Saturn-Mass Planet Circling a Low-Mass Star

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

Hartman et al. (2015) present the detection of HATS-6b, a warm gas giant planet in orbit around a low-mass M-dwarf star. HATS-6b is about the same size as Jupiter. It also has ~0.32 times the mass of Jupiter, about the same mass as Saturn. Circling its host star at a distance of 0.036 AU, HATS-6b takes 3.3 days to complete an orbit. The host star of HATS-6b has 0.57 times the Sun’s mass and only ~1/18th the Sun’s luminosity. HATS-6b and its host star are ~500 light-years away.

HATS-6b receives considerably less stellar irradiation than hot Jupiters, which is why it is termed a “warm” gas giant planet. Based on the luminosity of its host star and its distance from it, HATS-6b is estimated to have a temperature of about 700 K. Its atmospheric chemistry is expected to be quite different compared to hotter planets. HATS-6b appears to be a promising target for atmospheric characterization with the upcoming James Webb Space Telescope (JWST).

The host star of HATS-6b is one of only four stars with less than ~0.6 times the Sun’s mass known to host a gas giant planet in a relatively close-in orbit - the other three are WASP-80, WASP-43 and Kepler-45. Furthermore, the combination of a large planet with a relatively small star means that the transit depth (i.e. the fraction of starlight that is blocked by the planet when the planet transits) of HATS-6b is one of the deepest known.

Figure 2: Transit light curve of HATS-6b. The solid line shows the best-fit transit model. Hartman et al. (2015).

Figure 3: Comparison of HATS-6b with other known transiting gas giant planets in a plot showing host star mass versus planet mass. HATS-6b is indicated by the filled red triangle. Hartman et al. (2015).

Reference:
Hartman et al. (2015), “HATS-6b: A Warm Saturn Transiting an Early M Dwarf Star, and a Set of Empirical Relations for Characterizing K and M Dwarf Planet Hosts”, Astronomical Journal 149, 166.

Inflated Hot Jupiters Spun Out From Stellar Mergers

Over the years, wide area transit surveys have revealed a population of inflated hot Jupiters. These planets are basically Jupiter-mass planets that have remarkably large diameters and orbit very close to their host stars. It has been proposed that inflated hot Jupiters can form when stars in tightly-bound binary systems merger. The magnetic activity of stars in a tight binary system can act as a “brake” and cause the binary system to gradually lose angular momentum. This process brings the 2 stars closer to each other, further accelerating the rate of angular momentum loss. Eventually the 2 stars merge. The merging process can launch a substantial amount of material into orbit which settles into a disk around the newly formed star. Such a disk of material is known as an excretion disk.

Material in the excretion disk quickly coalescences to form one or more Jupiter-mass planets around the star. These planets, where newly formed, are known as inflated hot Jupiters. The reason for the term “inflated” is because these planets are still hot, and it will take time for them to cool and contract. Also, they are called “hot Jupiters” due to the intense stellar irradiation on their daysides as these planets orbit very close to their host star. An excretion disk has a much lower angular momentum compared to a normal protoplanetary disk. As a result, planets that formed out of an excretion disk will remain in close-in orbits around the star. Since a newly formed Jupiter-mass planet cools and contracts with time, measuring the degree of inflation of a hot Jupiter can shed light on how long ago did the merger event that led to the formation of the planet took place.

Hot Jupiters orbit very close to their host stars, typically around 0.05 AU. At such a distance, a hot Jupiter can raise strong tides on its host star. These tides cause the host star to spin-up by taking angular momentum away from the planet’s orbit. The outcome is that the planet’s orbit shrinks. If the planet is massive enough, it can spin-up its host star to co-rotate with its orbit around the star (i.e. the star’s spin period become the same as the planet’s orbital period), and the planet can delay or even avoid spiralling into its star. If the planet is not massive enough, it will continue to lose angular momentum, eventually spiralling into its star and get tidally destructed.

