Planets Captured by the Supermassive Black Hole

G2 is a dusty object in a highly eccentric orbit around the supermassive black hole in the center of the Milky Way galaxy. At closest approach, G2 is only ~200 AU from the supermassive black hole. G2 could be a low-mass star hosting a protoplanetary disk or a planet that was captured by the supermassive black hole. Trani et al. (2016) ran 10,000 simulations of a three-body hierarchical system comprised of a supermassive black hole, a star and a planet initially in a bound orbit around the star.

The simulations show that the planet can be removed from its host star and be captured into an independent orbit around the supermassive black hole. However, none of the simulated planets can achieve a highly eccentric orbit around the supermassive black hole. The smallest closest approach distance is 1750 AU, roughly 9 times larger than the closest approach distance of G2. Nevertheless, perturbations from other stars around the supermassive back hole can potentially perturb planets into highly eccentric orbits similar to the orbit of G2.

Reference:
Trani et al. (2016), "Dynamics of tidally captured planets in the Galactic Center", arXiv:1607.07438v1 [astro-ph.GA]

Earth-Mass Planets Can Form around Brown Dwarfs

Brown dwarfs are objects that formed in the same way as stars and they have masses between 0.01 to 0.08 times the mass of the Sun. However, brown dwarfs are not massive enough to sustain hydrogen fusion in their cores. Just like young stars, young brown dwarfs can also be surrounded by dusty disks. From a sample of 29 well-characterized brown dwarfs and very low mass stars with masses ranging from 0.03 to 0.2 times the mass of the Sun, Daemgen et al. (2016) found that more than half of them have disk mass greater than one Jupiter-mass. The dust in the disks is estimated to have temperatures in the range between 7 to 15 K. Jupiter-mass disks around brown dwarfs have the potential to form Earth-mass planets. This shows that brown dwarfs can harbour sufficient material in their disks to form Earth-mass planets.

Reference:
Daemgen et al. (2016), "Brown dwarf disks with Herschel: Linking far-infrared and (sub)-mm fluxes", arXiv:1607.07458 [astro-ph.SR]

The Tightly-Spaced Planets of Kepler-80

Kepler has discovered many planetary systems consisting of multiple small planets with orbital periods less than ~50 days. These compact planetary systems are known as Systems with Tightly-spaced Inner Planets (STIPs). Kepler-80 (KOI-500) is one such STIP. It consists of 5 transiting planets identified as planets "f", "d", "e", "b", and "c"; and their orbital periods are 1.0, 3.1, 4.6, 7.1, and 9.5 days, respectively. Additionally, the 5 planets have ~1.21, ~1.53, ~1.60, ~2.67, and ~2.74 times the radius of Earth, respectively.

Measurements of the transit times and transit timing variation (TTV) analysis indicate that the outer four planets ("d", "e", "b", and "c") have ~6.75, ~4.13, ~6.93, and ~6.74 times the mass of Earth, respectively. The similar masses but different radii is consistent with planets "d" and "e" having Earth-like compositions, and planets "b" and "c" with Earth-like cores surrounded by ~2 percent (by mass) hydrogen-helium envelopes. The orbits of the four outer planets are also in a rare dynamical configuration. The host star of this planetary system is a K5 main sequence star located ~1200 light years away. It has 0.678 times the radius, 0.730 times the mass and 0.170 times the luminosity of the Sun, and its effective temperature is 4540 K.

Reference:
MacDonald et al. (2016), "A Dynamical Analysis of the Kepler-80 System of Five Transiting Planets”, arXiv:1607.07540 [astro-ph.EP]

2015 RR245 is a Dwarf Planet in a 9:2 Orbital Resonance

2015 RR245 is a dwarf planet candidate detected by the Outer Solar System Origins Survey (OSSOS). It is in an eccentric orbit around the Sun. 2015 RR245 comes as close as 34 AU to the Sun and recedes as far as 130 AU from the Sun. If the albedo of 2015 RR245 is assumed to be 12 percent, then 2015 RR245 should have a diameter of approximately 670 km. 2015 RR245 is trapped in a 9:2 mean-motion resonance with Neptune and it is the first known Trans-Neptunian Object (TNO) to be in this orbital resonance.

