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]

A System of Two Warm Super-Earths

Affer et al. (2016) present the discovery of a planetary system consisting of two super-Earths in orbit around a red dwarf star with approximately half the mass and half the size of the Sun. Additionally, the red dwarf star has ~4 percent the Sun's luminosity and its effective temperature is 3722 ± 68 K. The two super-Earths are identified as GJ3998b and GJ3998c. Both planets were discovered via the radial velocity method. The inner planet, GJ3998b, has at least 2.47 ± 0.27 times the mass of Earth and its orbital period is 2.65 days. The outer planet, GJ3998c, has at least ~6.26 times the mass of Earth and its orbital period is 13.74 days. Estimates indicate that the equilibrium temperatures of GJ3998b and GJ3998c are ~740 K and ~420 K, respectively.

Reference:
Affer et al. (2016), "The HADES RV Programme with HARPS-N@TNG - GJ 3998: An early M-dwarf hosting a system of Super-Earths", arXiv:1607.03632 [astro-ph.EP]

Hot Planet Orbiting a Rapidly-Rotating A-Type Star

Zhou et al. (2016) present the discovery of a hot-Jupiter transiting a massive, rapidly-rotating A-type star. The planet is identified as KELT-17b. Transit and radial velocity observations indicate that KELT-17b has ~1.31 times the mass and ~1.525 times the radius of Jupiter. The planet's orbital period is 3.08 days. The host star of KELT-17b has ~1.635 times the mass, ~1.645 times the radius and ~7.51 times the luminosity of the Sun. It is a rapidly-rotating star with a rotation speed of at least 44.2 km/s. Also, its effective temperature is 7454 K. The host star of KELT-17b is one of the most massive, hottest, and most rapidly-rotating star with a known planet. Furthermore, the orbit of KELT-17b is severely misaligned. KELT-17b is only the fourth hot-Jupiter found transiting an A-type star, after WASP-33b, KOI-13b, and HAT-P-57b. All four hot-Jupiters orbiting A-type stars are in severely misaligned orbits.

Reference:
Zhou et al. (2016), "KELT-17b: A hot-Jupiter transiting an A-star in a misaligned orbit detected with Doppler tomography", arXiv:1607.03512 [astro-ph.EP]

Intermediate-Mass Planet beyond the Snow Line

Koshimoto et al. (2016) present the detection of a planet identified as OGLE-2012-BLG-0950Lb. This planet and its host star crossed the line-of-sight to a background source, and the combined gravitational field of the planet and its host star generated a gravitational microlensing event. OGLE-2012-BLG-0950Lb is the first planet to be discovered solely from the gravitational microlensing parallax due to the Earth’s orbital motion around the Sun and from detection of flux from the planet's host star.

OGLE-2012-BLG-0950Lb is estimated to have ~35 times the mass of Earth and it orbits around a host star with ~0.56 times the mass of the Sun. The planet's projected distance from its host star is ~2.7 AU and the planetary system is estimated to be located ~10,000 light years away. OGLE-2012-BLG-0950Lb orbits outside the snow line of its host star and its mass is between that of Neptune and Saturn. Such intermediate-mass planets beyond the snowline are predicted to be common in the core accretion model of planet formation.

Reference:
Koshimoto et al. (2016), "OGLE-2012-BLG-0950Lb: The Possible First Planet Mass Measurement from Only Microlens Parallax and Lens Flux", arXiv:1607.03267 [astro-ph.EP]

A Very Low Mass Star Transiting its Host Star

The low mass and intrinsic faintness of red dwarf stars make these objects difficult to study. As a consequence, the mass-radius relationship is poorly known for red dwarf stars. This is especially so for very low mass stars (VLMS) (i.e. stars with less than 10 percent the mass of the Sun). J2343+29A is a star with ~0.864 times the mass and ~0.854 times the radius of the Sun. The star's effective temperature is 5125 ± 67 K. Observations of J2343+29A show that it has a transiting companion in a 16.953 day orbit around it. Transit and radial velocity observations show that the companion is a VLMS with 0.098 ± 0.007 times the mass and 0.127 ± 0.007 times the radius of the Sun. With its mass and radius well constrained, the companion of J2343+29A is potentially a good benchmark for the study of VLMS.

