Ruth A. Murray-Clay

A super-Earth transiting a nearby low-mass star

A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9–10 times the Earth’s mass (M⊕), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8, 9), but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55M⊕ and a radius 2.68 times Earth’s radius (R⊕), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen–helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.

Atmospheric Escape from Hot Jupiters

Photoionization heating from ultraviolet (UV) radiation incidents on the atmospheres of hot Jupiters may drive planetary mass loss. Observations of stellar Lyman-α (Lyα) absorption have suggested that the hot Jupiter HD 209458b is losing atomic hydrogen. We construct a model of escape that includes realistic heating and cooling, ionization balance, tidal gravity, and pressure confinement by the host star wind. We show that mass loss takes the form of a hydrodynamic (Parker) wind, emitted from the planets dayside during lulls in the stellar wind. When dayside winds are suppressed by the confining action of the stellar wind, nightside winds might pick up if there is sufficient horizontal transport of heat. A hot Jupiter loses mass at maximum rates of ~2 × 1012 g s–1 during its host stars pre-main-sequence phase and ~2 × 1010 g s–1 during the stars main-sequence lifetime, for total maximum losses of ~0.06% and ~0.6% of the planets mass, respectively. For UV fluxes F UV 104 erg cm–2 s–1, the mass-loss rate is approximately energy limited and scales as . For larger UV fluxes, such as those typical of T Tauri stars, radiative losses and plasma recombination force to increase more slowly as F 0.6 UV. Dayside winds are quenched during the T Tauri phase because of confinement by overwhelming stellar wind pressure. During this early stage, nightside winds can still blow if the planet resides outside the stellar Alfven radius; otherwise, even nightside winds are stifled by stellar magnetic pressure, and mass loss is restricted to polar regions. We conclude that while UV radiation can indeed drive winds from hot Jupiters, such winds cannot significantly alter planetary masses during any evolutionary stage. They can, however, produce observable signatures. Candidates for explaining why the Lyman-α photons of HD 209458 are absorbed at Doppler-shifted velocities of ±100 km s–1 include charge-exchange in the shock between the planetary and stellar winds.

The effects of snowlines on C/O in planetary atmospheres

The C/O ratio is predicted to regulate the atmospheric chemistry in hot Jupiters. Recent observations suggest that some exoplanets, e.g., Wasp 12-b, have atmospheric C/O ratios substantially different from the solar value of 0.54. In this Letter, we present a mechanism that can produce such atmospheric deviations from the stellar C/O ratio. In protoplanetary disks, different snowlines of oxygen- and carbon-rich ices, especially water and carbon monoxide, will result in systematic variations in the C/O ratio both in the gas and in the condensed phases. In particular, between the H2O and CO snowlines most oxygen is present in icy grains—the building blocks of planetary cores in the core accretion model—while most carbon remains in the gas phase. This region is coincidental with the giant-planet-forming zone for a range of observed protoplanetary disks. Based on standard core accretion models of planet formation, gas giants that sweep up most of their atmospheres from disk gas outside of the water snowline will have a C/O ~ 1, while atmospheres significantly contaminated by evaporating planetesimals will have a stellar or substellar C/O when formed at the same disk radius. The overall metallicity will also depend on the atmosphere formation mechanism, and exoplanetary atmospheric compositions may therefore provide constraints on where and how a specific planet formed.

Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager

An exoplanet extracted from the bright Direct imaging of Jupiter-like exoplanets around young stars provides a glimpse into how our solar system formed. The brightness of young stars requires the use of next-generation devices such as the Gemini Planet Imager (GPI). Using the GPI, Macintosh et al. discovered a Jupiter-like planet orbiting a young star, 51 Eridani (see the Perspective by Mawet). The planet, 51 Eri b, has a methane signature and is probably the smallest exoplanet that has been directly imaged. These findings open the door to understanding solar system origins and herald the dawn of a new era in next-generation planetary imaging. Science, this issue p. 64; see also p. 39 The Gemini Planet Imager detects a Jupiter-like exoplanet orbiting the young star 51 Eridani. [Also see Perspective by Mawet] Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 × 10−6 and an effective temperature of 600 to 750 kelvin. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core-accretion process that may have formed Jupiter.

The Runts of the Litter: Why planets formed through gravitational instability can only be failed binary stars

Recent direct imaging discoveries suggest a new class of massive, distant planets around A stars. These widely separated giants have been interpreted as signs of planet formation driven by gravitational instability, but the viability of this mechanism is not clear cut. In this paper, we first discuss the local requirements for fragmentation and the initial fragment mass scales. We then consider whether the fragments subsequent growth can be terminated within the planetary mass regime. Finally, we place disks in the larger context of star formation and disk evolution models. We find that in order for gravitational instability to produce planets, disks must be atypically cold in order to reduce the initial fragment mass. In addition, fragmentation must occur during a narrow window of disk evolution, after infall has mostly ceased, but while the disk is still sufficiently massive to undergo gravitational instability. Under more typical conditions, disk-born objects will likely grow well above the deuterium burning planetary mass limit. We conclude that if planets are formed by gravitational instability, they must be the low mass tail of the distribution of disk-born companions. To validate this theory, on-going direct imaging surveys must find a greater abundance of brown dwarf and M-star companions to A-stars. Their absence would suggest planet formation by a different mechanism such as core accretion, which is consistent with the debris disks detected in these systems.

