Two Peculiar Low-Density Sub-Saturn-Mass Planets

Bakos et al. (2016) present the discovery of two transiting low-density sub-Saturn-mass planets identified as HAT-P-47b and HAT-P-48b. Both planets orbit relatively bright F-type stars and have masses half-way between those of Neptune and Saturn. HAT-P-47b and HAT-P-48b were discovered through a combination of transit and radial velocity observations. Both HAT-P-47b and HAT-P-48b are very low density planets, and they are currently the two lowest mass planets known to have radii exceeding that of Jupiter's.

Figure 1: Artist’s impression of an exoplanet.

HAT-P-47b has 0.206 ± 0.039 times the mass and 1.313 ± 0.045 times the radius of Jupiter, giving it only ~11 percent the density of water. Its orbital period around its host star is 4.732 days and its estimated equilibrium temperature is 1605 ± 22 K. The host star of HAT-P-47b has 1.387 ± 0.038 times the mass and 1.515 ± 0.040 times the radius of the Sun. It also has 4.15 ± 0.27 times the Sun's luminosity and its surface temperature is 6703 ± 50 K. HAT-P-47b and its host star are both located ~900 light years away.

HAT-P-48b has 0.168 ± 0.024 times the mass and 1.131 ± 0.054 times the radius of Jupiter, giving it only ~14 percent the density of water. Its orbital period around its host star is 4.409 days and its estimated equilibrium temperature is 1361 ± 25 K. The host star of HAT-P-48b has 1.099 ± 0.041 times the mass and 1.223 ± 0.046 times the radius of the Sun. It also has 1.67 ± 0.14 times the Sun's luminosity and its surface temperature is 5946 ± 50 K. HAT-P-48b and its host star are both located ~1000 light years away.

Figure 2: Transit light curves indicating the presence of HAT-P-47b and HAT-P-48b. Bakos et al. (2016)

Figure 3: Mass-radius diagram of sub-Saturn-mass transiting planets (i.e. planets with less than 0.3 times the mass of Jupiter). HAT-P-47b and HAT-P-48b are highlighted by large boxes. Solar System planets are indicated by blue triangles. Both HAT-P-47b and HAT-P-48b stand out by their very low densities. Bakos et al. (2016)

Figure 4: Planetary mean density versus mass for transiting planets that have masses known to less than 20 percent uncertainty. HAT-P-47b and HAT-P-48b are highlighted by large boxes. The size of the points scales with planetary radius, while the colour indicates equilibrium temperature. HAT-P-47b and HAT-P-48b fall in a relatively unpopulated region of the parameter space; they are the lowest density sub-Saturn-mass objects. Solar System planets are indicated by blue triangles. Bakos et al. (2016)

Reference:
Bakos et al. (2016), "HAT-P-47b AND HAT-P-48b: Two Low Density Sub-Saturn-Mass Transiting Planets on the Edge of the Period--Mass Desert", arXiv:1606.04556 [astro-ph.EP]

Transmission Spectrum from a Simultaneous Transit

Figure 1: Artist's impression of an Earth-sized planet.

TRAPPIST-1b and TRAPPIST-1c are two Earth-sized planets in orbit around a low-mass red dwarf star. The equilibrium temperatures of TRAPPIST-1b and TRAPPIST-1c are estimated to be ~366 K and ~315 K, respectively. On 4 May 2016, both TRAPPIST-1b and TRAPPIST-1c simultaneously transited their host star. This rare transit event was observed with the Hubble Space Telescope (HST). The simultaneous transits allowed the combined transmission spectrum of the two planets to be measured. The lack of features in the observed combined transmission spectrum indicates that TRAPPIST-1b and TRAPPIST-1c both do not have cloud-free hydrogen-dominated atmospheres.

If the planets have hydrogen-dominates atmospheres, they must contain clouds or hazes at pressures less than ~10 mbar to account for the featureless transmission spectrum. However, theoretical predictions for hydrogen-dominated atmospheres at the irradiation levels of TRAPPIST-1b and TRAPPIST-1c indicate that cloud formation only occurs at pressures greater than 100 mbar. At such pressures, the clouds are too deep in the atmosphere to noticeably affect the transmission spectrum. As a result, TRAPPIST-1b and TRAPPIST-1c are unlikely to have hydrogen-dominated atmospheres. Nevertheless, many denser types of atmospheres (e.g. cloud-free water-vapour atmosphere or Venus-like atmosphere with high-altitude hazes) remain consistent with the observed featureless transmission spectrum. Future observations will be needed to distinguish between such atmospheres.

Figure 2: Light curve showing the simultaneous transits of TRAPPIST-1b and TRAPPIST-1c. Julien de Wit et al. (2016)

Figure 3: Transmission spectra of TRAPPIST-1b and TRAPPIST-1c compared to models. The observed transmission spectrum from HST measurements is indicated by black circles with 1σ error bars. Julien de Wit et al. (2016)

Reference:
Julien de Wit et al. (2016), "A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c", arXiv:1606.01103 [astro-ph.EP]

Optical Absorbers in the Atmosphere of WASP-98b

WASP-98b is a rare example of a transiting hot-Jupiter in orbit around a metal-poor main-sequence star. Since “metal” in this context refers to elements heavier than hydrogen and helium, how metal-rich or metal-poor a star is depends on its abundance of such elements. Based on the core-accretion model of planet formation, Jupiter-like planets are more common around metal-rich stars. The metallicity of the host star of WASP-98b is only one-third the metallicity of the Sun. It is very rare for a Jupiter-like planet to form around a star with such a low metallicity.

