A Young and Massive Hot-Jupiter

Johns-Krull et al. (2016) present the identification of a massive hot-Jupiter in a ~9 day period orbit around the classical T Tauri star CI Tau. The detection of this planet was made through high-resolution infrared and optical radial velocity measurements. The amplitude of the radial velocity signal suggests that this massive hot-Jupiter has about 11 to 12 times the mass of Jupiter. CI Tau is a very young star with an age of only ~2 million years. The detection of more gas giant planets in close-in orbits around other young stars can shed more light on the planet formation process and on the survivability of massive planets in close-in orbits around young stars. A greater prevalence of hot-Jupiters around young stars compared to older stars may indicate that the destruction of hot-Jupiters around young stars is a common phenomenon.

Infrared and optical radial velocity measurements indicating the presence of a massive hot-Jupiter in a ~9 day period orbit around the classical T Tauri star CI Tau. Johns-Krull et al. (2016)

Reference:
Johns-Krull et al. (2016), "A Candidate Young Massive Planet in Orbit around the Classical T Tauri Star CI Tau", arXiv:1605.07917 [astro-ph.EP]

The Peculiar Orbit of a Hot-Jupiter

Močnik et al. (2016) present the discovery of WASP-157b, a hot-Jupiter in orbit around a G2V star. Transit and radial velocity observations indicate that WASP-157b has 1.045 ± 0.044 times the radius and 0.574 ± 0.093 times the mass of Jupiter. This gives WASP-157b a mean density of roughly half that of Jupiter's. WASP-157b takes only 3.95 days to orbit its host star and its estimated equilibrium temperature is 1339 ± 93 K.

Observations of the host star of WASP-157b show that it has a remarkably slow rotational speed of only ~1.0 km/s. This means that the host star of WASP-157b is either orientated pole-on or it is an exceptionally slow rotator. If the host star of WASP-157b is a slow rotator, then its rotation period is likely to be ~56 days. The average rotation period of stars similar to the host star of WASP-157b is 12.3 days, with a standard deviation of 7.2 days. If the host star is orientated pole-on, it means that WASP-157b has a highly inclined orbit that could even pass over the polar regions of its host star.

Reference:
Močnik et al. (2016), "WASP-157b, a Transiting Hot Jupiter Observed with K2", arXiv:1603.05638 [astro-ph.EP]

Earth-Sized Worlds of TRAPPIST-1

Figure 1: Artist's impression of a planet orbiting a red dwarf star.

TRAPPIST-1 is an M8.0 ± 0.5 type red dwarf star located about 40 light years away. It has only ~1/2000th the Sun's luminosity, ~8 percent the Sun's mass, ~11.5 percent the Sun's radius, and its effective temperature is only 2550 ± 55K. For comparison, the Sun's effective temperature is 5778 K. TRAPPIST-1 is slightly larger in size than the planet Jupiter and its low effective temperature classifies it as an ultracool dwarf. Ultracool dwarfs are a class of stellar-like objects with effective temperatures below 2700 K. They include extremely low mass red dwarf stars and brown dwarfs.

Observations of TRAPPIST-1 revealed the presence of three transiting Earth-size planets in orbit around it. The three planets are identified as TRAPPIST-1b, TRAPPIST-1c and TRAPPIST-1d. The two inner planets, TRAPPIST-1b and TRAPPIST-1c, orbit interior to the habitable zone around the star. Nevertheless, both planets are expected to be tidally-locked, with a permanent day-side hemisphere and a permanent night-side hemisphere. As a result, they can still support habitable regions, especially in the temperate regions around the day-night terminator. For TRAPPIST-1d, there are a number of solutions that place it within or beyond the habitable zone of the star.

Figure 2: Transit photometry of the TRAPPIST-1 planets. Gillion et al. (2016)

TRAPPIST-1b has a 1.51 day orbital period, receives 4.25 ± 0.38 times the amount of flux Earth gets from the Sun, has 1.113 ± 0.044 times the radius of Earth, has a predicted equilibrium temperature between 285 to 400 K, and its mass is roughly estimated to be ~1.38 times the mass of Earth.

TRAPPIST-1c has a 2.42 day orbital period, receives 2.26 ± 0.21 times the amount of flux Earth gets from the Sun, has 1.049 ± 0.050 times the radius of Earth, has a predicted equilibrium temperature between 242 to 342 K, and its mass is roughly estimated to be ~1.15 times the mass of Earth.

TRAPPIST-1c has an orbital period between 4.55 to 72.82 days, receives 0.02 to 1.0 times the amount of flux Earth gets from the Sun, has 1.168 ± 0.068 times the radius of Earth, has a predicted equilibrium temperature between 75 to 286 K, and its mass is roughly estimated to be ~1.60 times the mass of Earth.

With a host star only slightly larger than the planet Jupiter, the planets around TRAPPIST-1 are well suited for atmospheric characterisation. This is because the strength of a planet's transit signal is inversely proportional to the square of its host star's radius. Barstow & Irwin (2016) presented a study showing that if present-day Earth levels of ozone are present on these planets, the upcoming space-based James Webb Space Telescope (JWST) would be able to detect it if JWST observes 60 transits for the innermost planet TRAPPIST-1b, and observes 30 transits for TRAPPIST-1c and TRAPPIST-1d.

