* Astronomy

The Trans-Atlantic Exoplanet Survey (TrES) has announced the discovery of the first known transiting planet in Kepler field, TrES-2.
TrES-2 was identified from observations made with the 10-cm telescopes, Sleuth and PSST, pictured above. When the brightness of the star TrES-2 was monitored over several months, numerous 1.4% transits of the star were detected. The light curve derived from these TrES observations is shown below (click for a larger version). Also shown are the light curves from more recent observations of TrES-2, made at various optical wavelengths. These follow-up observations show a consistent transit depth, independent of the wavelength. (A wavelength-dependent depth would indicate that the eclipsing system was in fact an multiple-star system, not a planet orbiting a star.)

Read more  

Title: Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES-2
Authors: A. Sozzetti (1,2), G. Torres (1), D. Charbonneau (1), D.W. Latham (1), M.J. Holman (1), J.N. Winn (3), J.B. Laird (4), F.T. O'Donovan (5) ((1) Harvard-Smithsonian Center for Astrophysics, (2) Osservatorio Astronomico di Torino, (3) Massachusetts Institute of Technology, (4) Bowling Green State University, (5) California Institute of Technology)

We report on a spectroscopic determination of the atmospheric parameters and chemical abundance of the parent star of the recently discovered transiting planet TrES-2. A detailed LTE analysis of a set of Fe1 and Fe2 lines from our Keck spectra yields T_{eff} = 5850 ±50 K, \log g = 4.4 ±0.1, and (Fe/H) = -0.15±0.10. Several independent checks (e.g., additional spectroscopy, line-depth ratios) confirm the reliability of our spectroscopic T_{eff} estimate. The mass and radius of the star, needed to determine the properties of the planet, are traditionally inferred by comparison with stellar evolution models using T_{eff} and some measure of the stellar luminosity, such as the spectroscopic surface gravity (when a trigonometric parallax is unavailable, as in this case). We apply here a new method in which we use instead of \log g the normalized separation a/R_\star (related to the stellar density), which can be determined directly from the light curves of transiting planets with much greater precision. With the a/R_\star value from the light curve analysis of Holman et al. and our T_{eff} estimate we obtain M_\star = 0.980 ±0.062 M_\odot and R_\star = 1.000_{-0.033}^{+0.036} R_\odot, and an evolutionary age of 5.1^+2.7_{-2.3} Gyr, in good agreement with other constraints based on the strength of the emission in the Ca2 H & K line cores, the Lithium abundance, and rotation. The new stellar parameters yield improved values for the planetary mass and radius of M_p = 1.198 ±0.053 M_{Jup} and R_p = 1.220^{+0.045}_{-0.042} R_{Jup}, confirming that TrES-2 is the most massive among the currently known nearby (d\lesssim 300 pc) transiting hot Jupiters.

Read more  (112kb, PDF)

Title: The HARPS search for southern extra-solar planets. IX. Exoplanets orbiting HD 100777, HD 190647, and HD 221287
Authors: Dominique Naef, Michel Mayor, Willy Benz, Francois Bouchy, Gaspare Lo Curto, Christophe Lovis, Claire Moutou, Francesco Pepe, Didier Queloz, Nuno C. Santos, Stephane Udry
(version, v2))

The HARPS high-resolution high-accuracy spectrograph is offered to the astronomical community since the second half of 2003. Since then, we have been using this instrument for monitoring radial velocities of a large sample of Solar-type stars (~1400 stars) in order to search for their possible low-mass companions. Amongst the goals of our survey, one is to significantly increase the number of detected extra-solar planets in a volume-limited sample to improve our knowledge of their orbital elements distributions and thus obtain better constraints for planet-formation models.
In this paper, we present the HARPS radial-velocity data and orbital solutions for 3 Solar-type stars: HD 100777, HD 190647, and HD 221287. The radial-velocity data of HD 100777 is best explained by the presence of a 1.1 M_Jup planetary companion on a 384--day eccentric orbit (e=0.36). The orbital fit obtained for the slightly evolved star HD 190647 reveals the presence of a long-period (P=1038 d) 1.9 M_Jup planetary companion on a moderately eccentric orbit (e=0.18). HD 221287 is hosting a 3.1 M_Jup planet on a 456--day orbit. The shape of this orbit is not very well constrained because of our non-optimal temporal coverage and because of the presence of abnormally large residuals. We find clues for these large residuals to result from spectral line profile variations probably induced by stellar activity related processes.