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

WASP-98b orbits very close to its host star, with an orbital period of only 2.96 days. The planet is estimated to have ~0.8 times the mass and ~1.1 times the radius of Jupiter, and an equilibrium temperature of about 1180 K. New observations of two transit events of WASP-98b were done in four filters: g' (450.0 nm), r' (622.0 nm), i' (764.0 nm) and z' (898.9 nm). The observations show that WASP-98b appears larger in size when observed at g', r' and i' than at z'. The maximum difference is between r' and z', and it translates to about 5.5 pressure scale heights with a confidence level of roughly 6σ. The pressure scale height is the vertical distance through the planet’s atmosphere whereby the pressure decreases by a factor that is equal to the base of the natural logarithm (i.e. 2.71828…).

What this means is that there is an optical-absorbing molecular species in the upper atmosphere of WASP-98b that is making the planet’s atmosphere more opaque when observed in g', r' and i' than in z'. Gaseous titanium oxide (TiO) and vanadium oxide (VO) are optical-absorbing molecular species present in the atmospheres of the hottest hot-Jupiters where temperatures are too high for TiO and VO to condense out. However, the atmosphere of WASP-98b is not hot enough to support gaseous TiO and VO as they will simply condense out. Whatever that is absorbing optical radiation in the atmosphere of WASP-98b is currently unknown.

Figure 2: Variation of the ratio of the radii of WASP-98b and its host star (vertical axis) with wavelength (horizontal axis). The 4 black points with vertical and horizontal error bars indicate the observations in the 4 filters: g' (450.0 nm), r' (622.0 nm), i' (764.0 nm), z' (898.9 nm). The size of five atmospheric scale heights is shown on the right of the plot. Mancini et al. (2016)

Reference:
Mancini et al. (2016), “An optical transmission spectrum of the transiting hot Jupiter in the metal-poor WASP-98 planetary system”, arXiv:1606.00432 [astro-ph.EP]

Birth of a Compact Multi-Planet System

Osorio et al. (2016) present the discovery of a miniature protoplanetary disk around the star XZ Tau B. This discovery was made from observations using the Atacama Large Millimeter/Submillimeter Array (ALMA). XZ Tau B is a young red dwarf star estimated to be only ~4.6 million years old. It is located ~450 light years away, and it has ~1.2 times the radius and ~0.37 times the mass of the Sun. The large radius of the star indicates that it is still in the process of contracting to its final radius. The estimated effective temperature of XZ Tau B is 3550 K.

The protoplanetary disk around XZ Tau B extends out to ~3.4 AU from the host star. It also has a central cavity extending from the host star out to ~1.3 AU. This central cavity has been attributed to the presence of a compact system of newly-formed planets in orbit around the host star. Such a compact multi-planet system is consistent with observations by NASA's Kepler mission which has revealed that low-mass compact multi-planet systems are relatively common. In these planetary systems, the planets orbit within ~1 AU from their host star and have masses ranging from a fraction to a few times the mass of Earth.

The newly-formed planets hypothesised around XZ Tau B are likely to be super-Earths rather than sub-Earths. This is because the snowline around XZ Tau B is located ~0.5 AU from the host star, which places it well within the central cavity. The snowline is basically the distance from a star where temperatures start to become cool enough in the protoplanetary disk for volatiles such as water to condense into solid ice grains, thereby providing more planet-building material. Forming planets beyond the snowline increases the likelihood for more massive planets.

Reference:
Osorio et al. (2016), "A Dwarf Protoplanetary Disk around XZ Tau B", arXiv:1606.03118 [astro-ph.SR]

An Exceptionally Low-Mass “Brown Dwarf”

Free-floating, planetary-mass objects are predicted to be quite common in the galaxy. These objects hide in the darkness between stars and may even outnumber stars themselves. Using data from NASA’s Wide-field Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey (2MASS), Schneider et al. (2016) present the discovery of a free-floating, planetary-mass object identified as WISEA 1147-2040. This object is located ~100 light years away and it is part of a group of young stars known as the TW Hydrae association. Its membership in the TW Hydrae association means that it is a young object, only ~10 million years old.

Observations of WISEA 1147-2040 also provided additionally evidence consistent with its youth. For example, WISEA 1147-2040 is observed to have an unusually dusty atmosphere, and this indicates it has a relatively low surface gravity. A young brown dwarf like WISEA 1147-2040 is still in the process of cooling and contracting to its final radius. As a consequence, it is still somewhat “inflated”, giving rise to its relatively low surface gravity. WISEA 1147-2040 is estimated to be between 5 to 13 times the mass of Jupiter. Furthermore, its effective temperature is predicted to be ~1100 to 1200 K.

WISEA 1147-2040 most likely formed as a brown dwarf rather than a planet that got ejected from its natal planetary system. This is because WISEA 1147-2040 is only ~10 million years old, and such a span of time is probably too short for a planet to form and subsequently get ejected. Brown dwarfs are objects that formed in the same way as stars do, but they lack the mass to fuse hydrogen in their cores. Observing these isolated, planetary-mass objects can provide greater insights to the characteristics of gas giant planet around other stars. This is because, unlike planets, these isolated objects are much easier to study as they are not lost in the glare of a host star.

