Thursday, June 30, 2016

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]

Wednesday, June 29, 2016

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]

Tuesday, June 28, 2016

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]

Monday, June 27, 2016

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]

Sunday, June 26, 2016

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]

Saturday, June 25, 2016

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]

Friday, June 24, 2016

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]

Thursday, June 23, 2016

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]

Wednesday, June 22, 2016

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]

Tuesday, June 21, 2016

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]