Read more (158kb, PDF)

Astronomers at the Harvard-Smithsonian Centre for Astrophysics (CfA) announced that they have found the most massive known transiting extrasolar planet. The gas giant planet, called HAT-P-2b, contains more than eight times the mass of Jupiter, the biggest planet in our solar system. Its powerful gravity squashes it into a ball only slightly larger than Jupiter.
HAT-P-2b shows other unusual characteristics. It has an extremely oval orbit that brings it as close as 3.1 million miles from its star before swinging three times farther out, to a distance of 9.6 million miles. If Earth's orbit were as elliptical, we would loop from almost reaching Mercury out to almost reaching Mars. Because of its orbit, HAT-P-2b gets enormously heated up when it passes close to the star, then cools off as it loops out again. Although it has a very short orbital period of only 5.63 days, this is the longest period planet known that transits, or crosses in front of, its host star.

"This planet is so unusual that at first we thought it was a false alarm - something that appeared to be a planet but wasn't. But we eliminated every other possibility, so we knew we had a really weird planet" - CfA astronomer Gaspar Bakos.  

HAT-P-2b orbits an F-type star, which is almost twice as big and somewhat hotter than the Sun, located about 440 light-years away in the constellation Hercules. Once every 5 days and 15 hours, it crosses directly in front of the star as viewed from Earth-a sort of mini-eclipse. Such a transit offers astronomers a unique opportunity to measure a planet's physical size from the amount of dimming.
Brightness measurements during the transit show that HAT-P-2b is about 1.18 times the size of Jupiter. By measuring how the star wobbles as the planet's gravity tugs it, astronomers deduced that the planet contains about 8.2 times Jupiter's mass. A person who weighs 150 pounds on Earth would tip the scale at 2100 pounds, and experience 14 times Earth's gravity, by standing on the visible surface (cloud tops) of HAT-P-2b.

Read more

Title: HAT-P-2b: A Super-Massive Planet in an Eccentric Orbit Transiting a Bright Star
Authors: G. A. Bakos, G. Kovacs, G. Torres, D. A. Fischer, D. W. Latham, R. W. Noyes, D. D. Sasselov, T. Mazeh, A. Shporer, R. P. Butler, R. P. Stefanik, J. M. Fernandez, A. Sozzetti, A. Pal, J. Johnson, G. W. Marcy, B. Sipocz, J. Lazar, I. Papp, P. Sari

We report the discovery of HAT-P-2b, a massive (Mp=8.17+/-0.72 M_Jup) planet transiting the bright (V=8.7) F8 star HD 147506, with an orbital period of 5.63 days and an eccentricity of e=0.5. From the transit light curve we determine that the radius of the planet is Rp = 1.18+/-0.16 R_Jup. HAT-P-2b has a mass about 9 times the average mass of previously-known transiting exoplanets, and a density of rho = 6.6gcm^-3, similar to that of rocky planets like the Earth. Nevertheless, its mass and radius are in accord with theories of structure of massive giant planets composed of pure H and He. The high eccentricity causes a 9-fold variation of insolation of the planet between peri- and apastron.

Read more (255kb, PDF)

Title: Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits
Authors: Jonathan J. Fortney, Mark S. Marley, Jason W. Barnes
(version v3)

To aid in the physical interpretation of planetary radii constrained through observations of transiting planets, or eventually direct detections, we compute model radii of pure hydrogen-helium, water, rock, and iron planets, along with various mixtures. Masses ranging from 0.01 Earth masses to 10 Jupiter masses at orbital distances of 0.02 to 10 AU are considered. For hydrogen-helium rich planets, our models are the first to couple planetary evolution to stellar irradiation over a wide range of orbital separations (0.02 to 10 AU) through a non-grey radiative-convective equilibrium atmosphere model. Stellar irradiation retards the contraction of giant planets, but its effect is not a simple function of the irradiation level: a planet at 1 AU contracts as slowly as a planet at 0.1 AU. For hydrogen-helium planets, we consider cores up to 90% of the total planet mass, comparable to those of Uranus and Neptune. If "hot Neptunes" have maintained their original masses and are not remnants of more massive planets, radii of 0.30-0.45 times Jupiter's radius are expected. Water planets are ~40-50% larger than rocky planets, independent of mass. Finally, we provide tables of planetary radii at various ages and compositions, and for ice-rock-iron planets we fit our results to analytic functions, which will allow for quick composition estimates, given masses and radii, or mass estimates, given only planetary radii. These results will assist in the interpretation of observations for both the current transiting planet surveys as well as upcoming space missions, including CoRoT and Kepler.

Read more (98kb, PDF)