Reference:
Martin et al. (2011), “A binary merger origin for inflated hot Jupiter planets”, arXiv:1102.3336 [astro-ph.SR]

A Warm Gas Giant Planet Circling a Fast-Spinning Star

Bourrier et al. (2015) present the discovery of a transiting warm gas giant planet identified as KOI-12b. It was first detected by NASA’s Kepler space telescope. Follow-up observations were performed using the SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes) échelle spectrograph on the 1.93 m reflector telescope at the Haute-Provence Observatory in south-eastern France. KOI-12b has 1.43 ± 0.13 times the diameter of Jupiter and a 3σ upper mass limit of about 10 times the mass of Jupiter. The diameter of KOI-12b was directly measured from the amount of starlight the planet blocks as it transits in front of its host star.

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

KOI-12b orbits its host star at a distance of about 0.14 AU, with an orbital period of 17.86 days. The host star of KOI-12b is a fast-rotating star (spins with a speed of at least 60 km/s) with 1.45 ± 0.09 times the mass and 1.63 ± 0.15 times the diameter of the Sun. Furthermore, it is a relatively hot star with an effective surface temperature of 6820 ± 120 K. For comparison, the Sun has an effective temperature of 5778 K. The larger size and higher surface temperature means that the host star of KOI-12b is at least ~5 times more luminous than the Sun.

The distance of KOI-12b from its host star suggests that it is a moderately irradiated warm gas giant planet with an estimated temperature of just over 1000 K. However, the diameter of KOI-12b appears inflated given the moderate stellar irradiation it receives. KOI-12b is the largest exoplanet known that has an orbital distance greater than 0.1 AU and the host star of KOI-12b is one of the hottest known to host an exoplanet. Both KOI-12b and its host star are located at a distance of about 1400 light-years away.

Figure 2: Transit light curve of KOI-12b. The best fit to the data is displayed as a red line, with residuals in the lower panel. Bourrier et al. (2015).

Reference:
Bourrier et al. (2015), “SOPHIE velocimetry of Kepler transit candidates XVI. Tomographic measurement of the low obliquity of KOI-12b, a warm Jupiter transiting a fast rotator”, arXiv:1504.04130 [astro-ph.EP]

CoRoT May Have Detected a Binary Planet

CoRoT (French: COnvection ROtation et Transits planétaires; English: COnvection ROtation and planetary Transits) is a space-based observatory built by the French Space Agency (CNES). It detected an unusual transit event identified as SRc01 E2 1066. This single transit event has a depth of 4 percent and a remarkably long duration of 66 hours, more than twice the expected duration of the transit of Jupiter across the Sun. Additionally, the light curve of the transit event also features a “bump” in the middle.

From the long transit duration and the “bump” in the transit light curve, Erikson et al. (2012) suggests that the event was either the transit of a gas giant planet in front of a giant star or the transit of a distantly-orbiting gas giant planet in front of a Sun-like star. In both cases, the planet happened to pass in front of a starspot on the surface of its host star. Since the surface of a star at a starspot is cooler and less bright than the rest of the star’s surface, a planet passing in front of a starspot will block a slightly smaller proportion of the total starlight, resulting in a “bump” in the transit light curve.

Figure 1: Artist’s impression of a gravitationally bound pair of gas giant planets.

However, SRc01 E2 1066 could also be the transit of a gravitationally bound pair of gas giant planets. In this scenario, the “bump” in the transit light curve was due to one planet mutually eclipsing the other during the transit event, resulting in a slight drop in the total amount of starlight blocked by the binary planet, hence the “bump” in the light curve.

S. Ida et al. (2015) showed that SRc01 E2 1066 can be well fitted by a binary planet model. The model assumes a Sun-like host star, with P1 and P2 denoting the two planets. P1 has 0.221 times the radius of its host star and P2 has 0.157 times the radius of its host star. Both planets are tightly bound, separated by only 2.6 times the sum of their physical radii. The binary planet orbits at a distance of 12.5 AU from its host star.

If instead SRc01 E2 1066 was due to the transit of a single planet across a Sun-like host star, then the planet should have 0.187 times the radius of its host star and should orbit at a distance of 25 AU. Note that in the single planet model, the planet also needs to pass in front of a starspot to create the “bump” observed in the light curve.

