Survival of Far-Flung Planet Nine

Planet Nine is the name given to a possible planetary object in the outer Solar System. Its existence was inferred from the effect it has on the alignment of the orbits of a peculiar class of objects in the outer Solar System. Planet Nine is estimated to have ~10 times the mass of Earth, and it goes around the Sun in an eccentric orbit at an average distance of roughly 400 to 1500 AU from the Sun. With such a far-flung orbit, the orbit of Planet Nine is susceptible to disruption by passing stars. Li & Adams (2016) conducted a study to evaluate the survival rates of Planet Nine.

The Solar System is expected to have formed within a cluster of stars containing roughly 1,000 to 10,000 members. Planet Nine is likely to be ejected if the Sun had resided within its natal cluster for longer than ~100 million years. After the Sun had drifted out of its natal cluster, the probability of Planet Nine being ejected from the Solar System due to interactions with passing field stars is only ~3 percent over the age of the Sun. Finally, Planet Nine could be a planet that was captured from another star or it could be a captured free-floating planet. However, both these scenarios have relatively low probabilities.

Reference:
Li & Adams (2016), "Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine", arXiv:1602.08496 [astro-ph.EP]

Giant Planets that can Potentially Host Habitable Moons

Díaz et al. (2016) present the detection of 4 gas giant planets in long-period orbits around stars that are somewhat more luminous than the Sun. The 4 planets were detected using the radial velocity method, and they are identified as HD191806b, HD214823b, HD221585b and HD16175b. The suffix "b" denotes their planetary nature. All 4 planets have orbits that are largely within the habitable zone of their host stars. Furthermore, because all 4 planets are much more massive than Jupiter, they can host correspondingly more massive moons that might be habitable.

Figure 1: Artist's impression of a gas giant planet with a large habitable moon.

HD191806b is estimated to have at least 8.52 ± 0.63 times the mass of Jupiter. It orbits its host star every 1606.3 ± 7.2 days with an orbital eccentricity estimated to be 0.259 ± 0.017. Its host star has 1.14 ± 0.12 times the mass and 2.23 ± 0.16 times the luminosity of the Sun, and its effective temperature is 6010 ± 30 K. The average amount of insolation HD191806b receives is 0.30 ± 0.03 times of what Earth gets from the Sun. The relatively low average insolation that HD191806b receives places it on the cold side of the habitable zone.

HD214823b is estimated to have at least 19.2 ± 1.4 times the mass of Jupiter. It orbits its host star every 1877 ± 15 days with an orbital eccentricity estimated to be 0.154 ± 0.014. Its host star has 1.22 ± 0.13 times the mass and 4.35 ± 0.58 times the luminosity of the Sun, and its effective temperature is 6215 ± 30 K. The average amount of insolation HD214823b receives is 0.44 ± 0.07 times of what Earth gets from the Sun. The high mass of HD214823b suggests that it is more appropriate to classify it as a brown dwarf instead of a planet.

HD221585b is estimated to have at least 1.61 ± 0.14 times the mass of Jupiter. It orbits its host star every 1173 ± 16 days with an orbital eccentricity estimated to be 0.123 ± 0.069. Its host star has 1.19 ± 0.12 times the mass and 2.64 ± 0.18 times the luminosity of the Sun, and its effective temperature is 5620 ± 27 K. The average amount of insolation HD221585b receives is approximately half of what Earth gets from the Sun.

HD16175b is estimated to have at least 4.77 ± 0.37 times the mass of Jupiter. It orbits its host star every 995.4 ± 2.8 days with an orbital eccentricity estimated to be 0.637 ± 0.020. Its host star has 1.34 ± 0.14 times the mass and 3.22 ± 0.25 times the luminosity of the Sun, and its effective temperature is 6022 ± 34 K. The average amount of insolation HD16175b receives is 0.91 ± 0.10 times of what Earth gets from the Sun. The large eccentricity of the planet's orbit causes the amount of insolation it gets from its host star to vary by a factor of approximately 20. A hypothetical large rocky moon in orbit around HD16175b can still be habitable provided it can maintain a relatively constant temperature throughout the planet's orbit.

