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TOPIC: Extrasolar Planets
Blobrana


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HD 209458b
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Title: The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b Authors: Gilda E. Ballester, David K. Sing and Floyd Herbert

About ten per cent of the known extrasolar planets are gas giants that orbit very close to their parent stars. The atmospheres of these 'hot Jupiters' are heated by the immense stellar irradiation1. In the case of the planet HD 209458b, this energy deposition results in a hydrodynamic state in the upper atmosphere, allowing for sizeable expansion and escape of neutral hydrogen gas. HD 209458b was the first extrasolar planet discovered that transits in front of its parent star. The size of the planet can be measured using the total optical obscuration of the stellar disk during an observed transit, and the structure and composition of the planetary atmosphere can be studied using additional planetary absorption signatures in the stellar spectrum. Here we report the detection of absorption by hot hydrogen in the atmosphere of HD 209458b. Previously, the lower atmosphere and the full extended upper atmosphere of HD 209458b have been observed, whereas here we probe a layer where the escaping gas forms in the upper atmosphere of HD 209458b.

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Blobrana


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Posts: 131433
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HD 290458b
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The powerful vision of NASA's Hubble Space Telescope has allowed astronomers to study for the first time the layer-cake structure of the atmosphere of a planet orbiting another star. Hubble discovered a dense upper layer of hot hydrogen gas where the super-hot planet's atmosphere is bleeding off into space.
This is an artist's illustration of the extrasolar planet, designated HD 209458b, which is unlike any world in our solar system. It completes an orbit around its host star every 3.5 days. It is about the size of Jupiter. Unlike Jupiter, HD 290458b is so hot that its atmosphere is "puffed up." Starlight is heating the planet's atmosphere, causing hot gas to escape into space. Astronomers used Hubble to analyse the starlight that filtered through the planet's atmosphere. Imprinted on the starlight is information about the atmosphere's structure and chemical makeup.

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Blobrana


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RE: Extrasolar Planets
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During the last decade, the discovery of planets around stars other than the Sun has revealed a variety of kinds of planets, orbits, and systems, often very different to our Solar System. One of the most interesting examples are those of stars accompanied not only by planets, but also by other stars, making double "Sun" sunsets, and sunrises, no longer science fiction. A research team from the University of Jena (Germany) has recently made interesting discoveries at Calar Alto by performing a systematic search for such systems.

Since the discovery in 1995 of the first extrasolar planet around a normal star (51 Pegasi), it has become evident that there exists an unexpected diversity of planetary systems, most of them very different to our Solar System. First, it was found that there are many massive planets placed extremely close to their parent stars: the so-called "hot Jupiters". This is very different from what we observe in the Solar System, where massive planets are located far away from the Sun, and this finding required a revision of the theories of planetary formation. Second, it was shown that the process of planetary formation is not restricted to single stars as our Sun: indeed, under certain circumstances, it is possible to form planets around binary stars, which are a much more dynamically complex environment than our own Solar System.

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Blobrana


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Title: HD 97048's Circumstellar Environment as Revealed by a HST/ACS Coronagraphic Study of Disk Candidate Stars
Authors: R. L. Doering, M. Meixner, S. T. Holfeltz, J. E. Krist, D. R. Ardila, I. Kamp, M. C. Clampin, S. H. Lubow

We present the results of a coronagraphic scattered-light imaging survey of six young disk candidate stars using the Hubble Space Telescope Advanced Camera for Surveys. The observations made use of the 1.8" occulting spot through the F606W (broad V) filter. Circumstellar material was imaged around HD 97048, a Herbig Ae/Be star located in the Chamaeleon I dark cloud at a distance of 180 pc. The material is seen between ~2" (360 AU) and ~4" (720 AU) from the star in all directions. A V-band azimuthally-averaged radial surface brightness profile peaks at r = 2" with a value of 19.6 ± 0.2 mag arcsec^-2 and smoothly decreases with projected distance from the star as r^(-3.3 ± 0.5). An integrated flux of 16.8 ± 0.1 mag is measured between 2" and 4", corresponding to a scattered-light fractional luminosity lower limit of 8.4 x 10^-4. Filamentary structure resembling spiral arms similar to that seen in Herbig Ae/Be disks is observed. Such structure has been attributed to the influence of orbiting planets or stellar encounters. Average surface brightness upper limits are determined for the five non-detections: HD 34282, HD 139450, HD 158643, HD 159492, and HD 195627. Possible reasons for the non-detections are disks that are too faint or disks hidden by the occulter.

