Friday, January 1, 2016

A Data-Driven Year

Figure 1. Sunrise on Pluto with New Horizons: a once-in-a-lifetime view of the dwarf planet’s  icy mountains and plains.
During the year just ended, our own Solar System made the biggest headlines in astronomy and space exploration. At the top of the list were the amazing images and detailed physical data returned by the New Horizons mission to the double-dwarf system of Pluto and Charon (Stern et al. 2015). Close behind were the data sent back from Ceres by the Dawn spacecraft. Then came evidence from various sources elsewhere in our system for liquid water, whether flowing on the surface of Mars or sloshing in a global ocean beneath the ice shell of Enceladus (Thomas et al. 2015). Extrasolar astronomy offered nothing comparable. 

Yet the growth in our evidence for exoplanets – near and far, individually and in systems, at the present epoch and across galactic history – continued at a rapid pace in 2015. In November, the exoplanetary census maintained by the Extrasolar Planets Encyclopaedia passed 2000. Today it stands at 2041: double the count registered just two years ago, and an order of magnitude larger than the census in 2006. Most exoplanets (62%) are known by the shadows they cast while crossing the face of their parent stars, through the so-called transit method. Those in the next largest sample (31%) were revealed by their gravitational effects on the spectral lines of their parent stars, as shown by radial velocity observations. The remainder were found variously by direct imaging, microlensing, and pulsation timing.

As 2016 begins, the astronomical community is still digesting the massive haul of data collected by the primary Kepler mission from 2009 to 2013, even as new discoveries by K2 (Kepler’s successor) and other programs continue to accumulate. This posting will highlight some of the most remarkable extrasolar detections of the past year, focusing on contributions to our understanding of planet structure, planet formation, and system architectures. Then we’ll glance at a few other important studies of a more theoretical nature.

Figure 2. Aldebaran
The red giant Aldebaran appears left of center as a foreground object against members of the more distant Hyades Cluster. Also known as Alpha Tauri, Aldebaran is only about 20 parsecs away, while the distance to the Hyades is about 46 parsecs. Twenty-five years of radial velocity observations support the presence of a massive gas giant planet orbiting Aldebaran in a period of 629 days. Image credit: Wikipedia

Aldebaran is one of the brightest stars in northern skies, visible even in light-polluted environments (Figure 2). Its brightness is explained partly by its evolutionary status as a red giant of spectral type K5 III and partly by its proximity to Earth: it’s located just 66 light years away (20.4 parsecs). Back in 1993, when extrasolar planets were still science fiction, Artie Hatzes and William Cochrane reported six years of data demonstrating regular variations in the radial velocities of Aldebaran and two other nearby red giants, Arcturus and Pollux. As Hatzes & Cochrane noted at the time, “The exciting possibility that the variations are due to the presence of planetary companions should be treated with caution.” A planet around Pollux was eventually confirmed more than a decade later, in 2006. It’s a gas giant of 2.3 Jupiter masses (Mjup) orbiting at a semimajor axis of 1.6 AU (a little wider than the orbit of Mars). Although Arcturus is still awaiting confirmation, good tidings came from Aldebaran this past summer in the form of a Super Jupiter with an orbit similar to that of Pollux b (Hatzes et al. 2015). Details on the new system appear in Table 1.

In an era when splashy claims are sometimes rushed into press, only to be refuted within a few years, Hatzes & colleagues have upheld the most exacting scientific standards. They collected and analyzed data on Aldebaran b for more than 25 years – equivalent to about 15 planetary orbits – before ultimately confirming its reality. Their publications on this project over the past two decades offer a window on our progress in understanding nearby stars. When they reported their early findings, Pollux and Aldebaran were credited with respective masses of 2.8 and 2.5 Msol, and these values informed estimates of planetary masses. Since then, the numbers have been revised dramatically downward to 1.7 and 1.1 Msol, respectively. Evidently Pollux is a “retired A star” (Johnson et al. 2011), but Aldebaran seems to have begun its existence as a Sun-like star. Among planet-hosting red giants, which now number at least 60, that origin puts it in the minority. 