Reference:
Bannister et al. (2016), "OSSOS: IV. Discovery of a dwarf planet candidate in the 9:2 resonance", arXiv:1607.06970 [astro-ph.EP]

A High Mass Ratio Planetary System

Mróz et al. (2016) present the discovery of a high mass ratio system from a gravitational microlensing event. The planet to host star mass ratio of this system is 0.0117 ± 0.0004. However, the mass of the host star is not well constrained. If the host star has the same mass as the Sun, the planet's mass would be ~12.2 times the mass of Jupiter. With this mass, the planet would be just below the deuterium-burning limit, generally regarded as the boundary separating planets and brown dwarfs. If the host star has a lower mass, then the planet's mass would be smaller. Nevertheless, even if the host star has 0.18 times the mass of the Sun, the planet would still have roughly twice the mass of Jupiter. Having such a high planet to host star mass ratio makes this planetary system quite an extremely one. The planet is identified as OGLE-2016-BLG-0596Lb.

Reference:
Mróz et al. (2016), "OGLE-2016-BLG-0596Lb: High-Mass Planet From High-Magnification Pure-Survey Microlensing Event", arXiv:1607.04919 [astro-ph.EP]

Two Super-Earth-Sized Planets Transiting HD 3167

HD 3167 is a Sun-like star with ~0.88 times the mass and ~0.83 times the radius of the Sun. It is located ~150 light years away and its effective temperature is 5367 ± 50 K. Using data from the K2 mission, Vanderburg et al. (2016) present the discovery of two super-Earth-sized planets transiting HD 3167. The two planets are identified as HD 3167b and HD 3167c.

Figure 1: Artist's impression of an exoplanet.

Figure 2: Light curves indicating the presence of HD 3167b and HD 3167c. Vanderburg et al. (2016)

The inner planet, HD 3167b, has 1.595 ± 0.084 times the radius of Earth and its orbital period is only 23 hours. HD 3167b is an example of an ultra short period planet. Its equilibrium temperature is estimated to be 1560 ± 130 K. HD 3167b is expected to be predominantly rocky as the intense radiation from the host star is likely to have stripped away any thick gaseous envelope. The outer planet, HD 3167c, has 2.89 ± 0.20 times the radius of Earth and its orbital period is 29.845 days. The planet's equilibrium temperature is estimated to be 500 ± 40 K.

HD 3167 is one of the closest and brightest stars with multiple transiting planets, making it a good target for follow-up observations such as transmission spectroscopy and radial velocity observations. The two planets around HD 3167 have widely separated orbital periods. The orbital period of HD 3167c is more than 30 times larger than the orbital period of HD 3167b. This could indicate the presence of additional, non-transiting planets between HD 3167b and HD 3167c.

Reference: Vanderburg et al. (2016), "Two Small Planets Transiting HD 3167", arXiv:1607.05248 [astro-ph.EP]

Discovery of a Benchmark Brown Dwarf

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

Brown dwarfs are objects that are not massive enough to sustain hydrogen burning in their cores. As a result, brown dwarfs become gradually less luminous as they cool with time. Nevertheless, without additional information, the evolutionary state of a brown dwarf cannot be known because the mass and age of a brown dwarf are degenerate parameters. For example, an old, massive brown dwarf can appear similar to a young, low-mass brown dwarf. However, if a brown dwarf has a companion star, the presence the companion can help break the mass and age degeneracy.

Crepp et al. (2016) present the discovery of a brown dwarf in orbit around a Sun-like star with 0.82 ± 0.04 times the mass and 0.79 ± 0.03 times the radius of the Sun. The star is identified as HD 4747A and it is located ~60 light years away. Combining radial velocity measurements taken over 18 years with astrometric measurements, the brown dwarf around HD 4747A, identified as HD 4747B, is estimated to have ~60.2 times the mass of Jupiter.

Figure 2: Radial velocity measurements indicating the presence of HD 4747B. Crepp et al. (2016)

The average distance of 4747B from HD 4747A is ~16.4 AU and the orbital period of HD 4747B is ~38 years. Also, the eccentricity of the brown dwarf's orbit is estimated to be ~0.74, indicating it is in a rather eccentric orbit. HD 4747A is determined to have an age of roughly 3.3 billion years. Its rotational spin period of roughly 27 days is also consistent with such an age. Since HD 4747A and HD 4747B formed at the same time, both objects will have the same age. With a well constrained mass and age, HD 4747B is a good benchmark to test theoretical models of brown dwarfs.