Figure 3: Masses of the host stars and equilibrium temperatures of known sub-Neptune-sized exoplanets. The size of the symbols scales linearly with the radius of the planet. The background is colour-coded according to stellar mass (in units of the Sun’s mass). The TRAPPIST-1 planets are at the boundary between planets associated with hydrogen-burning stars and planets associated with brown dwarfs. Equilibrium temperatures are estimated neglecting atmospheric effects and assuming an Earth-like albedo of 0.3. The positions of the Solar System terrestrial planets are shown for reference. The range of possible equilibrium temperatures of TRAPPIST-1d is represented by a solid bar; the dot indicates the most likely temperature. Only the exoplanets with a measured radius equal to or smaller than that of GJ1214b are included. Gillion et al. (2016)

Figure 4: Potential for characterizing the atmospheres of known transiting sub-Neptune-sized exoplanets. Gillion et al. (2016)

References:
- Gillion et al. (2016), "Temperate Earth-sized planets transiting a nearby ultracool dwarf star", arXiv:1605.07211 [astro-ph.EP]
- Barstow & Irwin (2016), "Habitable worlds with JWST: transit spectroscopy of the TRAPPIST-1 system?", arXiv:1605.07352 [astro-ph.EP]

Gas Giant Planet Midway Between Two Stars

ν Octantis is a spectroscopic binary consisting of two stars identified as ν Octantis A (1.61 times the Sun's mass) and ν Octantis B (0.55 times the Sun's mass). Nearly 13 years of radial velocity measurements of ν Octantis indicate the presence of a Jupiter-like planet with ~2.6 times the mass of Jupiter. The remarkable fact about this planet is that it orbits ν Octantis A at half the separation between ν Octantis A and ν Octantis B. The planet takes ~415 days to orbit ν Octantis A, while ν Octantis A and ν Octantis B take ~1050 days to go around one another. The mean separation between v Octantis A and ν Octantis B is ~2.6 AU.

If the orbit of the planet around ν Octantis A is prograde, then its orbit can only be stable if its distance from ν Octantis A is less than ~0.25 times the separation between ν Octantis A and ν Octantis B. Nevertheless, if the orbit of the planet around ν Octantis A is retrograde, its orbit can remain stable even if its separation from ν Octantis A is as large as ~0.5 times the separation between ν Octantis A and ν Octantis B. Since the planet orbits ν Octantis A midway between v Octantis A and ν Octantis B, it clearly indicates a retrograde orbit. Dynamical modelling suggests the orbit of the planet can remain stable for ~100 million years or more.

Reference:
Ramm et al. (2016), "The conjectured S-type retrograde planet in nu Octantis: more evidence including four years of iodine-cell radial velocities", arXiv:1605.06720 [astro-ph.EP]

Irradiation from an Accreting White Dwarf

J1433 is a system comprised of a brown dwarf in a very close-in orbit around a white dwarf, taking only 78.1 minutes to orbit the white dwarf. The brown dwarf was once a star that transitioned from the stellar to the sub-stellar regime as the white dwarf accreted material from it. At present, the brown dwarf is estimated to have 0.055 ± 0.008 times the mass of the Sun and the white dwarf is estimated to have 0.80 ± 0.07 times the mass of the Sun. The mass of the white dwarf in J1433 is consistent with the average white dwarf mass of known accreting white dwarf systems, but is significantly higher than the average mass of isolated white dwarfs (i.e. ~0.6 times the mass of the Sun).

Figure 1: Artist’s impression of an accreting white dwarf.

The close-in orbit of the brown dwarf subjects it to intense irradiation from the white dwarf. For the brown dwarf, its average day-side temperature is 2401 ± 10 K and its average night-side temperature is 2344 ± 7 K. The irradiation-induced average temperature difference between the day-side and night-side on the brown dwarf is ~57 K, and the maximum temperature difference between the hottest and coolest parts on the brown dwarf is ~200 K. This suggests a modest level of heat transfer between the day-side to the night-side on the brown dwarf. As for the white dwarf, its estimated temperature is 13200 ± 200 K.

Since the brown dwarf is tidally-locked, its rotation period is the same as its orbital period. Its short orbital period of only 78.1 minutes means it is rotating very rapidly. The rotation speed at the equatorial region on the brown dwarf is estimated to be at least 131 ± 46 km/s. This is expected to generate interesting atmospheric dynamics and potentially extreme atmospheric phenomena on the brown dwarf.

Figure 2: Ellipsoidal and irradiation effects associated with the brown dwarf in J1433. Hernández et al. (2016)

Reference:
Hernández et al. (2016), "An irradiated brown-dwarf companion to an accreting white dwarf", arXiv:1605.07132 [astro-ph.SR]

A Super-Jupiter in an Extreme Planetary System

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

Santos et al. (2016) present the discovery of a super-Jupiter in a very eccentric long-period orbit around a host star that also has a hot-Neptune in a close-in orbit around it. The super-Jupiter is identified as HD 219828 c. Its orbital period is 13.1 years and it is estimated to have at least 15.1 times the mass of Jupiter. It also has a highly eccentric orbit with eccentricity 0.81. In the same planetary system, but much close to the host star, lies the hot-Neptune HD 219828 b. The orbital period of HD 219828 b is 3.83 days and it is estimated to have at least 21 times the mass of Earth.