Figure 2: Schematic view of the orbits of HD191806b, HD214823b, HD221585b and HD16175b. The filled green areas denote the habitable zone. Díaz et al. (2016)

Reference:
Díaz et al. (2016), "The SOPHIE search for northern extrasolar planets. XI. Three new companions and an orbit update: Giant planets in the habitable zone", arXiv:1604.07610 [astro-ph.EP]

Neptune-Sized Planet Orbiting a Baby Star

Mann et al. (2016) present the discovery of a super-Neptune-sized planet in a close-in orbit around a low-mass pre-main sequence star. This planet is identified as EPIC 205117205b. Its size is estimated to be ~5 times the diameter of Earth and it orbits its host star every 5.425 days. The host star of EPIC 205117205b is estimated to have 0.54 times the mass, 1.06 times the radius and 0.17 times the luminosity of the Sun. Notice that the radius and luminosity are both way too large for a star of this mass. This is because the host star of EPIC 205117205b is still in the process of contracting and settling down as a red dwarf star. Its age is estimated to be less than ~11 million years. The discovery of EPIC 205117205b suggests that close-in planets can form where they already are, or they can form further out and then migrate rapidly, within ~10 million years, to their present close-in orbits.

Reference:
Mann et al. (2016), "Zodiacal Exoplanets in Time (ZEIT) III: A Neptune-sized planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association", arXiv:1604.06165 [astro-ph.EP]

Two Different Interpretations of a Microlensing Signature

Figure 1: Artist's impression of a pair of red dwarf stars.

OGLE-2013-BLG-0723 is a gravitational microlensing event that was reported to be caused by a Venus-mass planet orbiting a brown dwarf in a binary system. Han et al. (2016) analysed the event again and found that a 2-body solution is more likely than the current 3-body solution. In this case, the 2-body solution involves a pair of low-mass red dwarf stars with ~0.2 and ~0.1 times the mass of the Sun, located ~3000 light years away. Both stars are spaced ~1.5 AU from each other. The fact that two very different solutions can approximate the same gravitational microlensing light curve shows that careful examination of all possible solutions is necessary.

Figure 2: Light curve of OGLE-2013-BLG-0723. The cyan and black curves plotted over the data are the best-fit models obtained from the previous 3-body model and the newly found 2-body model, respectively. The two lower panels show the residuals from the individual models. Han et al. (2016)

Reference:
Han et al. (2016), "A New Non-Planetary Interpretation of the Microlensing Event OGLE-2013-BLG-0723", arXiv:1604.06533 [astro-ph.EP]

Red Dwarf Stars and Saturn-Mass Planets

When a foreground star happens to cross the line-of-sight to a distant background star, the gravitational field of the foreground star can act as a lens, magnifying light from the background star. The brightening of the background star is recorded in the form of a light curve as the foreground star crosses its line-of-sight. If the foreground star hosts a planet, the gravitational field of the planet can induce a "blip" in the light curve. This phenomenon is known as gravitational microlensing and it is one of the methods used to detect planets around other stars.

Figure 1: Artist's impression of a Saturn-mass planet.

Using this technique, Hirao et al. (2016) present the discovery of a Saturn-mass planet around a red dwarf star with ~30 percent the mass of the Sun. The planet is identified as OGLE2012-BLG-0724Lb. By analysing the gravitational microlensing light curve, the planet is estimated to have ~0.47 times the mass of Jupiter and its estimated projected separation from its host star is ~1.6 AU.

The discovery of this Saturn-mass planet around a red dwarf star adds to the population of known sub-Jupiters (i.e. planets with 0.2 to 1.0 times the mass of Jupiter) around red dwarf stars. On the contrary, there appears to be a lack of Jupiter-mass planets (i.e. planets with 1 to 2 times the mass of Jupiter) around red dwarf stars. This supports the core accretion model of planet formation which predicts that Jupiter-mass planets, and planets that are even more massive, are unlikely to form around red dwarf stars.