Figure 2: Light curve for CoRoT target SRc01 E2 1066. The single planet fit (dotted red) and binary planet fit (solid purple line) are also shown. S. Ida et al. (2015).

References:
- A. Erikson et al. (2012), “Planetary transit candidates in the CoRoT-SRc01 field”, Astronomy & Astrophysics, Volume 539, A14
- S. Ida et al. (2015), “Extrasolar Binary Planets II: Detectability by Transit Observations”, arXiv:1504.06365 [astro-ph.EP]

Detecting Binary Planets by Transit Observations

In a planetary system hosting three or more gas giant planets, orbital instabilities can disrupt the planets’ orbits and cause two planets to encounter one another. Planet-planet tidal interactions during the encounter can result in the formation of a gravitationally bound pair of gas giant planets, referred to hereafter as binary planets. It is typical for binary planets to be tightly bound, with separations of only 3 to 5 times the sum of their physical radii. A binary planet around a Sun-like star can remain stable over the lifetime of its host star provided it is orbiting further than ~0.3 AU.

Figure 1: Artist’s impression of a gravitationally bound pair of gas giant planets.

Transit photometry appears to be a promising method for detecting binary planets. When a binary planet transits in front of its host star, the transit has a longer duration and a deeper transit depth than in the case of a single planet. Furthermore, the shape of the light curve from the transit of a binary planet is also different. In fact, some of the false positive detections by NASA’s Kepler space telescope and by the French CoRoT space observatory could turn out to be binary planets.

Typical fates of planetary systems containing three or more interacting gas giant planets are - the ejection of planets, planet-star collisions and planet-planet collisions. If planet-star tidal interactions are taken into account, most of the “planet-star collisions” result in the formation of hot Jupiters (i.e. gas giant planets that orbit very close to their host stars). Correspondingly, if planet-planet tidal interactions are taken into account, some of the “planet-planet collisions” result in the formation of binary planets.

Figure 2: Artist’s impression of a gravitationally bound pair of gas giant planets.

Assuming ~10 percent of all planetary systems host multiple gas giant planets in eccentric orbits, and ~10 percent of these planetary systems eventually form binary planets, then ~1 percent of all planetary systems should host binary planets. Additionally, if binary planets are on average ~0.5 AU from their host stars, then there is a ~1 percent probability that they will transit. As a result, one in ~10,000 stars should host transiting binary planets. A key characteristic of the transit light curve of a binary planet is that it varies from transit-to-transit depending on the positions of the two planets during the transit event. 4 cases have been identified.

Case A: During the transit event, one planet mutually eclipses the other. This causes the light curve to feature a “bump” due to the reduction in the total amount of starlight both planets block (Figures 3 and 4). Such a “bump” in the light curve cannot happen when a single planet transits its host star, unless the planet transits in front of a starspot that happens to be present on the surface of its host star.

Case B: The binary planet transits without any one planet mutually eclipsing the other. As a result, the transit light curve shows a deep dip near mid-transit (transit by both planets) and the deep dip is flanked on both sides by relatively shallower transit depths (transit by one planet) (Figure 4).

Case C: The binary planet transits its host star and one planet either ingresses into transit first or egresses out of transit first. This causes the light curve to feature a “step”. The transit depth steps from shallow to deep when one planet ingresses into transit first, or steps from deep to shallow when one planet egresses out of transit first (Figure 4).

Case D: If the orbital separation of the binary planet is larger, one planet can enter the transit around the same time the other planet leaves the transit. This can create a pair of side-by-side transits or two overlapping transits that overlap around a “bump” in the middle of the transit light curve (Figure 5).