The host star in this planetary system is a G0IV star that has evolved slightly beyond the main sequence. It has 3.08 times the Sun's luminosity, 1.23 ± 0.10 times the Sun's mass, 1.69 times the Sun's radius and an estimated effective temperature of 5891 ± 18 K. This extreme planetary system is estimated to be ~250 light years away. Dynamical simulations show that the orbits of HD 219828 b and HD 219828 c are both stable in the long run. Additionally, the large mass of HD 219828 c places it at the mass boundary between gas giant planets and brown dwarfs.

Figure 2: Top and middle plots: phase-folded radial-velocity curves and data for the planets HD 219828 b and HD 219828 c, respectively. Santos et al. (2016)

Reference:

Santos et al. (2016), "An extreme planetary system around HD219828. One long-period super Jupiter to a hot-neptune host star", arXiv:1605.06942 [astro-ph.EP]

Black Hole Tore Apart and Swallowed a Star

Sun-like stars, white dwarfs and giant stars can be wholly swallowed by black holes with masses greater than ~100 million, ~100 thousand and ~10 billion times the mass of the Sun, respectively. When a star is wholly swallowed by a black hole, no flares will be observed. In contrast, for a black hole that is not massive enough to swallow a star whole, a tidal disruption flare can be generated. A star approaching such a black hole on a low angular momentum orbit can be torn apart by tidal forces and a fraction of the star's mass can form an accretion disk around the black hole, powering a tidal disruption flare.

XMMSL1J063045.9603110 is a candidate tidal disruption event. This event was first detected in X-rays with an underlying soft X-ray thermal emission. Twenty days later, XMMSL1J063045.9603110 was again detected with soft X-ray thermal emission, this time ~10 times dimmer than when it was first detected. The X-ray emission over time appears to be consistent with the presence of an accretion disk around a black hole. In this case, the accretion disk represents a fraction of the material left over from a tidally disrupted star that got swallowed by the black hole.

Depending on assumptions, the black hole responsible for this tidal disruption flare is estimated to have between ~10 thousand to ~100 thousand times the mass of the Sun. Optical observations suggest that the black hole associated with XMMSL1J063045.9-603110 likely resides within either a very faint dwarf galaxy or a very bright globular cluster. If the black hole resides within a globular cluster, then XMMSL1J063045.9-603110 could be the first tidal disruption flare observed in a globular cluster.

Reference:
Mainetti et al. (2016), "XMMSL1J063045.9-603110: a tidal disruption event fallen into the back burner", arXiv:1605.06133 [astro-ph.HE]

Discovery of the First Transiting Jupiter Analog

Kipping et al. (2016) present the discovery of the first transiting Jupiter analog. This planet is identified as Kepler-167e and it is part of a planetary system which contains three known transiting super-Earths that are in relatively close-in orbits around their host star. Kepler-167 is the host star of this planetary system. It is a K3V star with ~0.77 times the Sun's mass, ~0.73 times the Sun's radius and an effective temperature estimated to be ~4890 K.

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

The three super-Earths in this planetary system are Kepler-167b, c and d. Kepler-167b has ~1.615 times the radius of Earth, a 4.393 day orbital period, an estimated equilibrium temperature of about 914 K and it receives 116 times the amount of flux Earth gets from the Sun. Kepler-167c has ~1.548 times the radius of Earth, a 7.406 day orbital period, an estimated equilibrium temperature of about 758 K and it receives 57.7 times the amount of flux Earth gets from the Sun. Kepler-167d has ~1.194 times the radius of Earth, a 21.804 day orbital period, an estimated equilibrium temperature of about 536 K and it receives 13.7 times the amount of flux Earth gets from the Sun.

Kepler-167e orbits much further out compared to the three inner super-Earths. Kepler-167e measures ~10.15 times the size of Earth, which is roughly 90 percent the radius of Jupiter. It orbits its host star every 1071 days, its estimated equilibrium temperature is about 130 K and it receives only ~7 percent the amount of flux Earth gets from the Sun. Kepler-167e is about twice as far from its host star as Earth is from the Sun and its orbit around its host star is close to being perfectly circular. With these properties, Kepler-167e bears a great deal of resemblance to Jupiter and hence, it is termed a Jupiter analog.

The compact trio of super-Earths around Kepler-167 raises the possibility that many of the currently known compact multi-planetary systems may host Jupiter analogs on distant orbits. Kepler-167e is a rare find because it transits its host star once every 2.9 years. Since one would need 2 transits to confirm its planetary nature and given that the primary Kepler mission ran for 4.3 years, the detection of Kepler-167e was indeed a fortunate one. Because Kepler-167e is a transiting planet, it also offers a unique opportunity for the characterisation of the atmosphere of a Jupiter analog.