Figure 2: The top panel shows the gravitational microlensing light curve. The middle panel shows a close up of the "blip" in the gravitational microlensing light curve indicating the presence of OGLE2012-BLG-0724Lb. The bottom panel shows the residual from the best fit model. Hirao et al. (2016)

Reference:
Hirao et al. (2016), "OGLE-2012-BLG-0724Lb: A Saturn-mass Planet around an M-dwarf", arXiv:1604.05463 [astro-ph.EP]

The Hot-Super-Earth Desert

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

The transition from a predominantly rocky composition to a volatile-rich composition occurs for planets roughly 1.6 to 1.8 times the radius of Earth. Planets around this size and larger are known as super-Earths since they are larger in size than Earth, but smaller than Neptune. For comparison, Neptune is about 3.8 times the size of Earth. Studies have shown that hot-super-Earth sized planets that orbit close to their host stars can have their volatile-rich envelopes stripped by photo-evaporation due to high incident fluxes. This results in a lack of super-Earth sized planets with high incident fluxes.

A study by Lundkvist et al. (2016) shows there appears to be no super-Earth sized planets between 2.2 and 3.8 times the size of Earth that receive incident flux above 650 times the incident flux on Earth. This is consistent with the prediction that the majority of planets between 2.2 and 3.8 times the size of Earth are expected to be volatile-rich and would be stripped of their outer envelopes when subjected to high incident fluxes. As a result, some fraction of hot-super-Earth sized planets smaller than 2.2 times the size of Earth started out as larger planets whose volatile-rich envelopes were stripped away by high incident fluxes.

Figure 2: Radius-flux diagram showing the distribution of exoplanets. The location of the four rocky solar-system planets; Mercury (Me), Venus (V), Earth (E) and Mars (M) is indicated with the green writing (no points). The location of the hot-super-Earth desert has been shaded. Lundkvist et al. (2016)

Reference:
Lundkvist et al. (2016), "Hot super-Earths stripped by their host stars", arXiv:1604.05220 [astro-ph.EP]

J1122+25 Could be the Fastest Spinning Brown Dwarf

As part of a search for flaring radio emissions from a sample of L and T brown dwarfs, Route & Wolszczan (2016) present the discovery of flares from a T6 brown dwarf identified as J1122+25. The flaring appears to be occurring with a stable recurrence period of 0.288 hours, or ~17 minutes. If this is indeed the rotation period of the brown dwarf, it is much shorter than the shortest rotation periods inferred from observations of photometric variability of brown dwarfs at optical/near-infrared wavelengths - 1.41 hours for the T6.5 brown dwarf J2228-43 and 1.55 hours for the T7 brown dwarf J0050-33.

Nevertheless, more work is still required to confirm if the flaring period of J1122+25 is really an indication of its rotation period. If J1122+25 is indeed rotating once every ~17 minutes, then its rotation velocity is likely to be much greater than 100 km/s. A Jupiter-sized brown dwarf with less than 80 times the mass of Jupiter can have a rotation period as short as ~20 minutes. In the case of J1122+25, its short rotation period of only ~17 minutes means that it has to be less than ~90 percent the size of Jupiter for it not to break apart.

Reference:
Route & Wolszczan (2016), "Radio Flaring from the T6 Dwarf WISEPC J112254.73+255021.5 with A Possible Ultra-short Periodicity", arXiv:1604.04543 [astro-ph.SR]

Low-Density Saturn-Mass Planets

Hellier et al. (2016) present the discovery of yet another low-density Saturn-mass planet identified as WASP-131b. The planet orbits a G0V star with an orbital period of 5.3 days. Its proximity to its host star causes it to be heated to an estimated equilibrium temperature of 1460 ± 30 K. Transit observations, together with follow-up radial velocity measurements, indicate that WASP-131b has 1.22 times the radius and 0.27 times the mass of Jupiter. This gives the planet a low density of only 15 ± 2 percent the density of Jupiter. Other known low-density Saturn-mass planets include WASP-21b, WASP-39b and Kepler-427b. All these planets appear to orbit low metallicity host stars (i.e. host stars with low abundance of elements heavier than hydrogen and helium).