Figure 3: Transit light curve of a binary planet illustrating “Case A”, where one planet mutually eclipses the other during the transit event. The 4 transit light curves shown here involve - a binary planet with each planet having the same mass as Jupiter and twice the diameter of Jupiter (solid purple line); a single planet with twice the mass and 2.8 times the diameter of Jupiter (dotted red line); a binary planet with each planet having the same mass and diameter as Jupiter (dashed blue line); and a single planet with twice the mass and 1.4 times the diameter of Jupiter (dash-dotted light-blue line). Note that a planet with 2.8 times the diameter of Jupiter is physically quite unlikely since the most inflated planet known does not exceed 2 times the diameter of Jupiter. K. Lewis, H. Ochiai, M. Nagasawa and S. Ida (2015).

Figure 4: Transit light curves of a binary planet illustrating “Case A”; “Case B”, where the binary planet transits without any one planet mutually eclipsing the other; and “Case C”, where the binary planet transits and one planet either ingresses into transit first or egresses out of transit first. The 4 curves in each panel are the same as the ones described in Figure 3. K. Lewis, H. Ochiai, M. Nagasawa and S. Ida (2015).

Figure 5: Transit light curve of a binary planet illustrating “Case D”, where one planet enters the transit around the same time the other planet leaves the transit. Here, the orbital separation of the binary planet is larger than in Figures 2 and 3. The 2 transit light curves shown here involve a binary planet with each planet having the same mass as Jupiter and twice the diameter of Jupiter (solid purple line); and a single planet with twice the mass and 2.8 times the diameter of Jupiter (dotted red line). K. Lewis, H. Ochiai, M. Nagasawa and S. Ida (2015).

The variations in shapes of the transit light curve from transit-to-transit may be the most noticeable signal of planet binarity. Since the transits of binary planets are generally longer in duration and deeper in depth than for single planets, some of the known hyper-inflated gas giant planets or light curves classified as false positive detections, may turn out to be binary planets. It is quite likely that binary planets are present in the large dataset of light curves collected by Kepler and CoRoT.

Reference:
K. Lewis, H. Ochiai, M. Nagasawa and S. Ida (2015), “Extrasolar Binary Planets II: Detectability by Transit Observations”, arXiv:1504.06365 [astro-ph.EP]

Three Super-Earths Orbiting HD 7924

HD 7924, a nearby K-dwarf star located ~55 light-years away, hosts a planetary system comprised of three super-Earths. High precision radial velocity measurements made using the High Resolution Echelle Spectrometer (HIRES) at Keck Observatory in Hawaii and the Automated Planet Finder Telescope (APF) at Lick Observatory in California robustly detected the three super-Earths around HD 7924. They are identified as HD 7924 b, HD 7924 c and HD 7924 d, with the lower case alphabetical suffixes denoting their planetary nature.

Super-Earths are a class of planets intermediate in size and mass between Earth and Neptune. Such planets do not exist in the Solar System even though observations have shown that they are ubiquitous around stars. The three super-Earths around HD 7924 were found using the radial velocity method. It involves measuring the tiny “wobbling” of a star due to gravitational tugging from an unseen planet that is orbiting the star. In the case of HD 7924, the way it “wobbles” indicates the presence of three super-Earths in orbit around it.

HD 7924 b has an orbital period of 5.40 days, is at least 8.7 times the mass of Earth, receives 114 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 830 K.

HD 7924 c has an orbital period of 15.30 days, is at least 7.9 times the mass of Earth, receives 28 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 580 K.

HD 7924 d has an orbital period of 24.45 days, is at least 6.4 times the mass of Earth, receives 15 times more insolation than Earth gets from the Sun and has an estimated equilibrium temperature of roughly 500 K.

Of the three super-Earths around HD 7924, HD 7924 c and HD 7924 d are new discoveries, while HD 7924 b was previously discovered in 2009. The orbits of all three super-Earths fit within a region of space less than half the size of Mercury’s orbit around the Sun. Although HD 7924 has only about one-third the Sun’s luminosity, the three super-Earths orbit way too close and receive too much stellar irradiation from HD 7924 to be habitable. 

Deconvoluted radial velocity curves of HD 7924 b (top), HD 7924 c (middle) and HD 7924 d (bottom). Open black squares indicate pre-upgrade Keck/HIRES data, open black circles are post-upgrade Keck/HIRES data, and filled green diamonds are APF data. Fulton et al. (2015).