Table 1. Parameters of the Aldebaran system
For Aldebaran (the host star), column 1 = mass in Solar units; column 2 = radius in Solar units; column 3 = metallicity as the ratio of iron to hydrogen; column 4 = effective temperature in Kelvin, column 5 = age in billions of years; column 6 = spectral type, column 7 = distance in parsecs. For Aldebaran b (the planet), column 1 = mass in Jupiter units; column 2 = orbital semimajor axis in astronomical units; column 3 = orbital eccentricity; column 4 = orbital period in days.
EPIC 210490365 Aldebaran is the right eye of the bull in the zodiacal constellation of Taurus. It’s surrounded by bluer, dimmer stars that trace the rest of the bull’s face (Figure 2). All the other stars comprise a completely unrelated structure known as the Hyades Cluster, which at 44 parsecs is more distant than Aldebaran, though still quite nearby in Galactic terms. The Hyades are also much younger than Aldebaran, representing stars of all colors born in a single hydrogen cloud about 650 million years ago. Data collected by the K2 Mission reveal that one member of the cluster – a red dwarf of type M4.5 with a fiendishly unmemorable catalog number – is accompanied by a gas dwarf of 3.43 Earth radii (Rea) transiting every 3.5 days (Mann et al. 2015). Given its membership in the Hyades Cluster, the host star’s characteristics are well constrained (0.29 Msol, [Fe/H] +0.15), but no data are available on the new planet’s mass.

Despite the cluster’s proximity and size (at least 200 star systems), EPIC 210490365b is only the third exoplanet ever detected in the Hyades. The first was a Super Jupiter of 7.5 Mjup orbiting the red giant Epsilon Tauri (the bull’s left eye) in a period of 595 days (Sato et al. 2007). The second was a Hot Jupiter of 0.92 Mjup orbiting HD 285507 (spectral type K4.5, 0.73 Msol) in a period of 6 days (Quinn et al. 2013). Two more giant planets – both Hot Jupiters – have been reported around Sun-like stars in another nearby cluster, Praesepe, which shares a common origin with the Hyades (Quinn et al. 2012). Finally, Kepler Mission data revealed two low-mass planets (Kepler-66b and -67b; Meibom et al. 2013) transiting G-type stars in NGC 6811, a star cluster aged about 1 billion years located 1107 parsecs away (3600 light years). With respective radii of 2.8 and 2.9 Rea, Kepler-66b and Kepler-67b appear typical of the large population of puffy Kepler planets with hydrogen/helium atmospheres and masses between 2 and 12 Mea. The new Hyades gas dwarf reported by Mann & colleagues has an even puffier profile, although it isn’t necessarily more massive. It’s the third low-mass planet detected in any star cluster and the first to be identified in the Hyades.

Figure 3. Bright stars within 15 parsecs of the center of the Hyades Cluster

The x-axis shows right ascension; the y-axis shows declination. The position of EPIC 210490365 is marked by a red six-pointed star at RA 04:13:06, Dec +15:14:52 Except for this M dwarf exoplanet host, only stars of type K or earlier appear in the diagram, color-coded by spectral classification. A halo around a star indicates multiplicity. The inner dot-dashed circle traces the cluster’s core radius (2.7 parsecs); the outer circle shows the tidal radius (10 parsecs). To date, three exoplanetary systems have been identified in the Hyades Cluster.
HD 219134 is still closer than Aldebaran and the Hyades, located just 21 light years (6.53 parsecs) away. With an enhanced metallicity of +0.11, this orange dwarf has long been considered a potential planetary host. Its spectral type of K3 (not too dim) and mass of 0.78 Msol (not too small) add to its appeal. This past year, two different groups (Motalebi et al., Vogt et al.) reported a fascinating system around this star, including at least three low-mass planets on short-period orbits and one cool gas giant orbiting outside 2 AU. Motalebi’s group even reported the detection of a single transit of the innermost planet (HD 219134 b), a likely Super Earth. If this transit is confirmed, the planet in question will join an exclusive club of small exoplanets with measured masses and radii. This system is discussed in more detail here and here.