Another planet whose discovery was reported in the same paper is WASP-130b. The planet orbits a G6V star with an orbital period of 11.6 days. Unlike WASP-131b, WASP-130b is a compact Jupiter-mass planet. Transit and radial velocity observations show that WASP-130b has 0.89 ± 0.03 times the radius and 1.23±0.04 times the mass of Jupiter. WASP-130b is similar to, but somewhat less compact than HATS-17b, a Jupiter-mass planet with 0.78 times the radius and 1.34 times the mass of Jupiter. The compactness of both planets suggests that they possess massive metallic cores. Correspondingly, both planets also orbit high metallicity host stars (i.e. host stars with high abundance of elements heavier than hydrogen and helium). WASP-130b is termed a warm-Jupiter as its estimated equilibrium temperature of 833 ± 18 K is not hot enough for it to be classified as a hot-Jupiter.

Reference:
Hellier et al. (2016), "WASP-South transiting exoplanets: WASP-130b, WASP-131b, WASP-132b, WASP-139b, WASP-140b, WASP-141b & WASP-142b", arXiv:1604.04195 [astro-ph.EP]

Predicting the Properties of Planet Nine

The strange clustering observed for the orbits of a class of trans-Neptunian objects in the outer solar system suggests that a massive planet with ~10 times the mass of Earth might exist in the outer solar system, a few hundred times further from the Sun than Earth. This hypothesised planet has been referred to as Planet Nine. Linder & Mordasini (2016) present models on the evolution and properties of Planet Nine.

Figure 1: Artist's impression of a planet that orbits far from its host star.

In the models, Planet Nine is assumed to have the same basic structure as Uranus and Neptune. It is also assumed to have 10 times the mass of Earth and it orbits the Sun at an average distance of 700 AU. The planet's core and envelope masses are 8.6 and 1.4 times the mass of Earth, respectively. The envelope is comprised of hydrogen and helium, while the composition of the planet's core is 50 percent water-ice, 33.3 percent silicates and 16.7 percent iron.

Given these properties, the present temperature of Planet Nine is estimated to be 43 K, far above the equilibrium temperature of about 10 K at 700 AU. This means that the planet's energy budget is dominated by the planet own emission and not by energy from the Sun. The planet's emission is largely driven by cooling and contraction of the planet's core. Also, the planet's present intrinsic luminosity is estimated to be ~0.006 times the intrinsic luminosity of Jupiter, and its present size is 3.66 times the diameter of Earth.

Figure 2: Evolution of the radius of a planet with 10 times the mass of Earth at 700 AU. The planet is comprised of an envelope of hydrogen and helium totalling 1.4 times the mass of Earth, and a core that is made up of 50 percent water-ice, 33.3 percent silicates and 16.7 percent iron. Linder & Mordasini (2016)

Figure 3: Evolution of the effective temperature for the same planet. Linder & Mordasini (2016)

Figure 4: Evolution of the intrinsic luminosity for the same planet. Linder & Mordasini (2016)

Suppose Planet 9 is a super-Earth instead of a planet like Uranus and Neptune. At 10 times the mass of Earth, but without an envelope of hydrogen and helium, Planet Nine is expected to have 1.9 times the diameter of Earth and a lower effective temperature of 38 K. The smaller size and lower temperature would give the planet a much lower intrinsic luminosity and hence make it much more difficult to detect than if it were more like Uranus and Neptune.

Suppose Planet Nine is similar to Uranus and Neptune, but instead of being 10 times the mass of Earth, the planet is 5, 20 or 50 times the mass of Earth. For these masses, the present intrinsic luminosities are ~0.0018, ~0.016 and ~0.078 times the intrinsic luminosity of Jupiter; the present effective temperatures are 40 K, 54 K and 69 K; and the present sizes are 2.92, 4.62 and 6.32 times the diameter of Earth, respectively.

Reference:
Linder & Mordasini (2016), "Evolution and Magnitudes of Candidate Planet Nine", arXiv:1602.07465 [astro-ph.EP]

Producing Black Holes via Runaway Mergers

In the cores of young dense star clusters (YDSCs), stars are tightly packed in very small regions of space. Dynamical interactions between stars can cause them to collide and merge with one another. The first pair of stars to collide and merge form what is referred to as the principal collision product (PCP). The more massive a PCP gets through subsequent mergers, the more efficient it becomes in participating in further mergers. This creates a runaway collision scenario that can lead to the formation of a very massive star. The very massive star can then collapse to form an intermediate massive black hole (IMBH). An IMBH is basically a black hole whose mass is larger than stellar mass black holes but smaller than supermassive black holes.