Reference:

Schneider et al. (2016), “WISEA J114724.10-204021.3: A Free-Floating Planetary Mass Member of the TW Hya Association”, arXiv:1603.07985 [astro-ph.SR]

Two Near-Resonance Super-Earths Orbiting a Cool Star

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

K2-21b and K2-21c are two super-Earths in orbit around a cool host star that has 0.64 ± 0.11 times the mass and 0.60 ± 0.10 times the radius of the Sun, and an effective temperature of 4043 ± 375 K. This planetary system is located ~210 light years away. K2-21b has 1.59 ± 0.43 times the radius of Earth. Its orbital period is 9.32 days and it receives ~11.0 times the amount of flux Earth gets from the Sun, giving it an estimated equilibrium temperature of about 510 K. K2-21c has 1.92 ± 0.53 times the radius of Earth. Its orbital period is 15.50 days and it receives ~5.6 times the amount of flux Earth gets from the Sun, giving it an estimated equilibrium temperature of about 430 K.

The ratio of the orbital periods of both K2-21b and K2-21c are very near to the 5:3 mean motion resonance. This means that for every 5 times K2-21b goes around its host star, K2-21c will go around close to 3 times. The sizes of K2-21b and K2-21c place them at the boundary between high-density planets with rocky compositions and low-density planets with thick gaseous envelopes. Transit timing variations (TTVs) due to gravitational interactions between K2-21b and K2-21c may be detectable in future using ground-based telescopes. This can allow the masses of both planets to be estimated, thereby constraining their compositions.

Figure 2: Transit light curves of K2-21b (red) and K2-21c (cyan). Petigura et al. (2015)

Reference:
Petigura et al. (2015), "Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star", arXiv:1507.08256 [astro-ph.EP]

Planet Candidates Re-Identified as Low-Mass Stars

Figure: Artist’s impression of a binary star system.

KOI-554.01, KOI-1074.01 and KOI-3728.01 are three transiting planet candidates detected by NASA's Kepler space telescope. More careful analysis of the data show that these three transiting planet candidates are actually false positive and should instead be classified as secondary components of binary star systems. Modulations detected in the out-of-transit region of the light curves of the host stars of KOI-554.01, KOI-1074.01 and KOI-3728.01 indicate that these three transiting planet candidates too massive to be planets. 

These modulations are comprised of three components. (1) Ellipsoidal variations are caused by tides raised on the host star by the gravitational pull of the companion object, causing the host star to stretch into an ellipsoidal shape. (2) The companion object reflects light from its host star and also gives off its own thermal emission. (3) The gravitational tugging on the host star by the companion object causes Doppler beaming whereby the host star brightens slightly with it is approaching the observer and dims slightly when it is receding away. These three effects are collectively called REB modulations and they modulate the light curve of the host star depending on the position of the companion object in its orbit around the host star.

The amplitude of the REB modulations depend strongly on the mass of the companion object and its distance from it host star. Measuring the REB modulations can allow the mass and orbital distance of the companion object to be measured. A massive object in a close-in orbit around its host star can produce strong REB modulations. The REB modulations seen in the out-of-transit light curves show that KOI-554.01, KOI-1074.01 and KOI-3728.01 are low-mass stars orbiting much more massive hosts in binary star systems.

Figure 2: Mass-radius relationship for the three detected eclipsing binaries in the Kepler sample, namely KOI-554 (blue), KOI-1074 (green), and KOI-3728 (red). Primary stars are represented by squares, while the close companions are represented by circles. Other low-mass eclipsing binaries are also plotted with black plus symbols. Massive planets from the Exoplanet Catalogue are plotted as black crosses. Lillo-Box et al. (2016)

KOI-554.01 is estimated to have ~88 times the mass and ~0.77 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 2500 K. Its orbital period around its host star is 3.66 days. KOI-554.01 is most likely a low-mass red dwarf star, although it could also be a brown dwarf since its mass is near the stellar-substellar boundary. Its host star has an effective temperature of about 6108 K, and has ~1.13 times the mass and ~1.08 times the radius of the Sun.

KOI-1074.01 is a low-mass red dwarf star estimated to have ~155 times the mass and ~1.38 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 1500 K. Its orbital period around its host star is 3.77 days. Its host star has an effective temperature of about 6302 K, and has ~1.09 times the mass and ~1.30 times the radius of the Sun.

KOI-3728.01 is estimated to have ~83 times the mass and ~2.38 times the radius of Jupiter, and its equilibrium temperature is estimated to be less than 2100 K. Its orbital period around its host star is 5.55 days. KOI-3728.01 is most likely a low-mass red dwarf star, although it could also be a brown dwarf since its mass is near the stellar-substellar boundary. Its host star has an effective temperature of about 7360 K, and has ~2.10 times the mass and ~4.36 times the radius of the Sun. KOI-3728.01 also appears to be somewhat inflated in size.

Figure 3: For KOI-554.01. The top panel shows a close up view of the out-of-transit time interval where the light curve modulations are detectable. The middle panel shows the complex light curve including the eclipse and an inset showing each contribution to the out-of-transit modulations. Blue line - ellipsoidal variation; purple line - Doppler beaming; green line - reflectance. The bottom panel show the residuals of the fit. Lillo-Box et al. (2016)

Figure 4: Same as Figure 3, but for KOI-1074.01. Lillo-Box et al. (2016)

Figure 5: Same as Figure 4, but for KOI-3728.01. Lillo-Box et al. (2016)

Reference:
Lillo-Box et al. (2016), "Search for light curve modulations among Kepler candidates. Three very low-mass transiting companions", arXiv:1606.02398 [astro-ph.EP]

Brown Dwarf Detected by Two Space Telescopes

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

OGLE-2015-BLG-1319 is the first gravitational microlensing event observed by two space telescopes, Spitzer and Swift, and from the ground. The foreground star responsible for this gravitational microlensing event is a K-type main sequence star that crossed the line-of-sight to a distant background star. The gravitational field of the foreground star acted as a "lens", magnifying light from the distant background star. The gravitational microlensing light curve observed from the ground shows a short anomaly over the peak due to a presence of a companion around the foreground star.