Figure 2: Folded transit light curves of Kepler-167b, Kepler-167c, Kepler-167d and Kepler-167e. For the upper three, data (gray points) are binned to a 10 minute cadence. Light curve of Kepler-167e uses 30 minute binning and uses circles to denote the first transit (Q4) and squares to denote the second transit (Q16). Note that all of the transits were fitted using the original unbinned data. Kipping et al. (2016)

Figure 3: Catalogue of known transiting exoplanets with colour depicting the peak wavelength colour of the parent star. Solar system planets are shown with black symbols, and the Kepler-167 planets with squares. The blue box depicts Jovian-sized planets beyond the snow-line, with Kepler-167e being the first transiting planet to be in this space. Kipping et al. (2016)

Figure 4: Schematic illustrating the scale of the Kepler-167 system. Planet sizes are scaled relative to the key, rather than the orbital distances in order to make them visible. The four known planets display remarkable coplanarity and near-circular orbits with the habitable-zone notably devoid of transiting planets. Kipping et al. (2016)

Reference:
Kipping et al. (2016), "A Transiting Jupiter Analog", arXiv:1603.00042 [astro-ph.EP]

A Red and Rough Kuiper Belt Object

1994 JR1 is a Kuiper Belt Object in a 3:2 orbital resonance with Neptune. That means for every three times Neptune goes around the Sun, 1994 JR1 will go around the Sun twice. 1994 JR1 was observed by the Long Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft on 2 November 2015 and on 7 April 2016. This represents the first close observations of a small KBO. Due to Earth's proximity to the Sun, only the dayside of 1994 JR1 is observable from Earth. As a result, the New Horizons spacecraft, which is beyond the orbit of Pluto, is the only means of observing 1994 JR1 at large solar phase angles.

Combining observations from the New Horizons spacecraft with observations from the Hubble Space Telescope and ground based observatories, 1994 JR1 appears to be a very red KBO with a high surface roughness (i.e. it is probably heavily cratered). 1994 JR1 has a relatively fast rotation period of 5.47 ± 0.33 hours and its diameter is assumed to be ~250 km. Simulations also indicate that the orbit of 1994 JR1 brings it close to Pluto every 2.4 million years. Each close encounter causes the orbit of 1994 JR1 to be gravitationally perturbed by Pluto.

Reference:
Porter et al. (2016), "Red, Rough, Fast, and Perturbed: New Horizons Observations of KBO (15810) 1994 JR1 from the Kuiper Belt", arXiv:1605.05376 [astro-ph.EP]

A Giant Planet in the Light Curve

Figure 1: Artist's impression of a giant planet.

Gould et al. (2016) present the discovery of a massive planet with 4.4 ± 1.6 times the mass of Jupiter in orbit around a red dwarf star with 0.37 ± 0.14 times the mass of the Sun. This system was discovered via gravitational microlensing, whereby the planet-star system crosses the line-of-sight to a background star and the gravitational field of the planet-star system acts as a lens, magnifying light from the background star. The projected separation between the planet and star is estimated to be ~1.2 AU.

This gravitational microlensing event is identified as OGLE-2015-BLG-0954 and it was observed by the Korea Microlensing Telescope Network (KMTNet), a system of three 1.6 m telescopes located in Chile, South Africa and Australia. The wide field of view and the high cadence (6 measurements per hour) of KMTNet allowed for this gravitational microlensing event to be measured despite the short line-of-sight crossing time of only 16 minutes.

Figure 2: Light curve and best-fit model for KMTNet observations of OGLE-2015-BLG-0954 with data from Chile (red), South Africa (blue) and Australia (magenta). Insets show the caustic which extends from 7164.62 to 7165.15. Error bars are omitted from the main figure to avoid clutter but are shown in the residuals. Gould et al. (2016)

Reference:
Gould et al. (2016), "A Super-Jupiter Microlens Planet Characterized by High-Cadence KMTNet Microlensing Survey Observations", arXiv:1603.00020 [astro-ph.EP]

Aquarius 2 is a Difficult to Detect Dwarf Galaxy

Torrealba et al. (2016) present the discovery of Aquarius 2, a dwarf galaxy that is a distant satellite galaxy of the Milky Way. Aquarius 2 was identified based on an overdensity of Red Giant Branch (RGB) stars and an overpopulation of Blue Horizontal Branch (BHB) stars. RGB and BHB stars are stars that have evolved off the main sequence and they are many times more luminous than their main sequence progenitors. These luminous evolved stars can indicate the position of what would otherwise be an invisible galaxy.

The estimated distance of Aquarius 2 from the Milky Way is ~360,000 light years and its estimated half-light radius is ~500 light years. The half-light radius of a galaxy is the size of the volume around the center of the galaxy which accounts for half the galaxy's brightness. The half-light radius of Aquarius 2 is estimated to enclose ~3 million times the Sun's mass.

Aquarius 2 has a low surface brightness of only 30.4 mag/arcsec², and the total luminosity of this dwarf galaxy is only ~4000 times the Sun's luminosity. Aquarius 2 has a large mass-to-light ratio of ~1300, indicating it is a dark matter dominated galaxy. The low surface brightness and low overall luminosity of Aquarius 2 makes it a particularly difficult galaxy to detect. The detection of Aquarius 2 suggests the presence of more dwarf galaxies lurking out there.

The red solid line denotes the elliptical half-light contour of the best fit model for Aquarius 2. The orange, red and blue circles/squares mark the locations of stars confirmed as foreground, RGB and BHB stars, respectively. Torrealba et al. (2016)

Absolute magnitude versus half-light radius for Milky Way satellite galaxies (red open circles), M31 satellite galaxies (black unfilled triangles), globular clusters (black dots), extended objects larger than ~300 light years in size (grey dots) and Local Group/nearby galaxies (grey crosses). Torrealba et al. (2016)

Reference:
Torrealba et al. (2016), "At the survey limits: discovery of the Aquarius 2 dwarf galaxy in the VST ATLAS and the SDSS data", arXiv:1605.05338 [astro-ph.GA]

Clouds of Water on a Cold Brown Dwarf

Figure 1: Artist's rendering of Jupiter.

