a distance of 43.5 AU from a nearby Sun-like star. The relatively large separation of the planet from its host star means that the planet is not lost in the glare of its host star. As a result, the planet can be directly imaged with existing high-contrast imaging systems. GJ 504 b was observed as part of the LEECH exoplanet imaging survey using the Large Binocular Telescope (LBT) in Arizona.

With an estimated temperature of roughly 550 K, GJ 504 b is currently the coldest directly imaged exoplanet. Gas giant planets and brown dwarfs cool with time as they radiate away heat acquired during their formation. A more massive gas giant planet will cool at a slower rate than a less massive gas giant planet. Given two isolated gas giant planets that have the same age but have different masses, the more massive gas giant planet, with its slower rate of cooling, will have a higher temperature than the less massive gas giant planet.

The mass of GJ 504 b can be predicted by two models. For a young GJ 504 b (0.1 to 0.5 billion years old), it is expected to have 3 to 8 times the mass of Jupiter. For an old GJ 504 b (3 to 6.5 billion years old), it is expected to have 19 to 30 times the mass of Jupiter. Observations of GJ 504 b indicate that it has 0.95 ± 0.06 times the diameter of Jupiter and a low surface gravity. This favours the low-mass interpretation, suggesting that GJ 504 b is a young, low-mass gas giant planet.

Observations of GJ 504 b also reveal that the planet has 4.0 ± 1.3 times the metallicity of the Sun. The metallicity of an object refers to its total metal abundance (i.e. all elements heavier than helium). The metallicity of GJ 504 b is much higher than its host star which has 1.3 to 1.9 times the metallicity of the Sun. The super-stellar metallicity of GJ 504 b indicates that the object formed like a planet and not by binary fragmentation. This is because pairs of objects that formed by binary fragmentation should have similar metallicities. Such pairs of objects include star-star binary systems and star-brown dwarf binary systems.

Reference:

Skemer et al. (2015), “The LEECH Exoplanet Imaging Survey: Characterization of the Coldest Directly Imaged Exoplanet, GJ 504 b, and Evidence for Super-Stellar Metallicity”, arXiv:1511.09183 [astro-ph.EP]