Parallax measurement of the gravitational microlensing event from Spitzer shows that the companion is a brown dwarf with 30 to 55 times the mass of Jupiter. The small separation of Swift from Earth means that the baseline is not long enough to allow Swift to provide an independent parallax measurement. As a result, there are two possible solutions for the projected separation of the brown dwarf from its host star. Due to the wide/close degeneracy, the brown dwarf is either 0.23 to 0.28 AU or 40 to 52 AU from its host star.

Figure 2: Light curve of OGLE-2015-BLG-1319 with data from Spitzer (red), and various ground-based observatories.  The inset shows the anomalous region over the peak of the event, revealing the presence of the companion brown dwarf. Shvartzvald et al. (2016)

Reference:
Shvartzvald et al. (2016), "First simultaneous microlensing observations by two space telescopes: Spitzer & Swift reveal a brown dwarf in event OGLE-2015-BLG-1319", arXiv:1606.02292 [astro-ph.EP]

Spreading Life from Star to Star

Interstellar panspermia refers to the transport and spread of life from one extrasolar system to another. If interstellar panspermia is the dominant mechanism for the origin of life in extrasolar systems, then life-bearing extrasolar systems should exhibit more clustering when compared to the case whereby life originated spontaneously. Future searches for biosignatures in the atmospheres of exoplanets can potentially test such a prediction.

Stars in the Milky Way drift relative to one another with a characteristic speed of a few tens of kilometres per second. Depending on the effective spreading speed of life, interstellar panspermia can fall within three possible regimes. (1) If interstellar panspermia takes place at speeds much greater than the characteristic speed of stars in the galaxy, then the drifting of stars is expected to be negligible in the clustering of related life-bearing extrasolar systems. This can happen if an intelligent species spreads life at high speeds.

(2) Even if interstellar panspermia takes place at speeds comparable to the characteristic speed of stars in the galaxy, the clustering of life-bearing extrasolar systems is still expected to hold. This is applicable for lithopanspermia, whereby fragments of the life-bearing crust of a planet can get ejected into space by large impacts and potentially seed other extrasolar systems with life. These ejected life-bearing crustal fragments have velocities comparable to the characteristic speed of stars in the galaxy.

(3) If interstellar panspermia occurs at speeds much less than the characteristic speed of stars in the galaxy, life can still spread to extrasolar systems within a continuous region of space until such a region of space becomes so large that the rotation of the Milky Ways starts to shear and break it apart.

If future searches for biosignatures in the atmospheres of exoplanets show large regions in the Milky Way saturated with life-bearing extrasolar systems and regions with close to no life-bearing extrasolar systems, it would indicate that interstellar panspermia, instead of a spontaneous origin, is the dominant mechanism for the origin of life in extrasolar systems. However, the detection of such a position-space correlation may not be possible if the timescale required for life to become observable once it has seeded an extrasolar system is longer than the timescale for stars to redistribute across the Milky Way.

Interesting, the position-space correlations of life-bearing extrasolar systems (i.e. the way in which life-bearing extrasolar systems are spatially distributed) can potentially indicate whether primitive life can spread naturally to other extrasolar systems (i.e. via lithopanspermia), or whether primitive life will have to wait for intelligent life to spread it to other extrasolar systems.

Interstellar panspermia is expected to be more effective if stars are closer to one another. In dense stellar systems such as globular clusters, all potentially habitable extrasolar systems can be life-bearing due to effective interstellar panspermia. As a result, it is possible that globular clusters are saturated with life-bearing extrasolar systems, while other places with lower stellar densities have a much lower abundance of life-bearing extrasolar systems.

Reference:
Lin & Loeb (2015), "Statistical Signatures of Panspermia in Exoplanet Surveys", arXiv:1507.05614 [astro-ph.EP]

Bow Shock of a Speeding Hot Jupiter

When a planet moves supersonically through the stellar wind or coronal plasma of its host star, a bow shock can form where the pressure between the plasma and the planet's magnetosphere balances out. This is generally ahead of the planet, in the direction of the planet's motion around its host star. If the planet has a strong magnetic field, the bow shock can be displaced several planetary radii ahead of the planet. Material from the stellar wind of the planet's host star can pile up at the bow shock. If the material is compressed sufficiently, the line-of-sight column density of the material in the bow shock can become high enough to generate a visible absorption signature. Furthermore, if the planet transits its host star, the visible absorption signature of the bow shock can be detected prior to the main transit event.

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

HD 189733b is a transiting hot Jupiter in a close-in orbit around its host star. It is 13 percent more massive than Jupiter and it orbits its host star once every 2.2 days. HD 189733b whizzes around its host star at 152.5 km/s. At this enormous speed, HD 189733b is moving supersonically through the stellar wind if its host star. Transit observations of the host star of HD 189733b revealed a pre-transit absorption feature occurring 125 minutes before the main transit event. This pre-transit absorption feature is consistent with the presence of a planetary bow shock that has a large standoff distance of 12.75 planetary radii from the planet itself. The large standoff distance is a clear indication of a very powerful magnetic field around the planet. In fact, the equatorial planetary magnetic field strength of HD 189733b is estimated to be 28 G. For comparison, the magnetic field strength on Earth is less than 1 G. The strong magnetic field around HD 189733b serves as the planet's first line of defence against the energetic stellar wind from its host star which can potentially erode the planet's atmosphere.