The metallicity of a YDSC can determine whether or not an IMBH can form. Metallicity is simply the abundance of elements heavier than hydrogen and helium. Mapelli (2016) show that for a YDSC with the same metallicity as the Sun, the very massive star generated from the runaway collision scenario loses so much mass through powerful stellar winds that the resulting black hole that is formed is no more than ~30 times the mass of the Sun. At low metallicity (i.e. 0.01 to 0.1 times the Sun's metallicity), the rate of mass loss becomes much lower and the resulting black hole can be up to a few hundred times the mass of the Sun, well within the mass range of IMBHs.

Reference:
Mapelli (2016), "Massive black hole binaries from runaway collisions: the impact of metallicity", arXiv:1604.03559 [astro-ph.GA]

Gaseous Metals on an Ultra-Hot Gas Giant Planet

WASP-121b is an ultra-hot gas giant planet in a 1.3 day polar orbit around an F6V host star. It has 1.2 times the mass of Jupiter, and it is a highly inflated planet with 1.8 times the radius of Jupiter. The equilibrium temperature of WASP-121b is estimated to be 2,400 K. WASP-121b is a transiting planet, which means it periodically passes in front of its host star and obscures some of its host star's light. Recent transit observations show that the planet appears larger when observed in optical wavelengths than when it is observed in near-infrared wavelengths.

Something high in the planet's atmosphere is absorbing optical radiation, causing the planet to appear larger when observed in optical wavelengths. This has been interpreted as evidence for the presence of titanium oxide (TiO) and vanadium oxide (VO) in the upper atmosphere. TiO and VO are strong absorbers of radiation, especially in optical wavelengths. The presence of TiO and VO should have a significant effect on the atmosphere of WASP-121b as they are expected to generate a strong thermal inversion. Basically a thermal inversion is a layer of hot air overlying a layer of cooler air. The lower density of the hot air prevents the cooler air below from rising.

Reference:
Evans et al. (2016), "Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere", arXiv:1604.02310 [astro-ph.EP]

Cold Giants Circling a Distant Sun

OGLE-2012-BLG-0026 is a gravitational microlensing event previously discovered by Han et al. 2013. A gravitational microlensing event occurs when a massive foreground object crosses the line-of-sight to a background star. The gravitational field of the foreground object can act as a lens and magnify the brightness of the background star. In the case of OGLE-2012-BLG-0026, the foreground object consists of a Sun-like star with two cold gas giant planets. Since its detection, the angular separation of the foreground star from the background star has increased sufficiently to allow light from the foreground star to be observed separately. As a result, new observations of the foreground star have enabled the physical parameters of the planetary system to be constrained to within 5 percent. The two gas giant planets are estimated to have 0.145 ± 0.008 and 0.86 ± 0.06 times the mass of Jupiter, and projected separations of 4.0 ± 0.5 AU and 4.8 ± 0.7 AU, respectively. Both planets are typical cold gas giant planets orbiting a Sun-like star, similar to Jupiter and Saturn.

Reference:

Beaulieu et al. (2016), "Revisiting the microlensing event OGLE 2012-BLG-0026: A solar mass star with two cold giant planets", arXiv:1601.01846 [astro-ph.EP]

Completing a Year in 4.2 Hours

Adams et al. (2016) present the detection of a planet candidate whose orbital period around its host star is one of the shortest known. Identified as EPIC 203533312, this planet candidate goes around its host star in only 4.2 hours. The host star of EPIC 203533312 has 0.73 ± 0.03 times the radius of the Sun and an effective temperature of 4630 K. By measuring how much light EPIC 203533312 obscures when it transits its host star, the planet candidate is found to have 1.92 ± 0.12 times the radius of Earth. EPIC 203533312 is so close to its host star that its density has to be at least 8.9 g/cm³ in order for it to be massive enough for its own self-gravity to keep it from being tidally disrupted by its host star.

Reference:
Adams et al. (2016), "Ultra Short Period Planets in K2: SuPerPiG Results for Campaigns 0-5", arXiv:1603.06488 [astro-ph.EP]