Kepler-138 The club of small planets with measured masses inducted several new members over the past year, including three from the same system. Analyses of transit data on a faint M dwarf known as Kepler-138, located at an unspecified distance, enabled the characterization of three undeniably terrestrial planets with orbital periods between 10 and 23 days. The host star has an estimated mass of 0.52 Msol and metallicity of -0.28. Analyses of transit timing variations enabled Jontof-Hutter & colleagues to calculate masses for all three planets, revealing a startling mismatch in densities.

Table 2. Parameters of the Kepler-138 system
Masses & radii are shown in Earth units; semimajor axis (a) in astronomical units (AU); period in days; and equilibrium temperature (Teq) in Kelvin.
Kepler-138b is a rare example of a subterrestrial exoplanet, almost identical to Mars in radius but even less massive. Despite its much larger radius, Kepler-138d is another subterrestrial; it is less massive than Venus. Both of these objects must include a volatile component (presumably water) to explain their unexpectedly large radii. Only Kepler-138c is a bona fide Super Earth, with an estimated mass that implies an Earth-like composition. Of course, all three planets are much hotter than Earth.

Kepler-444 Yet another remarkable system of small planets was announced at the beginning of 2015 (Campante et al.). Instead of its unprecedented architecture, though, the headlines emphasized the system’s advanced age: at an estimated 11.2 billion years, the host star is just a little younger than the Milky Way itself. As expected of such an ancient star, Kepler-444 is poor in metals, with [Fe/H] at -0.55. Nevertheless, it harbors five tightly packed, slow-roasted planets with a likely aggregate mass of 1.5 Mea – rather less than the total mass of the Solar System’s four terrestrial planets. All these objects are “Martians,” with radii larger than Mercury but smaller than Venus, and all are expected to be purely rocky and blazing hot, lacking volatiles. Even such dense objects are still too lightweight to be detected by radial velocity surveys, despite the brightness and proximity of the parent star. In fact, Kepler-444 is one of the closest stars in the entire Kepler sample, located just 35.7 parsecs away (116 light years). Its respectable mass of o.76 Msol and spectral type of K0 have enabled unusually precise estimates of system parameters, as summarized in Table 3.
Table 3. Parameters of the Kepler-444 system
Radii are shown in Earth units; semimajor axis (a) in astronomical units (AU); period in days; and equilibrium temperature (Teq) in Kelvin.

Campante & colleagues noted that all adjacent planet pairs in this system orbit just outside first-order mean motion resonances, suggesting that they migrated into their present configuration in the presence of a protoplanetary nebula. The orbits are also almost perfectly co-planar, consistent with dynamic stability over the lifetime of the Galaxy.

Adding to the intricacy of this celestial clockwork is the fact that Kepler-444 itself is the primary or “A” component of a hierarchical triple star system. Its companions, Kepler-444B and C, are M dwarfs with respective masses of 0.29 and 0.25 Msol. They are locked in a tight binary orbit with a semimajor axis of 0.3 AU or less. Currently, the binary is separated from Kepler-444A by a projected distance of about 66 AU. A new study by Dupuy & colleagues reports astrometric and radial velocity data on all three stars, arguing that the binary is engaged in a highly eccentric orbit around the system’s center of mass, sharing a semimajor axis of 36.7 AU and an eccentricity of 0.864 with Kepler-444A. These values imply that the BC binary and the A star have a minimum separation of 5 AU at each periastron passage during their shared orbital period of about 200 years. Such an orbit would seem to compromise the long-term survival of the planetary system, but Dupuy & colleagues argue that both the planetary and stellar orbits are co-planar, promoting dynamical stability over the age of the Milky Way. 

WASP-47 has been recognized as the host of an “entirely typical” Hot Jupiter since 2012. Located at a distance of about 200 parsecs, it’s a G-type star of 1.04 Msol with substantial enrichment in metals, given its [M/H] of +0.36. New data from two different sources – the spaceborne K2 Mission and the ground-based CORALIE program – now portray a unique mixed-mass system (Becker et al. 2015) that is already the focus of theoretical efforts to explain its singular architecture. Two short-period, low-mass planets were announced this summer, one inside and the other outside the orbit of the known Hot Jupiter (WASP-47b). Shortly afterward, a second gas giant (WASP-47c) was reported at a semimajor axis of 1.36 AU (Neveu-VanMalle et al. 2015). 