Figure 2: To-scale projections of the planet and bow shock in the orbital plane (top panel) and the view from Earth (bottom panel). Cauley et al. (2015)

Reference:
Cauley et al. (2015), "Optical hydrogen absorption consistent with a thin bow shock leading the hot Jupiter HD 189733b", arXiv:1507.05916 [astro-ph.EP]

Kepler-452b is a Potentially Earth-Like Planet

Kepler-452b is a transiting, potentially rocky planet in the habitable zone of its host star. Its discovery was announced by Jenkins et al. (2015) after a search through four years of data collected by NASA's Kepler space telescope which hunts for planets by looking for small dips in a star's brightness when a planet happens to pass in front of it. Kepler-452b is estimated to have ~1.63 times the radius of Earth and it orbits its Sun-like host star every 384.8 days. The transit of Kepler-452b in front of its host star has a transit depth of roughly 200 ppm (parts per million) and a transit duration of roughly 10.6 hours.

Kepler-452b orbits its host star within the habitable zone and at its distance from its host star, it receives only 10 percent more flux than Earth gets from the Sun. Kepler-452b has always been in the habitable zone of its host star and it is expected to remain there for another ~3 billion years. The equilibrium temperature on Kepler-452b is estimated to be ~265 K. For comparison, the equilibrium temperature of Earth is 255 K.

Interior models suggest that the likelihood of Kepler-452b being a rocky planet is somewhere between 49 to 62 percent. The host star of Kepler-452b has ~1.11 times the radius and ~1.037 times the mass of the Sun, and its estimated effective temperature is 5757 ± 85 K. Furthermore, the host star of Kepler-452b is estimated to be 6 ± 2 billion years old. Because Kepler-452b is expected to have a relatively low-mass and its host star is relatively faint, measuring the mass of Kepler-452b through radial velocity observations will be unlikely in the near future.

Reference:
Jenkins et al. (2015), "Discovery and Validation of Kepler-452b: A 1.6-Re Super Earth Exoplanet in the Habitable Zone of a G2 Star", arXiv:1507.06723 [astro-ph.EP]

More Brown Dwarfs in the Sun's Neighbourhood

There are 136 stars and 26 brown dwarfs known within ~20 light years of the Sun, giving a star to brown dwarf ratio of 5.2. The 26 brown dwarfs are divided into spectral classes (1 M-dwarf, 2 L-dwarfs, 18 T-dwarfs, and 5 Y-dwarfs). It appears that the brown dwarfs within ~20 light years of the Sun are not uniformly distributed. There are 21 brown dwarfs behind the Sun, but only 5 brown dwarfs ahead of the Sun in the direction of rotation of the galaxy. This non-uniform distribution of brown dwarfs is most likely due to an observation bias because brown dwarfs should be uniformly distributed like stars. What this means is that the brown dwarf census in the Sun's neighbourhood is incomplete. Assuming there are 5 more brown dwarfs in front of the Sun, the star to brown dwarf ratio decreases to 4.4. This brings it closer to the estimated star to brown dwarf ratio of ~3.3 for the Orion Nebula cluster, a star forming region.

Reference:
Bihain & Scholz (2016), "A non-uniform distribution of the nearest brown dwarfs", arXiv:1603.00714 [astro-ph.SR]

GW151226: Gravitational Waves from a Black Hole Merger

Figure 1: Artist’s impression of a black hole.

Gravitational waves are ripples propagating through the fabric of spacetime. Abbott et al. (2016) present the observation of a gravitational wave signal designated as GW151226. This is the second direct detection of a gravitational wave signal and it was observed by two detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) on 26 December 2015 at 03:38:53 UTC. The signal has significance greater than 5σ, and it was observable for approximately one second. The gravitational waves first hit the LIGO observatory in Livingston, Louisiana. 1.1 milliseconds later, they passed through the LIGO observatory in Hanford, Washington.

GW151226 was produced from the coalescence of two stellar-mass black holes, and the observed gravitational wave signal originated from the final stages of the two black holes spiralling into each other. The primary and secondary black holes have ~14.2 and ~7.5 times the mass of the Sun, respectively. After merger, the final black hole has ~20.8 times the mass of the Sun. The final mass is lower than the sum of the initial masses because approximately one solar-mass was radiated away as pure energy, in the form of gravitational waves. This is because it takes energy to distort spacetime and create gravitational waves. GW151226 is estimated to have occurred at a distance of approximately 1.4 billion light years away.

Figure 2: Gravitational signal from GW151226. Abbott et al. (2016)

LIGO can only detect gravitational waves generated from massive objects orbiting each other faster than ~30 times per second. GW151226 was observed for a full second. For comparison, the gravitational wave signal from the first binary black hole merger detected by LIGO, designated GW150914, was observable for only about 0.2 seconds, just 1/5 the duration of GW151226. This is because the black holes for GW150914 were bigger than the ones that generated GW151226, and bigger black holes run into each other before they can orbit around one another fast enough to be detectable for a longer period of time by LIGO. Smaller black holes, like the ones that generated GW151226, can get much closer together before they merge, allowing them to orbit sufficiently fast around one another to be detectable by LIGO over a longer period of time.

Reference:
Abbott et al. (2016), "GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence", Phys. Rev. Lett. 116, 241103

A Giant Planet Spinning Up its Host Star

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

HATS-18b is a very short period massive gas giant planet with 1.980 ± 0.077 times the mass and ~1.337 times the radius of Jupiter. It orbits a Sun-like host star with an orbital period of only 20.1 hours. The host star of HATS-18b has ~1.037 times the mass and ~1.020 times the radius of the Sun, and effective temperature 5600 ± 120 K. Additionally, the host star of HATS-18b is measured to have a rotation period of only 9.8 ± 0.4 days. Such a rotation period is remarkably fast given that the star is estimated to be 4.2 ± 2.2 billion years old. Sun-like stars at that age tend to have rotation periods around ~30 days. The host star of HATS-18b is basically spinning too fast for its age.