Reference:
Chaturvedi et al. (2016), "Detection of a very low mass star in an Eclipsing Binary system", arXiv:1607.03277 [astro-ph.SR]

Jupiter-Like Planet in a Triple Star System

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

HD 131399Ab is a young, Jupiter-like planet in a triple star system located ~320 light years away. Its host star, identified as HD 131399A, is an A-type star with 1.82 times the mass of the Sun and has an effective temperature of 9300 K. The projected separation of HD 131399Ab from its host star is 82 AU and the planet’s orbital period is roughly 550 years. HD 131399Ab is estimated to have 4 ± 1 times the mass of Jupiter and its effective temperature is 850 ± 50, making it one of the coldest directly imaged planets. Nevertheless, HD 131399Ab is still in the process of cooling down as it radiates away heat that was acquired during its formation. The triple star system that HD 131399Ab resides in is relatively young, estimated to be only ~16 million years old.

The two other stars in the triple star system are identified as HD 131399B and HD 131399C. Both stars circle around one another and together are referred to as HD 131399BC. HD 131399B is a G-type star with 0.96 times the mass of the Sun and an effective temperature of 5700 K. HD 131399C is a K-type star with 0.6 times the mass of the Sun and an effective temperature of 4400 K. The separation between HD 131399BC and HD 131399A is just over ~3 times the projected separation between HD 131399A and HD 131399Ab. With such a dynamically extreme orbital configuration, the orbit of HD 131399Ab around its host star is the widest known for a planet that orbits within a triple star system.

Figure 2: The orbital paths of HD 131399Ab and its three suns.

Reference:
Wagner et al. (2016), "Direct Imaging Discovery of a Jovian Exoplanet Within a Triple Star System", arXiv:1607.02525 [astro-ph.EP]

Two Inflated Hot-Jupiters with Contrasting Densities

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

Barros et al. (2016) present the discovery of two inflated hot-Jupiters with contrasting densities. The two hot-Jupiters are identified as WASP-113b and WASP-114b. Both hot-Jupiters orbit Sun-like host stars. The orbital period of WASP-113b is 4.542 days and the orbital period of WASP-114b is 1.549 days. Transit and radial velocity measurements indicate that WASP-113b has ~0.475 times the mass and ~1.409 times the radius of Jupiter, while WASP-114b has ~1.769 times the mass and ~1.339 times the radius of Jupiter.

The large radii indicate that both WASP-113b and WASP-114b are inflated. Nevertheless, they have contrasting densities. WASP-113b has ~0.172 times the density of Jupiter and WASP-114b has ~0.73 times the density of Jupiter. This means WASP-114b is over 4 times denser than WASP-113b. Finally, the equilibrium temperatures of WASP-113b and WASP-114b are ~1500 K and ~2050 K, respectively.

Figure 2: Phase folded transit light curves indicating the presence of WASP-113b (top) and WASP-114b (bottom). Barros et al. (2016)

Reference:
Barros et al. (2016), "Discovery of WASP-113b and WASP-114b, two inflated hot-Jupiters with contrasting densities", arXiv:1607.02341 [astro-ph.EP]

172 Planetary Candidates

Figure 1: Artist’s impression of an exoplanet.

Barros et al. (2016) present 172 planetary candidates and 327 eclipsing binary candidates from campaigns 1, 2, 3, 4, 5 and 6 of NASA's K2 mission. Histograms of the orbital periods and transit depths of the planetary and eclipsing binary candidates indicate that a large percentage of candidates have small transit depths. The smallest transit depth is 0.008 percent, which corresponds to a planet with 0.975 times the radius of Earth, assuming its host star is similar in size to the Sun. This planetary candidate is identified as EPIC 211784767b and its orbital period is 3.578 days.