Reference:
Fulton et al. (2015), “Three Super-Earths Orbiting HD 7924”, arXiv:1504.06629 [astro-ph.EP]

Neso - A Far-Flung Moon of Neptune

Figure 1: An artist’s rendition of the planet Neptune.

Neso is a small, irregular natural satellite in a far-flung orbit around the planet Neptune. Not much is known about Neso. It was discovered in August 2002 and given the provisional designation S/2002 N4. In February 2007, it was named “Neso” by the International Astronomical Union (IAU). Neptune’s large distance from the Sun means that its gravitational field can dominate an extremely wide region of space around it, giving it a large gravitational sphere of influence. As a consequence, Neptune can have moons that orbit around it at large distances and still remain gravitationally bound to Neptune.

Neso orbits Neptune at a mean distance of 50.26 million km. However, the orbit of Neso is fairly elongated. Neso comes as close as 28.93 million km to Neptune and recedes as far as 71.58 million km, its maximum distance from Neptune, making it the most distant known moon of any planet. Neso is in such a wide orbit around Neptune that it takes a whopping 26.67 years to go around Neptune just once. The orbital period of Neso is more than twice the orbital period of Jupiter and nearly as long as the orbital period of Saturn.

In fact, when Neso is furthest from Neptune, the distance between it and Neptune actually exceeds the maximum distance of Mercury from the Sun. Neso orbits Neptune in a retrograde orbit, which means it is in the opposite direction to the rotation of Neptune. Neso’s orbit appears rather similar to the orbit of Psamathe, another natural satellite of Neptune. Both objects might have originated from the break-up of a larger moon. From its observed brightness and assuming an albedo of 0.04, the diameter of Neso is estimated to be ~60 km.

Figure 2: A plan view showing the orbits of some of Neptune’s moons. The orbit of Neso is the one in red, elongated towards the lower left. The dashed circle shows the theoretical outer limit of stability for any moon around Neptune. The orbit of Triton, Neptune’s only large moon, is barely visible on this scale and is represented by the black dot at the center. Sheppard et al. (2006).

Reference:

Sheppard et al. (2006), “A Survey for “Normal” Irregular Satellites around Neptune: Limits to Completeness”, The Astronomical Journal 132: 171-176.

Brown Dwarf in a 3-Year Orbit around DE0630-18

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

With an estimated 0.086 ± 0.009 times the Sun’s mass, DE0630-18 is at the stellar/substellar mass boundary, so it is either a very low-mass star or a brown dwarf. DE0630-18 is estimated to be located ~60 light-years away. Using the FORS2 optical camera on the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in northern Chile, J. Sahlmann et al. (2015) report the discovery of a brown dwarf in a 3-year orbit around DE0630-18.

By precisely tracking the motion of DE0630-18 relative to a field of reference stars over a period of 1209 days between 7 December 2010 and 30 March 2014, DE0630-18 was found to “wobble” in a way that suggests the presence of an unseen companion in orbit around it. The magnitude of the wobbling indicates that the gravitational mass of the unseen companion around DE0630-18 is either ~0.060 or ~0.075 times the Sun’s mass, in the brown dwarf mass-regime.

This newfound object is designated DE0630-18B, with the suffix “B” indicating its secondary nature. The method by which the presence of DE0630-18B was detected is known as astrometry. DE0630-18 and its companion DE0630-18B make up either a brown dwarf / brown dwarf binary system or a very low-mass star (DE0630-18) / brown dwarf (DE0630-18B) binary system.

Figure 2: Motion of DE0630-18 relative to the field of reference stars. J. Sahlmann et al. (2015).