The relatively large mass and close-in orbit of HATS-18b is the reason for the short rotation period of its host star. HATS-18b raises tides on its host star and because the planet orbits faster than the star rotates, the tidal bulge raised on the star lags the planet. This causes the orbit of HATS-18b to decay due to tidal dissipation. As a result, the angular momentum that is taken out as the planet's orbit shrinks is deposited in the star, causing the star to spin up. Because HATS-18b orbits so close to its host star, the planet's equilibrium temperature is estimated to be 2060 ± 59 K.

Figure 2: Transit light curve indicating the presence of HATS-18b. Penev et al. (2016)

Reference:
Penev et al. (2016), "HATS-18 b: An Extreme Short-Period Massive Transiting Planet Spinning Up Its Star", arXiv:1606.00848 [astro-ph.EP]

A Giant Planet Orbiting Two Suns in the Habitable Zone

Figure 1: Artist’s impression of a giant planet hosting a large, potentially habitable moon.

Kepler-1647 is an eclipsing binary system consisting of a pair of somewhat Sun-like stars. One star is slightly large than the Sun (primary star) and the other is slightly smaller (secondary star). An eclipsing binary system is basically a binary star system whose brightness varies periodically as the orbital plane of the two stars is orientated such that the two stars periodically pass in front of one another. Observations by NASA’s Kepler space telescope revealed the presence of a giant planet, identified as Kepler-1647b, in orbit around the eclipsing binary system.

Kepler-1647b is known as a transiting circumbinary planet as it orbits two stars. It has a very long orbital period of about 1100 days, making it longest-period transiting circumbinary planet discovered to date and one of the longest-period transiting planets. In total, Kepler-1647b produced three transits. Two transits were across the secondary star and one was a heavily blended transit across the primary star. During the heavily blended transit, both Kepler-1647b and the secondary star were simultaneously crossing the line of sight to the primary star. Such an event is relatively rare, and it is referred to as a syzygy.

Figure 2: Comparison of the relative sizes of several transiting circumbinary planets, from the smallest (Kepler-47b) to the largest (Kepler-1647b). Kepler-1647b is the largest known transiting circumbinary planet. Image Credit: Lynette Cook

Kepler-1647b is estimated to have 1.06 ± 0.01 times the radius of Jupiter, making it the largest circumbinary planet discovered to date. In addition, Kepler-1647b is also the first known Jupiter-like transiting circumbinary planet. Before the discovery of Kepler-1647b, all known transiting circumbinary planets were Saturn-sized or smaller. By measuring the perturbations in the timings of the stellar eclipses of the eclipsing binary system due to the gravitational pull of the planet, Kepler-1647b is found to have 1.52 ± 0.65 times the mass of Jupiter, or 483 ± 206 times the mass of Earth.

It appears that the orbit of Kepler-1647b is within the circumbinary Habitable Zone (HZ) of the eclipsing binary system. In fact, the planet is well within the conservative HZ for the entire duration of its orbit. The time-averaged insolation that Kepler-1647b receives is estimated to be 0.71 ± 0.06 times the intensity of solar radiation Earth gets from the Sun. While Kepler-1647b itself is not habitable as it is a gas giant planet, it can host terrestrial-sized moons that are potentially habitable.

Figure 3: Top view of the orbital configuration of the Kepler-1647 system. The figure shows the location of the habitable zone (green) and the planet’s orbit (white circle). The binary star is in the center, surrounded by the critical limit for dynamical stability (red circle). The dark and light green regions represent the conservative and extended HZ, respectively. Kepler-1647b is inside the conservative HZ for its entire orbit. Kostov et al. (2016)

Reference:
Kostov et al. (2016), “Kepler-1647b: the largest and longest-period Kepler transiting circumbinary planet”, arXiv:1512.00189 [astro-ph.EP]

An Ultra-Dense Transiting Brown Dwarf

Figure 1: Artist’s impression of what could be a brown dwarf in orbit around a main-sequence star.

Bayliss et al. (2016) present the discovery of an ultra-dense brown dwarf in a 40.737 day orbit around its host star. This brown dwarf is identified as EPIC201702477b and it was first reported as a planet candidate based on two transit events observed by K2. Follow up observations, together with high precision radial velocity measurements, confirmed its nature as a brown dwarf. EPIC201702477b has 66.9 ± 1.7 times the mass and 0.757 ± 0.065 times the radius of Jupiter, giving it a remarkably high density of 191 ± 51 g/cm³, around ~25 times the density of iron.

EPIC201702477b has the smallest known radius for any brown dwarf, and it is also denser than any planet, substellar mass object or main-sequence star discovered so far. The host star of EPIC201702477b has 0.870 ± 0.013 times the mass and 0.901 ± 0.057 times the radius of the Sun. Its effective temperature is 5517 ± 20 K, and its estimated age is 8.8 ± 4.1 billion years old. Currently, there are only 12 known brown dwarfs (i.e. objects with 13 to 80 times the mass of Jupiter) that transit main-sequence stars.