Reference:
Crepp et al. (2016), "The TRENDS High-Contrast Imaging Survey. VI. Discovery of a Mass, Age, and Metallicity Benchmark Brown Dwarf", arXiv:1604.00398 [astro-ph.SR]

64 Newly Validated Planets from the K2 Mission

Figure 1: Artist's impression of an exoplanet.

Crossfield et al. (2016) present 197 planet candidates discovered using data from the K2 mission. Of these planet candidates, 104 are validated planets, 30 are false positives and 63 remain as planet candidates. Of the 104 validated planets, 64 are newly validated. They include several multi-planet systems and several small, roughly Earth-sized planets receiving Earth-like levels of irradiation. 37 planets are smaller than twice the size of Earth.

4 of the validated planets orbit a red dwarf star identified as K2-72. The 4 planets, referred to as planets "b", "c", "d" and "e", have radii between 1.2 to 1.5 times the radius of Earth, and their orbital periods are 5.58, 7.76, 15.19 and 24.16 days, respectively. Planets "c" and "d" orbit near the 2:1 mean motion resonance, and planets "b" and "c" orbit near the 7:5 mean motion resonance. The two outer planets receive similar amounts of insolation as Earth gets from the Sun.

Figure 2: Transit light curves indicating the presence of the 4 planets around the red dwarf star K2-72. Crossfield et al. (2016)

Other notable validated planets include K2-89b - a highly irradiated, roughly Earth-sized planet in a one-day orbit around a red dwarf star. Another planet is K2-65b. It has 1.58 times the radius of Earth and its orbital period is 12.65 days. It receives roughly 45 times the amount of insolation Earth gets from the Sun. Because K2-65b orbits a relatively bright star, it is a good target for follow-up radial velocity measurements to determine its mass. The sample of validated planets also includes four new two-planet systems - K2-80, K2-83, K2-84 and K2-90.

Figure 3: Orbital periods and radii of the 104 validated planets, 30 false positives, and 63 remaining planet candidates. Crossfield et al. (2016)

Figure 4: Planetary radii, incident insolation, and stellar effective temperature for the 104 validated planets (coloured points) and all planets at the NASA Exoplanet Archive (gray points). Crossfield et al. (2016)

Reference:
Crossfield et al. (2016), "197 Candidates and 104 Validated Planets in K2's First Five Fields", arXiv:1607.05263 [astro-ph.EP]

When Very Low-Mass Stars Settle Down

The minimum mass a star can have is roughly 0.08 times the mass of the Sun. A lower-mass object would be classified as a brown dwarf. Very low-mass stars (VLMS) and brown dwarfs have very low luminosities, making these objects difficult to detect. Furthermore, it can also be difficult to distinguish whether an object is a VLMS or a brown dwarf. It can take a long time for a VLMS to settle down and enter the main sequence (i.e. a state of steady nuclear burning).

A study shows that a VLMS with 0.08 times the mass of the Sun is estimated to take ~350 million years to settle on the main-sequence where it will shine with only ~1/52,600th the Sun's luminosity. A VLMS with a slightly higher mass of 0.09 times the mass of the Sun is estimated to take ~56 million years to settle on the main-sequence where it will shine with only ~1/4,290th the Sun's luminosity. In fact, a VLMS, depending on its mass, can take as long as a billion years or more to settle on the main-sequence.

Reference:
Auddy et al. (2016), "Analytic Models of Brown Dwarfs and the Substellar Mass Limit", arXiv:1607.04338 [astro-ph.SR]

Reflected Light from Giant Planets in the Habitable Zone

The detection of reflected light from a planet can allow for the study of the planet's atmosphere. However, the challenge is that the planet-to-star flux ratio is very small. Even for giant planets in close-in orbits, the flux ratio is still below ~1/10,000. This ratio decreases as the planet's orbital distance increases. Nevertheless, the reflected light from giant planets in the habitable zone of their host stars may be detectable with next generation telescopes such as ESO’s European Extremely Large Telescope (E-ELT). Even so, the planet-to-star flux ratio for giant planets in the habitable zone is less than ~1/10,000,000. The E-ELT is predicted to be able to detect the reflected light from several known giant planets in the habitable zone with less than 100 hours of observations for each planet.

Reference:

Martins et al. (2016), "Reflected light from giant planets in habitable zones: Tapping into the power of the Cross-Correlation Function", arXiv:1604.01086 [astro-ph.EP]