Figure 4. The three inner planets of WASP-47
The three transiting planets of WASP-47 are shown at their relative sizes. The number inside each sphere refers to the planet’s estimated mass in Earth units (Mea). The value for planet e is a guesstimate within narrow limits; the other two are supported by radial velocity and transit timing data. A second gas giant planet (WASP-47c) is also present on a wider orbit outside the scale of this diagram.
Still more recently, two theoretical studies that appeared as preprints offer findings relevant to the system’s evolution. The first, by Batygin & colleagues, proposes that the three inner planets of WASP-47 formed in situ, although it’s not clear how one of them ballooned into a giant while the other two stayed small. The second, by Hand & Alexander, reports the results of a suite of simulations of migration scenarios for systems of compact short-period planets. In their simulations, whole families of small planets assemble outside system snow lines and then migrate inward to warmer orbits. If the outermost or penultimate planet accretes enough mass during this process to grow into a gas giant (the growth mechanism is unclear), it will sculpt the orbits of the smaller inner planets in various ways. In extreme cases, the low-mass planets are ejected from the system or driven into the star. In more peaceful iterations, the gas giant squeezes the inner planets into extremely tight orbits, as we see in Kepler-90. Hand & Alexander also suggest that some compact low-mass systems harbor invisible giants on wider orbits (a notion conveyed in their title, "There Might Be Giants"). Their celebrity candidate for such a configuration is Kepler-11, but a similar scenario could also be applied to WASP-47, where the outer giant is actually visible.

Two Baby Giants from HATS Low-mass planets like Earth and Uranus have bulk compositions dominated by heavy elements, whereas gas giant planets like Saturn and Jupiter consist primarily of hydrogen. But the boundary between the two species is imprecise. Remarkably, a program other than Kepler or K2 recently contributed critical new data on this issue. The Hungarian Automated Telescope Network South, better known as HATS, reported two transiting planets, HATS-7b and HATS-8b, that appear transitional between the dwarfs and the giants. Although the two planets are similar in mass, they differ widely in radius and composition.

HATS-7b orbits a metallic K-type star in a period of 3.2 days (Bakos et al. 2015). Transit observations yield a radius of 6.31 Rea; radial velocity data provide a mass of 38.2 Mea. The bulk composition favored by the discovery team is about 82% rock/metal and 18% hydrogen/heliumbroadly similar to the composition of Uranus. They note that the minimum mass for a heavy element core that is consistent with the planet’s radius is 25 Mea – much higher than any purely rocky planet ever detected.

HATS-8b follows a similar orbit (3.5 days) around a metallic G-type star (Bayliss et al. 2015). At 43.9 Mea, this planet is less than half as massive as Saturn (95 Mea), but its puffy radius of 9.79 Rea makes it slightly larger. The discovery team suggests a hydrogen/helium envelope accounting for about 77% of its bulk composition. The corresponding heavy element core of about 10 Mea is predicted by current models of gas giant formation.

These two HATS join at least two previously reported objects that lie just at the threshold of the mass range typical of gas giant planets. They are Kepler-35b, which has a mass of 40 Mea, a radius of 8.16 Rea, and an orbital period of 131 days; and HAT-P-18b, which has a mass of 58 Mea, a radius of 10.6 Rea, and an orbital period of 5.5 days.

Theory of Small Planets

The sample of Mars- to Super Earth-size planets grew significantly in 2015. These objects consist entirely of heavy elements and are appropriately described as terrestrial. A selection appears in Figure 5. Although exoplanets in this mass range (0.05–10 Mea) are known mostly through transit observations, the full population of transiting planets is dominated by puffy gas dwarfs with hydrogen/helium envelopes and radii of 2–4 Rea. In contrast, as discussed in previous posts, most terrestrial planets in the Galaxy must be smaller than 2 Rea.