GIANT PLANETS ORBITING METAL-RICH STARS SHOW SIGNATURES OF PLANET-PLANET INTERACTIONS

Gas giants orbiting interior to the ice line are thought to have been displaced from their formation locations by processes that remain debated. Here we uncover several new metallicity trends, which together may indicate that two competing mechanisms deliver close-in giant planets: gentle disk migration, operating in environments with a range of metallicities, and violent planet-planet gravitational interactions, primarily triggered in metal-rich systems in which multiple giant planets can form. First, we show with 99.1% confidence that giant planets with semimajor axes between 0.1 and 1 AU orbiting metal-poor stars ([Fe/H] < 0) are confined to lower eccentricities than those orbiting metal-rich stars. Second, we show with 93.3% confidence that eccentric proto-hot Jupiters undergoing tidal circularization primarily orbit metal-rich stars. Finally, we show that only metal-rich stars host a pile-up of hot Jupiters, helping account for the lack of such a pile-up in the overall Kepler sample. Migration caused by stellar perturbers (e.g., stellar Kozai) is unlikely to account for the trends. These trends further motivate follow-up theoretical work addressing which hot Jupiter migration theories can also produce the observed population of eccentric giant planets between 0.1 and 1 AU.

Stability of the Directly Imaged Multiplanet System HR 8799: Resonance and Masses

A new era of directly imaged extrasolar planets has produced a three-planet system, where the masses of the planets have been estimated by untested cooling models. We point out that the nominal circular, face-on orbits of the planets lead to a dynamical instability in ~105 yr, a factor of at least 100 shorter than the estimated age of the star. Reduced planetary masses produce stability only for unreasonably small planets (2 M Jup). Relaxing the face-on assumption, but still requiring circular orbits while fitting the observed positions, makes the instability time even shorter. A promising solution is that the inner two planets have a 2:1 commensurability between their periods, and they avoid close encounters with each other through this resonance. The fact that the inner resonance has lasted until now, in spite of the perturbations of the outer planet, leads to a limit 10 M Jup on the masses unless the outer two planets are also engaged in a 2:1 mean-motion resonance. In a double resonance, which is consistent with the current data, the system could survive until now even if the planets have masses of ~20 M Jup. Apsidal alignment can further enhance the stability of a mean-motion resonant system. A completely different dynamical configuration, with large eccentricities and large mutual inclinations among the planets, is possible but finely tuned.

The circumbinary ring of KH 15D

The light curves of the pre-main-sequence star KH 15D from the years 1913-2003 can be understood if the star is a member of an eccentric binary that is encircled by a vertically thin, inclined ring of dusty gas. Eclipses occur whenever the reflex motion of a star carries it behind the circumbinary ring; the eclipses occur with a period equal to the binary orbital period of 48.4 days. Features of the light curve, including the amplitude of central reversals during mid-eclipse, the phase of the eclipse with respect to the binary orbit phase, the level of brightness out of eclipse, the depth of the eclipse, and the eclipse duty cycle, are all modulated on the timescale of nodal regression of the obscuring ring, in accord with the historical data. The ring has a mean radius near 3 AU and a radial width that is likely less than this value. While the inner boundary could be shepherded by the central binary, the outer boundary may require an exterior planet to confine it against viscous spreading. The ring must be vertically warped to maintain a nonzero inclination. Thermal pressure gradients and/or ring self-gravity can readily enforce rigid precession. In coming years, as the node of the ring regresses out of our line of sight toward the binary, the light curve from the system should cycle approximately back through its previous behavior. Near-term observations should seek to detect a mid-infrared excess from this system; we estimate the flux densities from the ring to be 3 mJy at wavelengths of 10-100 μm.

A Signature of Planetary Migration: The Origin of Asymmetric Capture in the 2:1 Resonance

The spatial distribution of Kuiper Belt objects (KBOs) in 2?:?1 exterior resonance with Neptune constrains that planets migration history. Numerical simulations demonstrate that fast planetary migration generates a larger population of KBOs trailing rather than leading Neptune in orbital longitude. This asymmetry corresponds to a greater proportion of objects caught into asymmetric resonance such that their resonance angles librate about values greater than ? (trailing) as opposed to less than ? (leading). We provide, for the first time, an explanation of this phenomenon, using physical, analytic, and semianalytic arguments. Central to our understanding is how planetary migration shifts the equilibrium points of the superposed direct and indirect potentials. Symmetric libration, in which librates about ~?, precedes capture into asymmetric resonance. As a particle transitions from symmetric to asymmetric libration, if exceeds the value ? at the unstable point of asymmetric resonance, then the particle is caught into trailing resonance, while if ? while in symmetric libration. This fractional time increases with faster migration because migration not only shifts ? to values less than ? but also shifts the stable point of symmetric libration to values greater than ?. Smaller eccentricities prior to capture strengthen the effect of these shifts. Large capture asymmetries appear for exponential timescales of migration ? shorter than ~107 yr. The observed distribution of 2?:?1 KBOs (two trailing and seven leading) excludes ? ? 106 yr with 99.65% confidence.

Fragment Production and Survival in Irradiated Disks: A Comprehensive Cooling Criterion

Accretion disks that become gravitationally unstable can fragment into stellar or substellar companions. The formation and survival of these fragments depends on the precarious balance between self-gravity, internal pressure, tidal shearing, and rotation. Disk fragmentation depends on two key factors: (1) whether the disk can get to the fragmentation boundary of Q = 1 and (2) whether fragments can survive for many orbital periods. Previous work suggests that to reach Q = 1, and have fragments survive, a disk must cool on an orbital timescale. Here we show that disks heated primarily by external irradiation always satisfy the standard cooling time criterion. Thus, even though irradiation heats disks and makes them more stable in general, once they reach the fragmentation boundary, they fragment more easily. We derive a new cooling criterion that determines fragment survival and calculate a pressure-modified Hill radius, which sets the maximum size of pressure-supported objects in a Keplerian disk. We conclude that fragmentation in protostellar disks might occur at slightly smaller radii than previously thought and recommend tests for future simulations that will better predict the outcome of fragmentation in real disks.