Figure 2: On the left panel: histogram of the orbital periods of the planetary candidates and the eclipsing binary candidates. On the right: histogram of the transit depths of the planetary candidates and the eclipsing binary candidates with transit depths less than 5 percent. Barros et al. (2016)

Reference:
Barros et al. (2016), "New planetary and EB candidates from Campaigns 1-6 of the K2 mission", arXiv:1607.02339 [astro-ph.EP]

KELT-11b is a Highly Inflated Sub-Saturn

Pepper et al. (2016) present the discovery of KELT-11b, a highly-inflated, low-mass gas giant planet in a 4.736529 ± 0.00006 day orbit around a sub-giant star. The host star of KELT-11b has ~1.44 times the mass and ~2.72 times the radius of the Sun, and its effective temperature is 5370 ± 51 K. As for KELT-11b itself, it has 0.195 ± 0.018 times the mass and ~1.37 times the radius of Jupiter, giving it an exceptionally low density of less than ~10 percent the density of water. The planet's remarkably low density makes it one of the most inflated planets known. Furthermore, the estimated equilibrium temperature on KELT-11b is ~1712 K, and its surface gravity is only ~1/4 as strong as the surface gravity on Earth.

The low gravity gives KELT-11b has an exceptionally large scale height of almost 2800 km. The scale height is basically the vertical distance in a planet's atmosphere over which the atmospheric pressure changes by a factor of approximately 2.718. As a result of its large scale height, the planet's atmospheric transmission signal is expected to be relatively large, making KELT-11b a good target for follow-up and atmospheric characterization. Currently, the host star of KELT-11b is the brightest star in the southern hemisphere with a known transiting planet.

Estimated atmospheric scale height of known transiting hot Jupiters versus the V-band brightness of the host star. KELT-11b is indicated by a filled green circle. Pepper et al. (2016)

Reference:
Pepper et al. (2016), "KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V=8 Subgiant HD 93396", arXiv:1607.01755 [astro-ph.EP]

Three Temperate Neptunes Orbiting Sun-Like Stars

Fulton et al. (2016) present the discovery of three moderately-irradiated, roughly Neptune-mass planets orbiting three relatively nearby Sun-like stars that are located no more than 80 light years away. The three planets are identified as HD 42618 b, HD 164922 c and HD 143761 c. All three planets were detected using the radial velocity method.

HD 42618 b has at least 15.4 ± 2.4 times the mass of Earth. It orbits its host star at ~0.55 AU and its orbital period is 149 days. HD 42618 b receives ~3.2 times the irradiation Earth gets from the Sun and its equilibrium temperature is ~337 K.

HD 164922 c has at least 12.9 ± 1.6 times the mass of Earth. It orbits its host star at ~0.34 AU and its orbital period is 75 days. HD 164922 c receives ~6.3 times the irradiation Earth gets from the Sun and its equilibrium temperature is ~401 K. Exterior to the orbit of HD 164922 c is a previously known Saturn-mass planet.

HD 143761 c has at least 25 ± 2 times the mass of Earth. It orbits its host star at ~0.41 AU and its orbital period is 39 days. HD 143761 c receives ~9.9 times the irradiation Earth gets from the Sun and its equilibrium temperature is ~448 K. Interior to the orbit of HD 143761 c is a previously known Jupiter-mass planet.

Reference:

Fulton et al. (2016), "Three Temperate Neptunes Orbiting Nearby Stars", arXiv:1607.00007 [astro-ph.EP]

Discovery of Three Transiting Hot-Saturns

Bhatti et al. (2016) present the discovery of three transiting hot-Saturns identified as HATS-19b, HATS-20b and HATS-21b.

HATS-19b orbits a slightly evolved star with ~3.31 times the Sun's luminosity. The planet has ~1.66 times the radius of Jupiter and its orbital period is 4.570 days. HATS-19b also has ~0.427 times the mass of Jupiter, giving it ~12 percent the density of water. The equilibrium temperature on HATS-19b is estimated to be ~1570 K.

HATS-20b orbits a Sun-like star with ~0.612 times the Sun's luminosity. The planet has ~0.776 times the radius of Jupiter and its orbital period is 3.799 days. HATS-20b also has ~0.273 times the mass of Jupiter, giving it ~73 percent the density of water. The equilibrium temperature on HATS-20b is estimated to be ~1147 K.

