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The detection of carbon monoxide absorption in the spectrum of the extrasolar planet τ Boötis b, and its tracing of the change in the radial velocity of the planet, demonstrates that atmospheric characterization is possible for non-transiting planets. Carbon monoxide on exoplanet τ Boötis b For more than a decade, the giant exoplanet orbiting τ Boötis has been closely observed. Its orbital inclination has been estimated on several occasions but with conflicting results. Now high-resolution infrared spectroscopy measurements from the Very Large Telescope array at the European Southern Observatory in Chile have been used to detect carbon monoxide in the thermal day-side atmosphere of the planet τ Boötis b while it was non-transiting. Previously a planet has had to be in transit across its host star for such observations to be made. From the spectral signature, the authors calculate an orbital inclination of about 44.5 degrees, and mass of about 5.95 times that of Jupiter. This new ground-based high-resolution spectroscopy technique should be generally applicable to the observation of atmospheres on other exoplanets. The giant planet orbiting τ Boötis (named τ Boötis b) was amongst the first extrasolar planets to be discovered 1 . It is one of the brightest exoplanets and one of the nearest to us, with an orbital period of just a few days. Over the course of more than a decade, measurements of its orbital inclination have been announced 2 and refuted 3 , and have hitherto remained elusive 4 , 5 , 6 , 7 , 8 . Here we report the detection of carbon monoxide absorption in the thermal dayside spectrum of τ Boötis b. At a spectral resolution of ∼100,000, we trace the change in the radial velocity of the planet over a large range in phase, determining an orbital inclination of 44.5° ± 1.5° and a mass 5.95 ± 0.28 times that of Jupiter, demonstrating that atmospheric characterization is possible for non-transiting planets. The strong absorption signal points to an atmosphere with a temperature that is decreasing towards higher altitudes, in contrast to the temperature inversion inferred for other highly irradiated planets 9 , 10 . This supports the hypothesis that the absorbing compounds believed to cause such atmospheric inversions are destroyed in τ Boötis b by the ultraviolet emission from the active host star 11 .

NRC publication: Yes

Whereas large planets, such as gas giants, are more likely to form around high-metallicity stars, terrestrial-sized planets are found to form around stars with a wide range of metallicities, indicating that they may be widespread in the disk of the Galaxy. Exoplanets around metal-poor stars A key discovery of the past decade in the field of exoplanet research was the realization that stars of high metallicity are those most likely to harbour giant exoplanets, supporting the model in which planets form by the accumulation of dust and ice particles. Whether the planet–metallicity correlation holds for terrestrial planets remained unclear, but the Kepler mission's discovery last year of hundreds of small exoplanet candidates provided an opportunity to find out. The spectroscopic metallicities of the host stars of 226 small exoplanet candidates have now been determined. The smaller ones, of less than four Earth radii, were found around stars with a wide range of metallicities, on average close to that of the Sun. Larger planets were more common around stars of high metallicity. These findings suggest that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation. The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a ‘fossil’ record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets 1 , 2 , 3 , 4 , supporting the model that planets form by accumulation of dust and ice particles 5 . Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets 4 , 6 , 7 , 8 , 9 . However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA’s Kepler mission 10 , including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.

In the solar system, the planets' compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal and that planets' orbits can change substantially after their formation. Here, we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10% and densities differing by a factor of 8. One planet is likely a rocky \"super-Earth,\" whereas the other is more akin to Neptune. These planets are 20 times more closely spaced and have a larger density contrast than any adjacent pair of planets in the solar system.

An exoplanet's mass Most of the known exoplanets were discovered using the radial velocity method, measuring the 'wobble' induced in the host stars by their orbiting companions. If the orbital velocity of the planet can also be determined, it becomes possible to calculate the masses of both the star and its exoplanet without the need for further assumptions or model dependencies. That has now been achieved for the well-studied 'hot Jupiter' HD 209458b, based on spectroscopic measurements of the changing Doppler shift of molecular absorption lines of carbon monoxide, observed as the planet passed between its host star and the Earth. The masses of the star and planet are 1.00±0.22 solar masses and 0.64±0.09 jovian masses respectively. Also revealed — as blueshift of the carbon monoxide signal with respect to host star velocity — a strong wind flowing at high altitude from the irradiated dayside to the non-irradiated nightside of the planet. If the orbital velocity of an extrasolar planet could be determined, the masses of both the planet and its host star could be calculated using Newton's law of gravity. Here, high-dispersion ground-based spectroscopy of a transit of the extrasolar planet HD 209458b is reported. This allowed the radial component of the planet's orbital velocity to be calculated, and thus the masses of star and planet. Moreover, a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet is suggested. For extrasolar planets discovered using the radial velocity method 1 , the spectral characterization of the host star leads to a mass estimate of the star and subsequently of the orbiting planet. If the orbital velocity of the planet could be determined, the masses of both star and planet could be calculated using Newton’s law of gravity, just as in the case of stellar double-line eclipsing binaries. Here we report high-dispersion ground-based spectroscopy of a transit of the extrasolar planet HD 209458b. We see a significant wavelength shift in absorption lines from carbon monoxide in the planet’s atmosphere, which we conclude arises from a change in the radial component of the planet’s orbital velocity. The masses of the star and planet are 1.00 ± 0.22 M Sun and 0.64 ± 0.09 M Jup respectively. A blueshift of the carbon monoxide signal of approximately 2 km s −1 with respect to the systemic velocity of the host star suggests the presence of a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet within the 0.01–0.1 mbar atmospheric pressure range probed by these observations. The strength of the carbon monoxide signal suggests a carbon monoxide mixing ratio of (1–3) × 10 −3 in this planet’s upper atmosphere.