WISE 0855 is the coldest brown dwarf known. It has an effective temperature of only ~250 K. This is comparable to the atmospheric temperatures found on Earth, Mars, Jupiter and Saturn. WISE 0855 is estimated to have between 3 to 10 times the mass of Jupiter, and it is located ~7.5 light years away. WISE 0855 provides a good opportunity for the direct study of an object with similar physical characteristics as Jupiter. Observations of WISE 0855 with the Gemini-North telescope located near the summit of Mauna Kea in Hawaii revealed the presence of atmospheric water vapour and clouds on WISE 0855. The spectrum of WISE 0855 appears remarkably similar to Jupiter's. 

Figure 2: Temperature-pressure profiles of Jupiter, WISE 0855 and Gl 570D. Two dashed lines show the boundaries where H2O gas and NH3 gas begin to condense into clouds composed of H2O ice and NH3 ice. Skemer et al. (2016)

Reference:
Skemer et al. (2016), "The First Spectrum of the Coldest Brown Dwarf", arXiv:1605.04902 [astro-ph.EP]

Two Giant Planets on Very Dissimilar Orbits

Raetz et al. (2016) present the first ever detection of a directly imaged planet in a wide-separation orbit around a star that also hosts a short-period transiting planet candidate. The directly imaged planet is identified as CVSO 30c and the short-period transiting planet candidate is identified as CVSO 30b. Both CVSO 30b and CVSO 30c are gas giant planets, each estimated to contain a few times the mass of Jupiter. The host star of CVSO 30b and CVSO 30c is a relatively young star estimated to be only a couple or so million years old. Its effective surface temperature is ~3470 K, its mass is 0.34 to 0.44 times the Sun's mass and its luminosity is about a quarter the Sun's luminosity.

CVSO 30b is short-period transiting planet candidate with an extremely short orbital period of only 10.8 hours. In contrast, the directly imaged planet CVSO 30c has an orbital period of about 27,000 years. The orbits of both planets could not have been more different. Both planets may have gotten into their current orbits following a violent planet-planet scattering event. CVSO 30c is 662 ± 96 AU from its host star, and this is far enough that the planet is not lost in the glare of its host star, allowing it to be directly detected. The direct observations indicate that the equilibrium temperature of CVSO 30c is ~1600 K.

Reference:
Raetz et al. (2016), "YETI observations of the young transiting planet candidate CVSO 30 b", arXiv:1605.05091 [astro-ph.EP]

Powerful White-Light Flare from a Red Dwarf Star

Schmidt et al. (2016) present the detection of a powerful white-light flare on a diminutive L0 spectral type red dwarf star. The event is given the designation ASASSN-16AE and the red dwarf star is identified as SDSS0533, estimated to be 315 ± 75 light years away. SDSS0533 appears to be near the stellar-substellar boundary. Nevertheless, SDSS0533 is most likely a red dwarf star rather than a brown dwarf because it is an old object.

Such a conclusion can be made because spectral observations indicate that SDSS0533 does not have a low surface gravity. This implies that SDSS0533 had sufficient time to contract to its current size, and the same mass in a smaller volume gives a higher surface gravity. Since SDSS0533 is a red dwarf star, it is hot enough to have an L0 spectral type despite its old age. If SDSS0533 is a brown dwarf, it would have already cooled too much to maintain an L0 spectral type.

The white-light flare that erupted on SDSS0533 is one of the strongest detected so far. During the flare, a significant area on the red dwarf star is predicted to have reached temperatures exceeding 10,000 K. For comparison, the normal temperature on such a red dwarf star is only ~2000 K or so. Based on the best-fit model, the flare's luminosity appears to have a half life of approximately 180 seconds. The detection of ASASSN-16AE shows that violent stellar-type activity can occur for objects belonging to the L spectral class. Objects in this spectral class include the least massive stars, and the youngest and/or most massive brown dwarfs.

Reference:
Schmidt et al. (2016), "ASASSN-16ae: A Powerful White-Light Flare on an Early-L Dwarf", arXiv:1605.04313 [astro-ph.SR]

Discovery of a Super-Sized Rocky Planet

Osborn et al. (2016) present the discovery of a super-sized rocky planet identified as EPIC212521166 b. This planet orbits an old metal-poor K3 host star every 13.86 days. A combination of high-precision transit and radial velocity observations indicate that EPIC212521166 b has 2.6 ± 0.1 times the radius and 18.3 ± 2.8 times the mass of Earth. This gives EPIC212521166 b a similar density as Earth, making it the most massive known planet with a sub-Neptune radius. The surface gravity on EPIC212521166 b is nearly 3 times the surface gravity on Earth.

Figure 1: Artist's impression of a rocky planet.

Interior models of EPIC212521166 b show that the planet can be comprised of an Earth-like core with 9 times the mass of Earth containing ~70 percent rock and ~30 percent iron, and an overlying layer of water amounting to 9 times the mass of Earth. Alternatively, EPIC212521166 b can be comprised of an iron core and rocky mantle totalling 18.1 times the mass of Earth, with a ~2500 km thick hydrogen-helium atmosphere measuring just 0.2 times the mass of Earth.