Reference:
J. Sahlmann et al. (2015), “Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18”, arXiv:1504.02469 [astro-ph.SR]

Two Eccentric Giant Planets around a Sun-Like Star

L. Mancini et al. (2015) report on the discovery of KOI-372, a planetary system consisting of two giant planets on wide and eccentric orbits around a Sun-like star. The two giant planets are identified as KOI-372 b and KOI-372 c, with the suffixes “b” and “c” indicating their planetary nature. KOI-372 b is a transiting giant planet that was first detected by NASA’s Kepler space telescope. Follow-up radial velocity measurements were then made using a ground-based telescope to determine the mass of KOI-372 b. The radial velocity measurements and observations by Kepler indicate that KOI-372 b is a dense Jupiter-like planet with 3.25 ± 0.20 times the mass and 0.882 ± 0.088 times the diameter of Jupiter.

Figure 1: Artist’s impression of a giant planet with a hypothetical moon in orbit around it.

KOI-372 b orbits its host star every 125.6 days in a fairly elongated orbit. The orbit of KOI-372 b brings the planet as close as 61 million km (0.41 AU) from its host star and out as far as 87 million km (0.58 AU). KOI-372 b is just interior to the habitable zone around its host star and any large Earth-size moon around KOI-372 b will be too hot to be habitable. Nevertheless, Kepler observed 12 transit events of KOI-372 b and analysis of the mid-transit times shows a clear variation in transit timing. This variation is due to the presence of a second planet in a wider orbit that is perturbing the orbit of KOI-372 b.

The second planet is identified as KOI-372 c and it has a mass between 0.13 to 0.31 times the mass of Jupiter. KOI-372 c also orbits its host star in an elongated orbit with an orbital period of roughly 460 days, at an average distance of about 1.2 AU from its host star. The orbit of KOI-372 c is right within the habitable zone. Any large Earth-size moon in orbit around KOI-372 c could potentially be habitable. Unlike its more massive sibling KOI-372 b, KOI-372 c does not transit its host star. Gyrochronological analysis shows that the planetary system of KOI-372 is relatively young, with an estimated age of 1.0 ± 0.3 billion years. In comparison, the Solar System is 4.57 billion years old.

Figure 2: When the known transiting planets are plotted in a semi-major axis versus planetary-mass diagram, it can be seen that KOI-372 b and KOI-372 c occupy sparsely-populated regions of the plot. The black circles indicate the positions of KOI-372 b and KOI-372 c (the size of the circle that denotes KOI-372 c is arbitrary since the planet does not transit its host star, so its size remains unknown). L. Mancini et al. (2015).

Reference:
L. Mancini et al. (2015), “KOI-372: a young extrasolar system with two giant planets on wide and eccentric orbits”, arXiv:1504.04625 [astro-ph.EP]

Using Supernovae to Detect the Faintest Galaxies

Low-mass dwarf galaxies with stellar masses totalling less than a million times the Sun’s mass are expected to exist in huge numbers. However, these dwarf galaxies are very difficult to detect due to their low luminosities and low surface brightness. Only several of these dwarf galaxies have been found so far. They appear as ultra-diffuse extended sources and detecting them requires imaging large areas of the sky. With the exception of the nearest ones, these dwarf galaxies will remain beyond detection limits even with upcoming wide-field ground-based survey telescopes such as the Large Synoptic Survey Telescope (LSST).

Surface brightness of known dwarf galaxies in the Local Group from data compiled by McConnachie (2012). The dotted line marks the surface brightness limit that can be detected by LSST. Conroy & Bullock (2015).

Galaxies contain stars, and stars are the progenitors of nova and supernova explosions. A study by Conroy & Bullock (2015) investigates the use of nova and supernova explosions to detect the faintest dwarf galaxies in the universe. The detection of such a luminous transient with no apparent host galaxy could indicate the presence of a faint dwarf galaxy. LSST will be able to detect nova explosions out to ~100 million light years and supernova explosions out to billions of light years.

References:
- Conroy & Bullock (2015), “Beacons In the Dark: Using Novae and Supernovae to Detect Dwarf Galaxies in the Local Universe”, arXiv:1504.04015 [astro-ph.GA]
- McConnachie (2012), “The Observed Properties of Dwarf Galaxies in and around the Local Group”, AJ, 144, 4