HATS-21b orbits a Sun-like star with ~0.98 times the Sun's luminosity. The planet has ~1.123 times the radius of Jupiter and its orbital period is 3.554 days. HATS-21b also has ~0.332 times the mass of Jupiter, giving it ~29 percent the density of water. The equilibrium temperature on HATS-21b is estimated to be ~1284 K.

Reference:
Bhatti et al. (2016), "HATS-19b, HATS-20b, HATS-21b: Three Transiting Hot-Saturns Discovered by the HATSouth Survey", arXiv:1607.00322 [astro-ph.EP]

No Transit Timing Variations for Kepler-421b

Observations by NASA's Kepler space telescope have revealed the presence of several planets with orbital periods exceeding 500 days. Many of these planets with long orbital periods are near or beyond the snow-line of their host stars. The snow-line is basically the region in the protoplanetary disk around a star where temperatures start to become cool enough for volatiles such as water to condense into solid ice grains. Kepler-421b is a transiting snow-line planet with 4.2 times the radius of Earth and its orbital period around its host star is ~704 days. The planet's equilibrium temperature is estimated to be only ~185 K. Two transits of Kepler-421b were observed by Kepler.

Dalba & Muirhead (2016) present observations of the third transit of Kepler-421b by the 4.3-meter Discovery Channel Telescope on 19 February 2016. Observations of the third transit show no transit timing variations (TTVs). The lack of TTVs suggests Kepler-421b is either the only planet in its planetary system or the dynamical interactions with unseen planetary companions are too weak to noticeably perturb the orbit of Kepler-421b. Future transits of Kepler-421b will occur on 24 January 2018, 29 December 2019, 2 December 2021, and so on.

Reference:
Dalba & Muirhead (2016), "No Timing Variations Observed in Third Transit of Snow-Line Exoplanet Kepler-421b", arXiv:1606.09246 [astro-ph.EP]

Supernovae & Magnetars that Collapse into Black Holes

Figure 1: Artist’s impression of a neutron star.

When a massive star undergoes core collapse in a supernova explosion, the collapsed core of the star can form a rapidly rotating, strongly magnetized neutron star. Such a neutron star can be referred to as a “millisecond magnetar". The tremendous reservoir of rotational energy possessed by the millisecond magnetar can be rapidly released through electromagnetic dipole spin-down of the millisecond magnetar. This can greatly energise and boost the luminosity of the supernova explosion, potentially resulting in a superluminous supernova explosion.

Furthermore, a millisecond magnetar formed following core collapse of a massive star can have a large enough mass to be classified as a supramassive neutron star. Such an overly-massive neutron star is supported against gravitational collapse by its rapid rotation and it is only temporarily stable. As the supramassive neutron star, in the form of a millisecond magnetar, loses rotational energy though electromagnetic dipole spin-down, it can collapse and transform into a black hole. The formation of a black hole results in the sudden loss of the central engine (i.e. the millisecond magnetar) to energise the supernova explosion.

Figure 2: Light curves of supernovae explosions powered by magnetar spin-down. The black hole transformation time ranges from 0.25 to 7.5 days. This is basically the time required for a magnetar to spin down sufficiently and collapse into a black hole. The dotted line is the input magnetar spin-down energy without black hole formation. The magnetar-driven break out bump is more prominent in the light curve in cases of early black hole formation. Moriya et al. (2016)

Moriya et al. (2016) modelled the light curves of supernovae explosions powered by the spin-down of millisecond magnetars that eventually collapse to form black holes. Although the light curves can reach peak luminosities that are within the regime of superluminous supernovae explosions, they decline very rapidly after peaking because of the abrupt loss of the central engine. Basically, the light curve of a supernova explosion powered by the spin-down of a supramassive neutron star which then collapses into a black hole declines much quicker than the light curve of a supernova explosion powered by the spin-down of an indefinitely stable, lower-mass magnetar.