Figure 2: Transit light curves indicating the presence of EPIC201702477b, phase-folded to the best fitting orbital period of 40.73691 ± 0.00037 days. Bayliss et al. (2016)

Figure 3: Top Panel: Radial velocity measurements indicating the presence of EPIC201702477b from the HARPS (solid squares) and SOPHIE (empty circles) spectrographs plotted against time. The black line shows the best fit global model. Lower inset panel shows the residuals from the best fit model. Bottom Panel: Same as above but phase-folded to the best fitting orbital period of 40.73691 ± 0.00037 days. Bayliss et al. (2016)

Figure 4: The density-mass relationship for the known transiting brown dwarfs. EPIC201702477b stands out as the highest density object yet discovered, very near to the peak density predicted by the model. Bayliss et al. (2016)

Reference:
Bayliss et al. (2016), "EPIC201702477b: A Long Period Transiting Brown Dwarf from K2", arXiv:1606.04047 [astro-ph.EP]

The Faint Emission from Isolated Neptunes

Figure 1: Artist’s impression of what could be an isolated Neptune-like planet.

The orbits of some Kuiper Belt Objects suggest the presence of a planet with ~10 times the mass of Earth on a very distant ~700 AU semimajor axis orbit around the Sun. Assuming the hypothesized planet is Neptune-like (i.e. the planet is modelled as a simple two-layer model that consists of a rocky/icy core and a hydrogen/helium envelope), Ginzburg et al. (2016) show that the planet can potentially be detected via the internal flux it gives off as it cools over time. Because the planet is so far from the Sun, its own internal flux is much greater than the incident irradiation it gets from the Sun. As a result, the thermal evolution of the planet progresses as if the planet were an isolated object. Measuring the temperature of the planet can allow the mass of its hydrogen-helium envelope to be constrained. This is because a thick hydrogen-helium envelope acts as an insulator, slowing down the planet’s rate of cooling. If the planet’s effective temperature is ~50 K, its wavelength at peak emission will be ~60 μm.

Figure 2: Effective temperature as a function of age for a planet with ~10 times the mass of Earth and whose hydrogen-helium envelope accounts for 14 percent of its total mass. Ginzburg et al. (2016)

Figure 3: Effective temperature of 4.5 billion year old Neptune-like planets as a function of their atmospheric mass. The curves are for three values of the planet’s radius in units of Neptune’s radius: 3/4 (bottom black line), 1.0 (middle blue line), and 4/3 (top red line). These radii correspond to planet masses of about 5.4, 17, and 54 times the mass of Earth, respectively, assuming Neptune’s composition and accounting for the gravitational compression of the planet’s core. Ginzburg et al. (2016)

Figure 4: Contours of the effective planetary temperature (solid blue lines) and of the temperature at the atmosphere-core boundary (dashed black lines), at an age of 4.5 billion years, as a function of the planet’s radius and atmospheric mass. A solid crust forms below the bottom dashed black line and cooling is no longer dominated by the insulating effect of the atmosphere as the atmosphere is too thin. An atmospheric mass fraction of 0.5 is also indicated (dotted red line), assuming Neptune’s composition and accounting for the gravitational compression of the planet’s core. Ginzburg et al. (2016)

Reference:
Ginzburg et al. (2016), “Blackbody Radiation from Isolated Neptunes”, arXiv:1603.02876 [astro-ph.EP]

Detecting Exomoons by Gravitational Microlensing

Figure 1: Artist’s impression of a large moon.

When a foreground star hosting a planet crosses the line-of-sight to a distant background star, the gravitational field of the foreground star and its planet can act as a "lens", magnifying light from the background star. The observed change in brightness of the background star is in the form of an asymmetric gravitational microlensing light curve. The asymmetric shape of the light curve is due to the presence of the planet. If the planet happens to be far enough from its host star and also happens to have a moon in orbit around it, the moon can induce a very short-duration perturbation on the smooth asymmetric gravitational microlensing light curve.

Furthermore, if the projected star-planet separation is large enough, the planet can generate a gravitational microlensing light curve without any noticeable contribution from its host star, and if the planet has a moon around it, the moon will induce a very short-duration perturbation on the light curve. The shape of the light curve from the planet-moon system will be somewhat similar in shape to a light curve generated by a star-planet system. The main difference is that the gravitational microlensing event caused by the planet-moon system will occur over a much shorter timescale.

Figure 2: Synthetic gravitational microlensing light curves involving a triple lens system (i.e. star-planet-moon system) whereby the moon has the same mass as Mars and the planet has the same mass as Jupiter. The short-duration perturbation on each light curve is due to the presence of the moon. Chung & Ryu (2016)

Reference:
Chung & Ryu (2016), "Properties of microlensing events by wide separation planets with a moon", arXiv:1606.00565 [astro-ph.EP]

KOI 408.05 is a Warm Neptune in the Habitable Zone

Kunimoto et al. (2016) present the discovery of a warm Neptune in the habitable zone around a Sun-like star. This planet is identified as KOI 408.05. It orbits a host star with ~0.963 times the mass and ~0.931 times the radius of the Sun, and the effective temperature of the host star is ~5862 K. The planet's orbital period is 637 days and it is 1.472 times further from its host star than Earth is from the Sun, in a Mars-like orbit. KOI 408.05 is estimated to receive ~60 percent the amount of flux Earth gets from the Sun and its equilibrium temperature is predicted to be 252 K. For comparison, Earth's equilibrium temperature is 255 K.

The transit of KOI 408.05 in front of its host star has 1555 ppm (parts-per-million) transit depth and transit duration of about 16.7 hours. From the transit depth, the size of KOI 408.05 is estimated to be ~4.92 times the radius of Earth. With these properties, KOI 408.05 is a warm Neptune-sized planet in the habitable zone of its host star. Although KOI 408.05 is expected to be like Neptune with a think hydrogen-helium envelope, a large moon in orbit around it can potentially be habitable.