The remarkably high mean density of EPIC212521166 b suggests that rocky planets with 10 to 20 times the mass of Earth can form without accreting significant amounts of hydrogen and helium. EPIC212521166 b is unlikely to be the massive core of a gas-giant planet whose hydrogen and helium envelope was stripped away by stellar radiation because radiation from its host star is incapable of removing any substantial amount of hydrogen from the planet. The proximity of EPIC212521166 b from its host star gives it an equilibrium temperature of 640 ± 20 K.

Figure 2: EPIC212521166 b (solid cross, right) compared to other super-Earth, sub-Neptune and Neptunian planets. The mass-radius curves shown here are for planets with 100 percent iron, Earth-like, 50 percent water and 100 percent water compositions (dashed lines from bottom to top). Osborn et al. (2016)

Reference:
Osborn et al. (2016), "EPIC212521166 b: a Neptune-mass planet with Earth-like density", arXiv:1605.04291 [astro-ph.EP]

Detecting the Presence of an Unseen Hot-Jupiter

Hot-Jupiters are a class of Jupiter-like planets that orbit very close to their host stars. They have orbital periods of only a few days. The most commonly accepted mechanism regarding the formation of hot-Jupiters is that they formed at larger distances from their host stars before migrating inwards. Alternatively, hot-Jupiters may also form in situ via gas accretion onto massive cores with 10 to 20 times the mass of Earth. Hot-Jupiters that formed in situ are expected to be accompanied by low-mass companion planets with orbital periods of less than ~100 days.

Millholland et al. (2016) present the possible detection of a non-transiting hot-Jupiter in a planetary system consisting of two low-mass transiting planet candidates with longer orbital periods. The technique employed to detect this non-transiting hot-Jupiter is a novel one which combines optical phase curve analysis and astrometric transit timing variations (TTVs). Optical phase curve analysis involves measuring the reflected light from the hot-Jupiter as its dayside rotates in and out of view.

Astrometric TTVs occurs when the transit timings of the outer planets are not perfectly periodic as the star they orbit is a "moving target". This is because the mass of the hot-Jupiter is sufficiently large to be non-negligible in comparison to the mass of the host star. As a result, the gravitational influence of the hot-Jupiter causes the star to wobble as both the star and hot-Jupiter are orbiting their common center-of-mass.

The non-transiting hot-Jupiter in this study was detected around a Sun-like star identified as KOI-1858. Orbiting KOI-1858 are two known transiting planet candidates - KOI-1858.01 and KOI-1858.02. KOI-1858.01 has ~3.53 times the radius of Earth and has a 116.3 day orbital period. KOI-1858.02 has ~2.06 times the radius of Earth and has a 86.0 day orbital period. Both the observed optical phase curve and astrometric TTVs are mutually consistent with the presence of a non-transiting hot-Jupiter with 1.5 ± 0.4 times the mass of Jupiter and a 2.991 day orbital period.

Reference:
Millholland et al. (2016), "On the Detection of Non-Transiting Hot Jupiters in Multiple-Planet Systems", arXiv:1602.05674 [astro-ph.EP]

Possible Detection of Ground Fog on Titan

On 25 December 2004, the Huygens probe was released from the Cassini spacecraft. The probe descended through the atmosphere of Titan and landed on the surface of Titan on 14 January 2005. The probe's descent through Titan's atmosphere took 2.5 hours, and the probe continued to take measurements and relay data for an hour after landing. Smith et al. (2016) present results from the analysis of data taken with the Descent Imager/Spectrometer Radiometer (DISR) on the Huygens probe.

The analysis involved 82 images taken with the Side Looking Imager (SLI), a sub-instrument on the DISR. These images were all taken while the Huygens probe was on the surface of Titan. Out of the 82 images, 6 images appear to show an extended, horizontal feature above the horizon that seems to rise and fall over the course of the observations. The most likely interpretation is that this feature is a fog bank near the horizon that rose and fell over a span of time.

Other than a fog bank near the horizon, a number of other explanations might potentially account for the observed feature. However, none of these alternative explanations appear likely. One alternative is that the observed feature is a low-level cloud. This is unlikely because the shape and position of the feature did not appear to change much throughout the observation duration of over 20 minutes. For comparison, clouds are known to change shape and move on time spans of less than a minute. Nevertheless, the possibility that the observed feature is a low-level cloud cannot be ruled out entirely.

Another explanation is that the observed feature is a mirage. A superior mirage occurs when the image appears above the horizon because the ground is colder than the surrounding air, while an inferior mirage occurs when the image appears below the horizon because the ground is warmer than the surrounding air. If the observed feature is a mirage, it would be a superior mirage as it is above the horizon. However, as the Huygens probe descended though Titan's atmosphere, its measurements indicate increasing temperature with decreasing altitude, and that the air above ground is cooler than the ground itself. As a result, a mirage is an unlikely explanation for the observed feature.

Reference:
Smith et al. (2016), "Possible ground fog detection from SLI imagery of Titan", arXiv:1603.04413 [astro-ph.EP]

1284 Newly Validated Planets

Figure 1: Artist's impression of an exoplanet.