A notable feature in these light curves is the presence of a magnetar-driven breakout bump. This "bump" in the light curve is basically energy released from the spin-down of the millisecond magnetar. The magnetar-driven breakout bump is more noticeable when the black hole formation occurs early because the sudden cessation of energy input from the abrupt lost of the central engine allows the breakout bump to shine more prominently on its own.

Reference:
Moriya et al. (2016), "Supernovae powered by magnetars that transform into black holes", arXiv:1606.09316 [astro-ph.HE]

Giant Planets and the Core Accretion Mass Limit

As part of the Calan-Hertfordshire Extrasolar Planet Search, Jenkins et al. (2016) present the discovery of eight giant planets. The planets have masses between 1.1 to 5.4 times the mass of Jupiter, and orbital periods ranging from 40 to 2900 days. The study also show a sharp decline in the number of planets with more than 3 times the mass of Jupiter, suggesting that the core accretion model of planet formation is efficient at forming giant planets with up to a few times the mass of Jupiter, but has problems forming super-Jupiters (i.e. planets with several times the mass of Jupiter).

This sample of newly discovered planets includes a planetary system, located ~340 light years away, consisting of two giant planets identified as HD 147873b and HD 147873c. Both planets orbit a host star with ~1.38 times the mass and ~2.29 times the radius of the Sun. The host star also has ~6 times the Sun's luminosity and its effective temperature is ~5972 K. HD 147873b and HD 147873c have at least ~5.14 and ~2.30 times the mass of Jupiter, and their orbital periods are 116.6 and 491.5 days, respectively.

Another notable planet is HD 224538b. It is a giant planet with at least ~5.97 times the mass of Jupiter and its orbital period around its host star is ~1200 days. The orbit of HD 224538b has a large eccentricity of about 0.46. The host star of HD 224538b is an F-type star with ~1.34 times the mass, ~1.54 times the radius and ~2.95 times the luminosity of the Sun. Furthermore, the star's effective temperature is ~6097 K and it is a metal-rich star with almost twice the Sun's metallicity. HD 224538b is basically a massive eccentric giant planet orbiting a super metal-rich star, a relatively rare type of planet.

Reference:
Jenkins et al. (2016), "New Planetary Systems from the Calan-Hertfordshire Extrasolar Planet Search and the Core Accretion Mass Limit", arXiv:1603.09391 [astro-ph.EP]

Five Planets Transiting a Bright Star

HIP 41378 is a relatively bright, slightly metal-poor F-type star located ~380 light years away. It has ~1.15 times the mass and ~1.4 times the radius of the Sun, and its effective temperature is ~6200 K. Using new data from the K2 mission, Vanderburg et al. (2016) present the discovery of a planetary system consisting of five transiting planets around HIP 41378.

Figure 1: Artist's impression of an exoplanet.

The two innermost planets are sub-Neptune-sized planets with 2.90 ± 0.44 and 2.56 ± 0.40 times the size of Earth, and their orbital periods are 15.57 and 31.70 days, respectively. The three outer planets each transit once during the 75 days of K2 observations. One is a sub-Saturn-sized planet with 5.51 ± 0.77 times the size of Earth in a ~131 day orbit; one is a Neptune-sized planet with 3.96 ± 0.59 times the size of Earth in a ~156 day orbit; and one is a Jupiter-sized planet with 10.2 ± 1.4 times the size of Earth in a ~324 day orbit.

HIP 41378 is a good target for future radial velocity measurements to determine the masses of its five transiting planets. Furthermore, the outer Jovian planet, identified as HIP 41378 f, is one of the first known gas giant planets with a relatively cool equilibrium temperature that transits a star that is sufficiently bright for transit transmission spectroscopy. HIP 41378 f is an excellent candidate for follow-up transit observations to characterise its atmosphere and measure its oblateness.

Figure 2: Phase-folded light curve for each of the five transiting planets in the HIP 41378 system. Vanderburg et al. (2016)

Reference:
Vanderburg et al. (2016), "Five Planets Transiting a Ninth Magnitude Star", arXiv:1606.08441 [astro-ph.EP]

Dwarf Planets to Low-Mass Stars

Figure 1: Artist's impression of an exoplanet.