This population attracted considerable attention in the past year, especially the subset with masses measured by transit timing or radial velocity variations. Notable studies of small planets either published or in press in 2015 include Dawson, Chiang & Lee, A metallicity recipe for rocky planets; Dressing et al., The mass of Kepler-93b and the composition of terrestrial planets; Howe & Burrows, Evolutionary models of Super Earths and Mini Neptunes incorporating cooling and mass loss; Jontof-Hutter, Ford, et al.,  Robust TTV mass measurements: Ten Kepler exoplanets between 3 and 8 Mea; Owen & Wu, Atmospheres of low-mass planets: The Boil-Off; Rogers, Most 1.6 Earth-radius exoplanets are not rocky; and Unterborn et al., Scaling the Earth. Bibliographic details appear in the reference section below.

Figure 5. Terrestrial planets near and far with measured masses and radii
These imaginative renderings of eight well-studied extrasolar planets appear alongside photographs of Mercury, Venus, and Earth. All are represented at their relative sizes, with masses (Mea) and radii (Rea) indicated in Earth units. 55 Cancri e is assigned the new radius reported by Demory et al. 2015, who speculate that this Hellworld experiences frequent volcanic eruptions. Three of the exoplanets shown here (55 Cancri e, Kepler-10b, Kepler-78b) complete a single orbit in less than 24 hours.

Twenty Years of Hot Jupiters

2015 was the 20th anniversary of the discovery of the first Hot Jupiter, 51 Pegasi b. A Hot Jupiter is a gas giant planet that orbits its parent star in a period shorter than 10 days. Before 51 Pegasi b was announced, few people even dreamed that such planets were possible. Now about 300 are known.

Theorists have been trying to explain these unexpected objects ever since the initial detections. Until recently, all theories assumed that they formed outside their system ice lines (at about 2 AU or so) and were then transported somehow to their present hot orbits. Proposed mechanisms included 1) migration by interaction with the primordial gas nebula (Type II migration); 2) migration by gravitational interactions with another gas giant in the same system, either during a brief period of dynamic instability or by long-term perturbations through so-called Kozai cycles; and 3) migration by gravitational interactions with a misaligned stellar companion of the host star in another type of Kozai regime.

The first mechanism has generally been the most popular, in part because it doesn’t depend on assuming the past or present existence of a potentially undetected (and possibly undetectable) companion planet or star. If Type II migration is indeed the dominant delivery mechanism for Hot Jupiters, then we would expect all Hot Jupiters that took this pathway to follow orbits that are well-aligned with the spin axis of their parent stars.

However, orbital misalignment appears to be quite common among transiting Hot Jupiters (the only subset in which this phenomenon can be observed). Accordingly, a team of astronomers launched the “Friends of Hot Jupiter” study to test whether one of the other two mechanisms might be responsible for most of this population. They selected a sample of 50 Hot Jupiters in two subgroups: 23 with well-aligned orbits and 27 with misaligned orbits.

The results of this impressive program have now been published in three successive articles. The first, by Knutson et al., appeared in 2014. This installment returned the remarkable finding that half of the systems in each group revealed radial velocity evidence for a second gas giant on a wider orbit. Some of these were already known, but almost half were newly determined through radial velocity trends. Nevertheless, analyses found no statistically significant association between evidence for an outer giant and the alignment of the known Hot Jupiter.

The second and third installments appeared in 2015; both involved searches for companion stars in Hot Jupiter systems (Ngo et al., Piskorz et al.) Although about half the host stars in the sample showed evidence of a binary companion, the presence or absence of such a companion had no association with the orbital alignment of the Hot Jupiter. The authors also noted that Hot Jupiters appear to reside preferentially in binary systems, despite the indifference of alignment to binarity.

If nothing else, the Friends of Hot Jupiters seem to have falsified the claim that stellar Kozai cycles are the primary cause of observed misalignments in transiting giants. Where exactly Hot Jupiters form and how exactly they got where they are now, however, remain open questions.

Happy New Year!

Figure 6. The sample of Hot Jupiters studied by Sing & colleagues
This pretty array of Hot Jupiters was published at the start of the holiday season, calling to mind a set of ornaments for a cosmic Christmas tree. The image illustrates a study by Sing and colleagues, who conducted a spectroscopic search for water in Hot Jupiter atmospheres.


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