Having Two Suns Makes Little Difference

Circumbinary planets are planets that orbit two host stars. As a result, they experience a time-varying irradiation pattern. Circumbinary planets appear to be relatively common. It has been predicted that planets larger than 6 times the radius of Earth occur for at least 10 percent of circumbinary systems. A number of circumbinary planets have already been detected and most of them are around the size of Neptune.

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

May & Rauscher (2016) present a study of the atmospheric effects of the time-varying irradiation pattern on known and hypothetical gaseous Neptune-like circumbinary planets. In the study, the parameter η defines the difference in temperature for the circumbinary case as compared to the single-star case. The results from the study show that for circumbinary planets on stable orbits around their host stars, η does not exceed 1 percent even for the most extreme cases.

For example, Kepler-47b is a Neptune-sized planet in a 49.5 day orbit around a pair of stars. For Kepler-47b, η is only 0.2 percent. This means that the maximum temperature deviation from the single-star model is only 6 K. Basically, such temperature differences are not large enough to induce discernable changes in the atmospheric circulation between circumbinary planets and planets orbiting single stars. This means that Neptune-like circumbinary planets can be treated as planets orbiting single stars when it comes to atmospheric modelling.

Figure 2: η values for 7 of the 10 known circumbinary systems. All have η values less than 0.1 percent. May & Rauscher (2016)

Reference:

May & Rauscher (2016), "Examining Tatooine: Atmospheric Models of Neptune-Like Circumbinary Planets", arXiv:1605.08785 [astro-ph.EP]

Transiting Worlds Beyond the Snow Line

Figure 1: Artist's impression of an exoplanet.

By visually inspecting the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) caused by giant planets on long-period orbits, Uehara et al. (2016) present the detection of seven transiting planet candidates that are consistent with Neptune-sized to Jupiter-sized planets with orbital periods ranging from a few to ~20 years.

These 7 planet candidates orbit their host stars beyond the snowline. The snowline is basically the distance from a star where temperatures in the protoplanetary disk start to become cold enough for volatiles such as water to condense into solid grains.

Of the 7 KOIs in this study, 2 are host to compact multi-transiting planetary systems. This suggests that over 20 percent of compact multi-transiting planetary systems also host cool gaint planets with orbital periods exceeding 3 years. The 7 KOIs in this study are KOI-847, KOI-671, KOI-2525, KOI-1108, KOI-693, KOI-435 and KOI-1421.

Figure 2: Artist's impression of an exoplanet.

KOI-847 is a planetary system with a known Neptune-sized planet candidate whose orbital period is 80.9 days. Two STEs were found which indicate the presence of two Neptune-sized planet candidates whose orbital periods are ~840 days and ~930 days. Since the two transits are nearly identical, they are likely to be caused by just one Neptune-sized planet candidate rather than two. In this case, the orbital period of the Neptune-sized planet candidate is ~1106 days.

KOI-671 is a known compact multi-transiting planetary system with four transiting planet candidates orbiting their host star closer than Mercury is to the Sun. The STE observed for this system indicates the presence of a Neptune-sized planet candidate with 3.9 ± 2.5 times the radius of Earth and a remarkably long orbital period of roughly 7700 days (~20 years). If confirmed, this should be the longest period transiting planet found to date.

KOI-2525 is known to host one super-Earth candidate. The STE observed for this planetary system indicates the presence of a second planet candidate with 12.7 ± 1.3 times the radius of Jupiter in a ~1200 day orbit around its host star.

KOI-1108 is a planetary system with three known transiting super-Earth candidates in close-in orbits around the host star. The STE observed for KOI-1108 corresponds to a Neptune-sized planet with 5.5 ± 1.9 times the radius of Earth and an orbital period of roughly 1160 days.

KOI-693 is a planetary system hosting two confirmed super-Earths. The STE observed for KOI-693 is consistent with a Neptune-sized planet with 3.5 ± 1.5 times the radius of Earth and an orbital period of roughly 980 days.

KOI-435 is a planetary system with two confirmed planets and three planet candidates. All five planets and planet candidates are in the super-Earth/sub-Neptune regime. The STE observed for this planetary system corresponds to a planet candidate with 7.8 ± 3.3 times the radius of Earth in a ~910 day orbit around its host star.

KOI-1241 only has a STE observed for it. The STE corresponds to a planet candidate with 10.2 ± 4.5 times the radius of Earth, in a ~2230 day orbit around its host star.