Mass and radius are two of the most important properties that define a planet. However, in the effort to detect planets, usually only the mass or radius of the planet is measured. As a result, there is a need to predict a planet's radius based on its mass or a planet's mass based on its radius. Chen & Kipping (2016) present a forecasting model based on a probabilistic mass-radius relation from a sample of 316 objects with well-constrained masses and radii. The objects span nine orders-of-magnitude in mass, from dwarf planets to low-mass stars. They are classified into 4 classes - Terran worlds (i.e. Earth is in this category), Neptunian worlds, Jovian worlds and stars. With this classification, dwarf planets are simply low-mass Terran worlds and brown dwarfs are simply high-mass Jovian worlds.

In the model by Chen & Kipping (2016), there is a transition in the mass-radius relation at ~2.0 times the mass of Earth. This transition marks the divide between solid Terran worlds and gas-rich Neptunian worlds. What this means for solid Super-Earths is that they are expected to have masses not much greater than the mass of Earth (i.e. within ~2.0 times the mass of Earth).

Figure 2: The mass-radius relation from dwarf planets to low-mass stars. Chen & Kipping (2016)

There appears to be no change in the mass-radius relation from Jupiter-mass planets to brown dwarfs. Based only on their mass and radius, brown dwarfs can be seen as high-mass Jovian worlds. Also, there appears to be no change in the mass-radius relation from dwarf planets to Earth-mass planets, indicating that based on mass and radius alone, dwarf planets are simply low-mass Terran worlds. The transition from Neptunian worlds to Jovian worlds occurs at ~0.4 times the mass of Jupiter. With this classification, Saturn is close to being the largest Neptunian world.

Reference:
Chen & Kipping (2016), "Probabilistic Forecasting of the Masses and Radii of Other Worlds", arXiv:1603.08614 [astro-ph.EP]

Gamma Rays from a Region of Stellar Oddities

Using the High Energy Spectroscopic System (H.E.S.S.) telescopes, H.E.S.S. Collaboration et al. (2016) present the discovery of a steady and extended very high-energy (VHE) gamma-ray source originating from a region containing a number of rare stellar oddities. This VHE gamma-ray source is identified as HESSJ1808-204. The source region of HESSJ1808-204 contains a luminous blue variable candidate LBV 1806-20, a massive stellar cluster Cl*1806-20, and a magnetar SGR 1806-20. High-energy particle acceleration from LBV 1806-20, Cl*1806-20 and/or SGR 1806-20 could be responsible for this VHE gamma ray source. HESSJ1808-204 is estimated to shine with ~4 times the Sun's luminosity in the form of TeV VHE gamma rays.

Reference:
H.E.S.S. Collaboration et al. (2016), "Extended VHE gamma-ray emission towards SGR1806-20, LBV1806-20, and stellar cluster Cl*1806-20", arXiv:1606.05404 [astro-ph.HE]

A Dark Galaxy Hosting ~100 Globular Clusters

Dragonfly 44 is 2nd largest ultra diffused galaxy (UDG) in the Coma cluster of galaxies, and the only one that has been spectroscopically confirmed to be a member of the Coma cluster. UDGs are a population of large, very low surface brightness, spheroidal-shaped galaxies. Observations of Dragonfly 44 indicate it has a dynamical mass of about 7 billion times the mass of the Sun within a half light radius of approximately 15,000 light years. A galaxy's half light radius is basically the size of the area centred on the galaxy that contributes to half the galaxy's overall brightness. A whopping 98 percent of the galaxy's mass within its half light radius is in the form of dark matter.

Dragonfly 44 is also accompanied by a large population of globular clusters. Images of Dragonfly 44 show a population of approximately 100 globular clusters. These globular clusters are distributed in a halo around Dragonfly 44. Based on the distribution of globular clusters, the total mass enclosed within the halo of Dragonfly 44 is estimated to be roughly one trillion times the mass of the Sun, making Dragonfly 44 similar in mass to the Milky Way. Even though Dragonfly 44 is similar in mass to the Milky Way, its abundance of stars is ~100 times less than the Milky Way. The night sky from a hypothetical planet in Dragonfly 44 would appear much emptier of stars.