NASA's Kepler space telescope has so far detected ~5000 planets and planet candidates. However, only 20 or so have orbital periods longer than the orbital period of KOI 408.05. In addition to KOI 408.05, Kunimoto et al. (2016) also presented the detection of three other planets around three different host stars. KOI 205.02 is a Mercury-sized planet with ~0.449 times the radius of Earth in a 4.813 day orbit around its host star at 0.052 AU. Its equilibrium temperature is predicted to be 902 K and it receives ~100 times the amount of flux Earth gets from the Sun.

KOI 290.02 is a scorchingly hot Earth-sized planet with ~1.29 times the radius of Earth in a 6.828 day orbit around a relatively luminous host star at 0.078 AU. Its equilibrium temperature is predicted to be 1654 K and it receives ~1015 times the amount of flux Earth gets from the Sun. KOI 488.02 is also an Earth-sized planet with ~1.14 times the radius of Earth in a 17.39 day orbit around its host star at 0.119 AU. Its equilibrium temperature is predicted to be 736 K and it receives ~43 times the amount of flux Earth gets from the Sun.

Reference:
Kunimoto et al. (2016), “Lifting Transit Signals from the Kepler Noise Floor: I. Discovery of a Warm Neptune”

Inflated Hot-Jupiters by the Half-Dozen

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

Espinoza et al. (2016) present the discovery of six inflated hot-Jupiters identified as HATS-25b through HATS-30b. All six planets transit their host stars. The planets have masses between 0.5 to 0.7 times the mass of Jupiter, radii between 1.17 to 1.75 times the radius of Jupiter and orbital periods between 3.2 to 4.6 days. HATS-26b and HATS-27b stand out in having remarkably low densities of only 15.3 and 18.0 percent the density of water, respectively. All planets, with the exception of HATS-27b, appear to be good targets for future atmospheric characterisation. Nevertheless, HATS-27b is a good target for measuring the Rossiter-McLaughlin to determine its spin-orbit alignment due to the relatively high brightness and high stellar rotational velocity of its host star.

Figure 2: Mass-radius diagram for all the transiting hot-Jupiters discovered to date (grey points). Red points indicate the discovered exoplanets presented in this work. The black lines show the mass-radius relations of 4.5 billion year old planets at 0.045 AU from the Sun for core-free giant planets (solid line) and for giant planets with 100 Earth-mass cores (dashed line), which are appropriate for the insolation levels received by HATS-25b, HATS-28b, HATS-29b and HATS-30b. The blue lines show the same relations but for planets at 0.02 AU, more (but not exactly) appropriate for the insolation levels received by HATS-26b and HATS-27b. These relations imply insolation levels around 2500 times the solar insolation level at Earth, while the actual insolation levels for HATS-26b and HATS-27b are closer to 2250 and 1250 times the solar flux at Earth, respectively. Espinoza et al. (2016)

Figure 3: Equilibrium temperature-radius diagram for all the transiting hot-Jupiters discovered to date along with these six newly discovered ones. Espinoza et al. (2016)

Reference:
Espinoza et al. (2016), "HATS-25b through HATS-30b: A Half-dozen New Inflated Transiting Hot Jupiters from the HATSouth Survey", arXiv:1606.00023 [astro-ph.EP

Microlensing Detection of a Saturn-Like Planet

OGLE-2014-BLG-1760 is a gravitational microlensing event involving a star with 0.51 ± 0.44 times the mass of the Sun that hosts a planet with 180 ± 110 times the mass of Earth. Both the star and its planet were detected when they crossed the line-of-sight to a background star, and their combined gravitational fields acted as a “lens”, magnifying the brightness of the background star. Assuming the planetary system is 22.5 ± 3.6 thousand light years away, the projected star-planet separation will be 1.7 ± 0.3 AU. From its estimated mass, this planet is probably a low mass gas giant planet like Saturn. Since both the star and its planet have a high proper motion relative to the background star, they are expected to become resolvable in the near future when their angular separation is larger, allowing the properties of the planetary system to be better determined.

Reference:
Bhattacharya et al. (2016), “Discovery of a Gas giant Planet in Microlensing Event OGLE-2014-BLG-1760”, arXiv:1603.05677 [astro-ph.EP]

Planet in a Close-In Orbit around a Subgiant Star

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

K2-39b is a transiting giant planet in a close-in orbit around a subgiant star with ~3.88 times the radius and ~1.53 times the mass of the Sun. Transit and radial velocity observations indcate that K2-39b has ~50.3 times the mass and 8.3 ± 1.1 times the radius of Earth. This gives K2-39b a mean density that is only half the density of water. The orbital period of K2-39b around its host star is only 4.6 days.

Currently, K2-39b is the shortest-period planet known orbiting a subgiant star. However, only a handful of evolved stars (i.e. stars larger than 3.5 times the Sun's radius) are known to host short-period (i.e. planets with orbital periods less than 100 days) transiting planets. The existence of a planet like K2-39b that is orbiting so close to its host star suggests that tidal destruction may not be that effective in removing planets in close-in orbits around subgiant stars.

Figure 2: Transit light curve indicating the presence of K2-39b. Van Eylen et al. (2016)

Figure 3: Radial velocity curve indicating the presence of K2-39b. Van Eylen et al. (2016)

Figure 4: K2-39b compared with other confirmed planets - transiting planets (open circles) and non-transiting planets (open stars). Short-period transiting planets orbiting evolved stars are indicated with filled blue circles. This figure shows stellar radius versus semi-major axis, where the dotted line indicates the surface of the host star. Van Eylen et al. (2016)

Reference:
Van Eylen et al. (2016), "The K2-ESPRINT Project V: a short-period giant planet orbiting a subgiant star", arXiv:1605.09180 [astro-ph.EP