Figure 3: Artist's impression of an exoplanet.

Reference:
Uehara et al. (2016), "Transiting Planet Candidates Beyond the Snow Line Detected by Visual Inspection of 7557 Kepler Objects of Interest", arXiv:1602.07848 [astro-ph.EP]

Abundance of Objects Similar to WISE J0855-0714

Free-floating planetary-mass objects with masses around and below ~10 times the mass of Jupiter, and with temperatures below ~2200 K, are known to exist in star-forming regions and in clusters of stars younger than ~150 million years. These young planetary-mass objects are relatively luminous because they are cooling rapidly and are still in the process of contacting. This makes them much easier to detect than their older counterparts.

WISE J0855-0714 is currently the coldest known free-floating object beyond the Solar System. Using data from the Hubble Space Telescope (HST) and the Very Large Telescope (VLT), Osorio et al. (2016) show that the temperature of WISE J0855-0714 is somewhere between 225 to 250 K, consistent with previous observations. Additionally, WISE J0855-0714 has a low luminosity and a relatively high surface gravity. These properties indicate that WISE J0855-0714 is an old object.

WISE J0855-0714 is estimated to have 2 to 10 times the mass of Jupiter for an age of between 1 to 12 billion years. A lower mass would indicate a younger age since a lower-mass would allow WISE J0855-0714 to cool more rapidly to its current observed luminosity. If WISE J0855-0714 has a similar age as the Sun, its mass would be ~5 times the mass of Jupiter.

WISE J0855-0714 appears to represent an old version of similar mass objects that have been discovered in star-forming regions and in young clusters of stars. By extrapolating the known population of free-floating planetary-mass objects in young star clusters, it is estimated that there are around 15 to 60 objects like WISE J0855-0714 within ~20 light years of the Sun.

Reference:
Osorio et al. (2016), "Near-infrared photometry of WISE J085510.74-071442.5", arXiv:1605.08620 [astro-ph.EP]

A Planet Seemingly too Massive for its Host Star

Koshimoto et al. (2014) present the detection of a massive gas giant planet from a gravitational microlensing event identified as OGLE-2008-BLG-355. This planet is estimated to have ~4.6 times the mass of Jupiter and it orbits a star whose mass is estimated to be ~0.37 times the mass of the Sun. The planet is at a projected separation of roughly 1.7 AU from its host star. Such a planetary system poses a challenge to the core-accretion model of planet formation which predicts that such a massive planet should only form around a more massive host star. Nevertheless, the mass of the host star is only weakly constrained, and it can range anywhere from 0.20 to 0.67 times the mass of the Sun. If the core-accretion model of planet formation stands true for this planetary system, future observations should show that the mass of the host star is closer to the upper part of the estimated range.

Similar to OGLE-2008-BLG-355, two previously gravitational microlensing events, OGLE-2003-BLG-235 and MOA-2011-BLG-293, involve planets much more massive than Jupiter in orbit around host stars that initially appeared too low in mass. However, subsequent observations found that the host stars of both planetary systems have masses closer to the upper limit of their initial predicted mass ranges, and they are therefore massive enough to support the formation of such massive planets around them via the core-accretion model of planet formation.

Reference:
Koshimoto et al. (2014), “OGLE-2008-BLG-355Lb: A Massive Planet around A Late type Star”, arXiv:1403.7005 [astro-ph.EP]

The Abundance of Snowline Planets

Figure 1: Artist’s impression of a Neptune-like planet.

Gravitational microlensing is sensitive to the detection of planets that lie roughly between 1 to 10 AU from their host stars, making it good for detecting planets that lie beyond the snowline. Basically, the snowline is the distance from a star where it becomes cold enough for volatiles such as water to condense into solid ice grains. Over the years, data gathered by the Optical Gravitational Lensing Experiment (OGLE), the Microlensing Observations in Astrophysics (MOA) and the Wise microlensing surveys have shown that ~55 percent of microlensed stars host a snowline planet. Additionally, Neptune-like planets appear to be ~10 times more common than Jupiter-like planets.