Two double-sun exoplanets have been discovered by the Kepler spacecraft, establishing a new class of ‘circumbinary’ exoplanets and suggesting that at least several million such systems exist in our Galaxy. Dual 'Suns' a common phenomenon The Kepler spacecraft's haul of newly discovered extrasolar planets continues to grow. Most Sun-like stars in the Milky Way are found in gravitationally bound pairs or binaries. The discovery of exoplanet Kepler-16 b showed that planets can exist in orbits around a binary, and now two further such 'circumbinary' planets have been found: Kepler-34 b and Kepler-35 b. Each is a low-density gas giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 b orbits two Sun-like stars every 289 days, and Kepler-35 b orbits a pair of smaller stars every 131 days. The observed rate of circumbinary planets implies that 1% of close binary stars have giant planets in nearly coplanar orbits, equivalent to a population of at least several million such bodies in the Milky Way. Most Sun-like stars in the Galaxy reside in gravitationally bound pairs of stars 1 , 2 (binaries). Although long anticipated 3 , 4 , 5 , 6 , 7 , 8 , the existence of a ‘circumbinary planet’ orbiting such a pair of normal stars was not definitively established until the discovery 9 of the planet transiting (that is, passing in front of) Kepler-16. Questions remained, however, about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we report two additional transiting circumbinary planets: Kepler-34 (AB)b and Kepler-35 (AB)b, referred to here as Kepler-34 b and Kepler-35 b, respectively. Each is a low-density gas-giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 b orbits two Sun-like stars every 289 days, whereas Kepler-35 b orbits a pair of smaller stars (89% and 81% of the Sun’s mass) every 131 days. The planets experience large multi-periodic variations in incident stellar radiation arising from the orbital motion of the stars. The observed rate of circumbinary planets in our sample implies that more than ∼1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.

When an extrasolar planet passes in front of (transits) its star, its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal much more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star, which we call Kepler-11, that reveal six transiting planets, five with orbital periods between 10 and 47 days and a sixth planet with a longer period. The five inner planets are among the smallest for which mass and size have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation. Edge-on view of Kepler-11 planetary system NASA's Kepler mission, a space observatory designed to detect and study extrasolar planets that transit across the disk of their host star, has hit the jackpot with the discovery of a six-planet system orbiting a Sun-like star now named Kepler-11. Five of the planets have orbital periods of between 10 and 47 days, and these are among the smallest for which size and mass have both been measured. The sixth and outermost transiting planet has been less well characterized thus far. Only one other star has more than one confirmed transiting planet (Kepler-9, which has three). This newly discovered system resembles our own Solar System in being close to coplanar, but Kepler-11's planets orbit much closer to their star. Kepler is due to continue to return data on Kepler-11 and its planets for some time yet, and it should provide many valuable constraints on models of the formation and evolution of solar systems in general. When an extrasolar planet passes in front of its star (transits), its radius can be measured from the decrease in starlight and its orbital period from the time between transits. This study reports Kepler spacecraft observations of a single Sun-like star that reveal six transiting planets, five with orbital periods between 10 and 47 days plus a sixth one with a longer period. The five inner planets are among the smallest for which mass and size have both been measured, and these measurements imply substantial envelopes of light gases.

A statistical analysis of microlensing data from 2002–07 reveals that stars in the Milky Way are orbited by planets as a rule, rather than an exception. Planets common in the Milky Way Most of the extrasolar planets known so far were discovered using methods biased towards planets that are relatively close to their parent stars, and in this population about 17–30% of solar-like stars host a planet. A rather different picture emerges from an analysis of gravitational microlensing data collected between 2002 and 2007. This method probes planets that are farther away from their stars. The data reveal that it is the rule, rather than the exception, for stars in our Galaxy to host one planet or more. 'Super-Earths' are the most abundant type, being associated with around 62% of stars; 52% host cool Neptune-like planets; and 17% host 'Jupiters'. Most known extrasolar planets (exoplanets) have been discovered using the radial velocity 1 , 2 or transit 3 methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% (refs 4 , 5 ) of solar-like stars host a planet. Gravitational microlensing 6 , 7 , 8 , 9 , on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing 10 . These planets are at least as numerous as the stars in the Milky Way 10 . Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10  au (Sun–Earth distance) from their stars. We find that of stars host Jupiter-mass planets (0.3–10  M J , where M J = 318  M ⊕ and M ⊕ is Earth’s mass). Cool Neptunes (10–30  M ⊕ ) and super-Earths (5–10  M ⊕ ) are even more common: their respective abundances per star are and . We conclude that stars are orbited by planets as a rule, rather than the exception.

The questions of how planets form and how common Earth-like planets are can be addressed by measuring the distribution of exoplanet masses and orbital periods. We report the occurrence rate of close-in planets (with orbital periods less than 50 days), based on precise Doppler measurements of 166 Sun-like stars. We measured increasing planet occurrence with decreasing planet mass (M). Extrapolation of a power-law mass distribution fitted to our measurements, df/dlogM = 0.39 M⁻⁰.⁴⁸, predicts that 23% of stars harbor a close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses). Theoretical models of planet formation predict a deficit of planets in the domain from 5 to 30 Earth masses and with orbital periods less than 50 days. This region of parameter space is in fact well populated, implying that such models need substantial revision.

Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step toward imaging Earth-like planets. Imaging detections are challenging because of the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our solar system.