Reference:
van Dokkum et al. (2016), "A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44", arXiv:1606.06291 [astro-ph.GA]

A Re-Inflated Planet Orbiting a Red Giant Star

Inflated gas giant planets have been known for quite awhile. These planets orbit very close to their host stars and consequently receive high incident fluxes. However, it is unclear whether the inflated size of such a planet is primarily caused by the deposition of energy from the host star into the interior of the planet or by an inhibition in the planet’s cooling process. Grunblatt et al. (2016) present the discovery of EPIC 211351816.01. This planet appears to be an inflated gas giant planet in orbit around a red giant star. EPIC 211351816.01 has 1.27 ± 0.09 times the radius and 1.10 ± 0.11 times the mass of Jupiter. Its orbital period around its host star is 8.408 days. The host star of EPIC 211351816.01 has 4.20 ± 0.14 times the radius and 1.16 ± 0.12 times the mass of the Sun. Its age is estimated to be 7.8 ± 2.0 billion years old and its effective temperature is 4790 ± 90 K.

EPIC 211351816.01 seems to have been re-inflated when its host star evolved into a much more luminous red giant star. This is because when its host star has yet to evolve into a red giant star, the planet is at a large enough distance that the amount of irradiation it received was probably not sufficient to have kept the planet inflated. At that time, the planet would have received only ~200 times the flux Earth gets from the Sun, and that was probably not enough to keep the planet inflated. In a way, the discovery of EPIC 211351816.01 provides evidence that the inflation of a planet is driven by the deposition of energy from the planet’s host star rather than by a slowdown in the planet’s cooling process.

Reference:
Grunblatt et al. (2016), “EPIC 211351816.01: A (Re-?)Inflated Planet Orbiting a Red Giant Star”, arXiv:1606.05818 [astro-ph.EP]

Qatar-3b, Qatar-4b and Qatar-5b

Mislis et al. (2016) present the discovery of three massive hot-Jupiters identified as Qatar-3b, Qatar-4b and Qatar-5b. The three planets were detected by the Qatar Exoplanet Survey (QES). The orbital periods of Qatar-3b, Qatar-4b and Qatar-5b are 2.508 days, 1.805 days and 2.879 days, respectively. Follow-up spectroscopic observations also show that Qatar-3b, Qatar-4b and Qatar-5b have 4.31, 5.85 and 4.32 times the mass of Jupiter, respectively. All three planets are classified as massive hot-Jupiters (i.e. hot-Jupiters with more than 4 times the mass of Jupiter).

The equilibrium temperatures of Qatar-3b, Qatar-4b and Qatar-5b are estimated to be 1681 ± 84 K, 1570 ± 26 K and 1415 ± 31 K, respectively. These three massive hot-Jupiters orbit host stars that are quite similar to the Sun. The host star of Qatar-5b has a relatively high metallicity (i.e. it has a high abundance of elements heavier than hydrogen and helium), and it is one of the most metal-rich stars known to host a planet.

Reference:
Mislis et al. (2016), "Qatar Exoplanet Survey: Qatar-3b, Qatar-4b and Qatar-5b", arXiv:1606.06882 [astro-ph.EP]

A Cold Planet near the Galactic Center

Han et al. (2016) present the discovery of a giant planet in orbit around a low-mass red dwarf star based on data acquired by two surveys which detected a gravitational microlensing event identified as OGLE-2015-BLG-0051/KMT-2015-BLG-0048. Models indicate the planet has ~0.72 times the mass of Jupiter and its projected separation from its host star is 0.73 ± 0.08 AU. At that distance, the planet is beyond the snow-line of its host star. The planet's host star is a low-mass red dwarf star with roughly one-tenth the mass of the Sun, and it is estimated to be ~26700 light years away, near the center of the galaxy.