Morton et al. (2016) present calculations of the astrophysical false positive probability (FPP) for every Kepler object of interest (KOI) detected by NASA's planet-hunting Kepler space telescope. Out of 7056 KOIs, 1935 KOIs have FPP of less than 0.01, validating them as true planets with a confidence level better than 99 percent. Among them, 1284 KOIs have not been validated or confirmed previously. These 1284 newly validated planets more than doubles the number of confirmed Kepler exoplanets and brings the total number of known exoplanets to well over 3000.

Of these 1284 newly validated planets, over 500 of them are believed to be rocky planets. Among them, 9 planets are potentially habitable because they are small enough (i.e. they are not gas-giant planets) and appear to be situated within the habitable zones of their host stars. These potentially habitable planets are between 1 to 2 times the radius of Earth, and they receive between one-third to 1.5 times the amount of flux Earth gets from the Sun.

Figure 2: Properties of the 9 KOIs validated by Morton et al. (2016) that lie within the habitable zones of their host stars. Morton et al. (2016)

Figure 3: Periods and radii of candidate and confirmed KOIs. Blue circles represent confirmed KOIs. Orange circles represent candidate KOIs, and they are shaded by false positive probability (FPP), with a transparent circle representing a high FPP. Morton et al. (2016)

Reference:
Morton et al. (2016), "False positive probabilties for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives", arXiv:1605.02825 [astro-ph.EP]

Magnesium and Calcium from Evaporating Trojans

WASP-12b is a hot-Jupiter with a remarkably high equilibrium temperature of over 2500 K. It orbits its host star only two stellar radii from the star's surface. Observations of the host star of WASP-12b in ultraviolet wavelengths indicate that the host star appears dark at certain wavelengths that correspond to absorption by magnesium and calcium. This points to the presence of a cloud of magnesium ions, calcium ions, and various other ions that is surrounding the star and absorbing stellar radiation.

A study by Kislyakova et al. (2016) suggests that material outgassed from the surface of rocky Trojan satellites on tadpole orbits near the L4 and L5 Lagrange points of WASP-12b can account for such a cloud of magnesium and calcium ions surround the star. At the location of WASP-12b, temperatures are high enough to melt the surface of rocky Trojan satellites to create magma oceans that can thermally released materials such as magnesium, silicon, calcium, iron, etc. These materials are then ionised by stellar radiation and energetic electrons to form a cloud of plasma around the host star.

The result is a cloud of magnesium and calcium ions along the orbit of WASP-12b that is expected to be extended enough to cover the entire stellar disk. The absorption of stellar emission at some ultraviolet wavelengths is also predicted to occur in other exoplanetary systems that have transiting hot-Jupiters with extremely high equilibrium temperatures that exceed 2000 K.

Two configurations were considered in this study. The first configuration consists of two rocky Trojan satellites similar in size to Jupiter's moon Io around the L4 and L5 Lagrange points. The second configuration is comprised of two Trojan swarms consisting of 100 rocky bodies, each measuring 100 km in radius. The configuration with two Io-sized Trojan satellites has greater stability on long time scales, while the configuration with two Trojan swarms can more readily generate an extended cloud of magnesium and calcium ions.

Reference:

Kislyakova et al. (2016), "On the ultraviolet anomalies of the WASP-12 and HD 189733 systems: Trojan satellites as a plasma source", arXiv:1605.02507 [astro-ph.EP]

Two Giant Planets Orbiting Rapidly Rotating Stars

KELT-7b and HAT-P-56b are two hot-Jupitets that orbit rapidly rotating F-type host stars. KELT-7b has 1.28 ± 0.18 times the mass and 1.53 times the radius of Jupiter, while HAT-P-56b has 2.18 ± 0.25 times the mass and 1.47 times the radius of Jupiter. KELT-7b orbits its host star every 2.73 days and the maximum rotation period of its host star is 1.08 ± 0.03 days. HAT-P-56 orbits its host star every 2.79 days and the maximum rotation period of its host star is 1.8 ± 0.2 days. Both KELT-7b and HAT-P-56b belong to a rare class of super-synchronous hot-Jupiters whose orbital periods are longer than the rotation periods of their host stars.

A recent study by Zhou et al. (2016) show that the spin-orbit alignments of KELT-7b and HAT-P-56b are 2.8 ± 0.6 degeees and 8 ± 2 degrees, respectively. This means that the orbital planes of both planets are closely aligned with the equatorial planes of their host stars. Planets found in such well-aligned orbits are believed to have migrated towards their host star in the protoplanetary disk via planet-gas interactions. On the contrary, planets found in highly inclined orbits experienced dynamical interactions such as planet-planet scattering events.

In the study by Zhou et al. (2016), there appears to be no evidence that the rotation rates of the host stars of KELT-7b and HAT-P-56b have been modified by star-planet tidal interactions. This indicates that the current spin-orbit angles of KELT-7b and HAT-P-56b most likely represent their initial configurations right after they have migrated towards their host stars via planet-gas interactions in the protoplanetary disk.

Reference:
Zhou et al. (2016), "Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES", arXiv:1605.01991 [astro-ph.EP]

Spin-Orbit Alignment of KOI-2138.01

When a star rotates fast enough, centrifugal forces will cause the star to assume a detectably oblate spheroidal shape. This means the star has a larger radius at its equator than at its poles. A consequence of this is that the equatorial regions will have a lower surface gravity, and thus lower temperature and brightness; while the polar regions will have a higher surface gravity, and thus higher temperature and brightness. This darkening around the star's equator is known as gravity darkening.