Figure 2: The distribution of estimated mass ratios, before (dark gray) and after (light gray) correcting for detection efficiency. The right-hand vertical axis shows the corresponding frequency. The top horizontal axis indicates the masses of Earth, Neptune, Jupiter and the brown dwarf range, for a typical host star with 0.3 times the mass of the Sun. Shvartzvald et al. (2016)

Reference:
Shvartzvald et al. (2016), “The frequency of snowline-region planets from four-years of OGLE-MOA-Wise second-generation microlensing”, arXiv:1510.04297 [astro-ph.EP]

A Pair of Stars with the Shortest Known Orbital Period

The Optical Gravitational Lensing Experiment (OGLE) is a long-term photometric survey of the sky. Soszynski et al. (2015) present 242 ultra-short-period eclipsing and ellipsoidal binary stars identified in the OGLE field-of-view. One eclipsing binary system, identified as OGLE-BLG-ECL-000066, has an orbital period of less than 0.1 days. This binary system likely consists of a pair of red dwarf stars orbiting one another in a near-contact configuration. The red dwarf stars are each estimated to have ~1/5 the mass and ~1/4 the radius of the Sun.

Nevertheless, more observations will still be required to confirm the status of OGLE-BLG-ECL-000066. If confirmed, OGLE-BLG-ECL-000066 will probably be the shortest-period known binary system with non-degenerate components. The phrase “non-degenerate components” means that the system is not associated with objects such as white dwarf stars, neutron stars and black holes. A pair of red dwarf stars can have an orbital period of less than 0.1 days because red dwarf stars are small and compact, allowing them to be in very close proximity to one another without merging.

Reference:
Soszynski et al. (2015), “Ultra-Short-Period Binary Systems in the OGLE Fields Toward the Galactic Bulge”, arXiv:1503.02080 [astro-ph.SR]

A Brown Dwarf 0.6 AU from a Red Dwarf Star

OGLE-2014-BLG-0257 is a gravitational microlensing event involving a binary composed of a brown dwarf and a low-mass red dwarf star crossing the line-of-sight to a background star. Analysis of the gravitational microlensing lightcurve indicates that the brown dwarf has 37.7 ± 5.2 times the mass of Jupiter and the low-mass red dwarf star has 0.19 ± 0.02 times the mass of the Sun. Assuming the binary is located 4.08 ± 0.42 light years away, the projected separation between the binary components is 0.61 ± 0.07 AU. If the mass of the low-mass red dwarf star is scaled up to the mass of the Sun, and if the projected separation between the brown dwarf and the low-mass red dwarf star is scaled up by the same factor, then the separation between the binary components becomes ~3.2 AU per solar mass. Such a separation places the brown dwarf in the “brown dwarf desert”. Basically, the “brown dwarf desert” is a region of space within ~5 AU of Sun-like stars where it is exceedingly rare to find brown dwarfs residing there.

Reference:
Han et al. (2016), “OGLE-2014-BLG-0257L: A Microlensing Brown Dwarf Orbiting a Low-mass M Dwarf”, arXiv:1603.04567 [astro-ph.SR]

Gas Content of Interacting Pairs of Dwarf Galaxies

Pearson et al. (2016) present a study of 10 interacting pairs of dwarf galaxies in the Local Universe. The gas content of these galaxies appears more extended compared to isolated dwarf galaxies. For interacting pairs of dwarf galaxies that evolve in isolation, the gas content is bound and not lost to the surroundings. However, this is not the case for interacting pairs of dwarf galaxies located near massive host galaxies as their gas content can become lost to the surroundings.

Interacting pairs of dwarf galaxies can have a large amount of their gas content moved to the outskirts. Over ~50 percent of their gas content can be moved to the outskirts. For interacting pairs of dwarf galaxies that are evolving in isolation, the gas content "parked" in the outskirts can be re-accreted to drive future star formation. As for interacting pairs of dwarf galaxies that evolve in the vicinity of massive host galaxies, not only can they lose their gas content, they are also morphologically shaped by their environment and they can take on asymmetric profiles.

Of the 10 interacting pairs of dwarf galaxies in this study, 7 appear to have dense bridges of gas connecting them. The density of the gas content in these bridges is higher than the gas content in the surrounding regions. This explains why star formation can occur in these regions but not elsewhere. Ultimately, it is the proximity with massive host galaxies that drives the lost of gas content from interacting pairs of dwarf galaxies, and not due to mutual interactions between the dwarf galaxies themselves.

Reference:
"Local Volume TiNy Titans: Gaseous Dwarf-Dwarf Interactions in the Local Universe", arXiv:1603.09342 [astro-ph.GA]

A Young and Massive Hot-Jupiter

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

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

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

The Peculiar Orbit of a Hot-Jupiter

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

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

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