Reference:
Han et al. (2016), "OGLE-2015-BLG-0051/KMT-2015-BLG-0048Lb: a Giant Planet Orbiting a Low-mass Bulge Star Discovered by High-cadence Microlensing Surveys", arXiv:1606.09352 [astro-ph.EP]

The Present Distribution of Globular Clusters

Figure 1: Artist’s impression of a planet in a globular cluster.

Galaxies with larger luminosities tend to host big populations of globular clusters. At the same time, galaxies with larger luminosities are also less common than galaxies with lower luminosities. Dwarf galaxies are low luminosity galaxies that individually have very few globular clusters, but they are very common in the universe. In contrast, giant elliptical galaxies are very luminous galaxies with huge populations of globular clusters, but these galaxies are extremely rare.

It is worth asking how globular clusters are distributed amongst galaxies and what kind of galaxies account for most of the globular clusters in the universe. A plot of the distribution of globular clusters show that half the population of blue (metal-poor) globular clusters are found in galaxies with less than ~13 billion times the Sun's luminosity, while half the population of red (metal-rich) globular clusters are found in galaxies with less than ~28 billion times the luminosity of the Sun. Basically, blue (metal-poor) globular clusters tend to reside around lower mass galaxies. A major contributing factor for this is that there are almost no red (metal-rich) globular clusters present around dwarf galaxies.

Figure 2: The fraction of all globular clusters within galaxies with luminosities ≤ L. The large solid dots indicate the median points along each curve. Harris (2016)

It appears that most globular clusters belong to galaxies with between a trillion to a hundred trillion solar-masses, with a peak near ~10 trillion solar-masses. Since galaxies grow by consuming their smaller neighbours, this peak is expected to shift gradually towards a higher mass as the universe gets older.

Overall, blue (metal-poor) globular clusters outnumber red (metal-rich) globular clusters by 4 to 1. In the early universe, matter was predominantly in the form of low-metallicity gas (i.e. gas with low abundance of elements heavier than hydrogen and helium). Those conditions were very favourable for the formation of dense massive star clusters that went on to become the blue (metal-poor) globular clusters observed in the present universe. That was the period when most of the blue (metal-poor) globular clusters formed.

Red (metal-rich) globular clusters formed at a later period when matter became more enriched with elements heavier than hydrogen and helium. This explains why red (metal-rich) globular clusters are associated with more massive galaxies as massive galaxies generally appeared during the later stages of galaxy formation.

Reference:
Harris (2016), "Where Are Most of the Globular Clusters in Today's Universe?", arXiv:1603.00348 [astro-ph.GA]

Water Vapour in the Atmospheres of Hot-Jupiters

Hot-Jupiters are a class of planets similar in size to Jupiter but orbit very close to their host stars. It is common for hot-Jupiters to have cloud or haze layers in their atmospheres that can mask the detection of atmospheric constituents that lie deeper down in the atmosphere. These cloud or haze layers have been found to be concealing the presence of water vapour in the atmospheres of hot-Jupiters.

When a transiting planet passes in front of its host star, it blocks some of the light from its host star. The size of the transiting planet can be determined by measuring the amount of starlight it blocks. If the transiting planet is cloudy or hazy, it is expected to appear larger when observed in optical wavelengths than in infrared wavelengths. This is because infrared radiation can penetrate clouds more readily than optical radiation (i.e. visible light). As a result, the difference in the planet's radius when its transit is observed in optical and infrared wavelengths can be used to determine whether the planet's atmosphere is cloudy or clear.

Using data from the Hubble Space Telescope (HST), Iyer et al. (2016) present a study of 19 hot-Jupiters. 10 of these hot-Jupiters show the presence of water vapour in their atmospheres, while the other 9 hot-Jupiters show no water vapour at all. The datasets for all 19 hot-Jupiters were combined to create one average overall spectrum. The overall spectrum agrees with models showing that cloud and haze layers are hiding the presence of water vapour deeper in the atmosphere. Such a finding means that hot-Jupiters did not form in an environment deprived of water.

Reference:
Iyer et al. (2016), "A Characteristic Transmission Spectrum dominated by H2O applies to the majority of HST/WFC3 exoplanet observations", arXiv:1512.00151 [astro-ph.EP]