KOI-2138.01 is a transiting super-Earth planet candidate in orbit around a rapidly rotating host star that has 2.29 times the radius and 2.34 times the mass of the Sun. The star's rapid rotation spins it into an oblate spheroid with a gravity darkened equator. Observing the way KOI-2138.01 transits its host star and how it passes in front of the gravity darkened equator allows the planet's orbital inclination to be determined. KOI-2138.01 is found to have a relatively flat orbit with a projected spin-orbit alignment of only 1 ± 13 degrees. The close proximity of KOI-2138.01 from its host star also gives it an estimated dayside temperature of up to ~1300 K.

Reference:
Barnes et al. (2016), "Probable Spin-Orbit Aligned Super-Earth Planet Candidate KOI-2138.01", arXiv:1512.03855 [astro-ph.EP]

Thermal Expansion of Water-Rich Super-Earths

Super-Earths are a relatively common class of planets that have masses between 1 to 10 times the mass of Earth. Internal structure models of super-Earths usually do not include thermal effects due to the understanding that the thermal expansion of a solid Earth-like planet is negligible. Thomas & Madhusudhan (2016) present temperature-dependent internal structure models of super-Earths and found that thermal effects can induce significant changes in the radii of water-rich super-Earths.

Figure 1: Artist's impression of a super-Earth.

The total mass of water on Earth forms a negligible fraction of the planet's total mass. Unlike Earth, water-rich super-Earths can have water mass fractions exceeding one percent. In the study by Thomas & Madhusudhan (2016), water-rich super-Earths are assumed to be comprised of Earth-like cores (i.e. 33 percent iron and 67 percent silicates) beneath heated water layers. At low temperatures and pressures, water exists as a liquid, vapour or solid (Ice Ih). At the high pressures and temperatures expected on these water-rich super-Earths, water can take on a number of alternate forms such as exotic high pressure ices (i.e. Ice V, VI, VII, X, etc), a supercritical fluid, or a superheated vapour.

Consider a water-rich super-Earth with 4 times the mass of Earth. Its internal structure is comprised of an Earth-like core beneath a layer of water constituting 5 percent of the planet's mass. If the pressure on the planet's surface is 100 bar, the increase in the planet's radius when the planet's surface temperature increases from 300 K to 1000 K is approximately 0.3 times the radius of Earth. The planet's radius is expected to increase by a larger amount with lower surface pressures and/or higher surface temperatures.

Surface pressure is an important factor in determining the radius of a water-rich super-Earth because the thermal expansion of water decreases under higher pressures. Take a water-rich super-Earth with 4 times the mass of Earth, a water mass fraction of 10 percent, and a surface temperature of 1000 K. If the planet's surface pressure is increased from 10 bar to 1000 bar, its water layer will be compressed significantly, and this is expected to decrease the planet's radius by a factor of two.

Figure 2: The radius of a water-rich super-Earth depends on its surface temperature and internal temperature profile (i.e. adiabatic or isothermal). A higher surface temperature leads to a larger planetary radius. This figure shows temperature-dependent internal structure models of water-rich super-Earths that are comprised of an Earth-like core under a water layer that makes up 30 percent of the planet's mass. Also, the surface pressure is assumed to be 100 bar. Thomas & Madhusudhan (2016)

Figure 3: The radius of a water-rich super-Earth depends a lot on its surface pressure. However, for pressures above 1000 bar the effect temperature has on the radius of a water-rich super-Earth becomes insignificant. This figure shows temperature-dependent internal structure models of water-rich super-Earths that are comprised of an Earth-like core under a water layer that makes up 30 percent of the planet's mass. Thomas & Madhusudhan (2016)

Nevertheless, the water content of a water-rich super-Earth does not have much of an effect on the planet's radius. For example, a water-rich super-Earth with 10 times the mass of Earth and a total water content of 50 percent, can increase its radius by 0.5 times the radius of Earth when its surface temperature is increased from 300 K to 1000 K. Given the same increase in surface temperature and the same planet, but now with a total water content of just 1 percent, the increase in the planet's radius is only slightly smaller, at 0.4 times the radius of Earth.

Reference:
Thomas & Madhusudhan (2016), "In hot water: effects of temperature-dependent interiors on the radii of water-rich super-Earths", arXiv:1602.02758 [astro-ph.EP]

Globular Clusters in Dark Galaxies

Globular clusters are dense, spherical concentrations of stars. They tend to reside in the halos of galaxies. The Milky Way is surrounded by a halo that is populated by globular clusters and satellite galaxies. A study by Zaritsky et al. (2016) suggests that some of these globular clusters actually belong inside the halos of undetected satellite galaxies of the Milky Way. This is supported by the recent confirmation of a globular cluster that is associated with the ultrafaint galaxy Eridanus II, a satellite galaxy of the Milky Way. The high surface brightness of a globular cluster due to its high concentration of stars can make it much easier to identify than its host galaxy. This is especially true if the host galaxy is strongly dominated by dark matter and has an extremely tiny population of stars scattered over a large area. As a result, some of the globular clusters in the halo of the Milky Way galaxy could be residing within the halos of undetected dark satellite galaxies.

Reference:
Zaritsky et al. (2016), "Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies", arXiv:1604.08594 [astro-ph.GA]