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Blobrana


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Title: Planets of young stars
Authors: E.W. Guenther, E. Esposito

Since the first massive planet in a short period orbit was discovered, the question arose how such an object could have formed. There are basically two formation scenarios: migration due to planet-disk or planet-planet interaction. Which of the two scenarios is more realistic can be found out by observing short-period planets of stars with an age between 10E7 and 10E8 yrs. The second aim of the survey is to find out how many planets originally formed, and how many of these are destroyed in the first Gyrs: Do most young, close-in planets evaporate, or spiral into the host stars? In here we report on the first results of a radial-velocity search program for planets of young stars which we began in 2004. Using HARPS, we currently monitor 85 stars with ages between 10E7 and 10E8 yrs. We show that the detection of planets of young stars is possible. Up to now, we have identified 3 planet-candidates. Taking this result together with the results of other surveys, we conclude that the frequency of massive-short period planets of young stars is not dramatically higher than that of old stars.

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Blobrana


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Title: Terrestrial Planet Formation Around Individual Stars Within Binary Star Systems
Authors: Elisa V. Quintana (NASA ARC), Fred C. Adams (U. Michigan), Jack J. Lissauer (NASA ARC), John E. Chambers (Carnegie/DTM)

We calculate herein the late stages of terrestrial planet accumulation around a solar type star that has a binary companion with semimajor axis larger than the terrestrial planet region. We perform more than one hundred simulations to survey binary parameter space and to account for sensitive dependence on initial conditions in these dynamical systems. As expected, sufficiently wide binaries leave the planet formation process largely unaffected. As a rough approximation, binary stars with periastron q_B > 10 AU have minimal effect on terrestrial planet formation within ~2 AU of the primary, whereas binary stars with q_B \la 5 AU restrict terrestrial planet formation to within ~ 1 AU of the primary star. Given the observed distribution of binary orbital elements for solar type primaries, we estimate that about 40 -- 50 percent of the binary population is wide enough to allow terrestrial planet formation to take place unimpeded. The large number of simulations allows for us to determine the distribution of results -- the distribution of plausible terrestrial planet systems -- for effectively equivalent starting conditions. We present (rough) distributions for the number of planets, their masses, and their orbital elements.

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Blobrana


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A study published in this week’s edition of Astrophysical Journal Letters, led by University of Texas at Austin graduate student Jacob Bean with research scientists Michael Endl and Fritz Benedict, brings new insight into how planets form around the most populous stars in our Milky Way galaxy. Bean’s work shows that the chemical make-up of these “red dwarf” stars with orbiting planets is different from most of Sun-like stars that harbour planets — and indicates that astronomers must take chemical composition into account in their planet searches around these stars.
Red dwarfs have lower mass than any other type of star, ranging from just 8 per cent of the Sun’s mass to as much as 60 per cent. They also give off correspondingly less light, making them more difficult to study. Despite their stature, though, red dwarfs are the most numerous stars in the galaxy. Of the hundreds of billions of stars in our Milky Way, at least 70 per cent are red dwarfs.

"This factor alone makes them a crucial sample for determining the fraction of stars that are orbited by planets" - Jacob Bean.

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Blobrana


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Super-Earths
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They are out there, and they are unlike anything previously discovered in the galaxy. Dubbed "super-Earths" because of their size, these planets are the first predominantly rocky worlds found outside our solar system.
Big deal? You bet. The discovery of this new class of planet takes us a significant step closer to finding other Earth-like worlds that might be suitable for life. It also sheds new light on the physics of planetary formation. Indeed, the race to find Earth's bigger cousins is hotting up: a French-led space telescope called Corot is scheduled to launch later this month, with the promise of detecting more of these mysterious worlds.
What is most interesting is not that super-Earths are big, but that they are relatively small. Since the discovery in 1995 of a Jupiter-sized planet around the star 51 Pegasi, astronomers have located more than 200 extrasolar planets.

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Blobrana


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Posts: 131433
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M Dwarf GJ 436
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Title: The M Dwarf GJ 436 and its Neptune-Mass Planet
Authors: H. L. Maness, G. W. Marcy, E. B. Ford, P. H. Hauschildt, A. T. Shreve, G. B. Basri, R. P. Butler, S. S. Vogt
(revised v2)

We determine stellar parameters for the M dwarf GJ 436 that hosts a Neptune-mass planet. We employ primarily spectral modelling at low and high resolution, examining the agreement between model and observed optical spectra of five comparison stars of type, M0-M3. Modelling high resolution optical spectra suffers from uncertainties in TiO transitions, affecting the predicted strengths of both atomic and molecular lines in M dwarfs.

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Blobrana


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RE: Extrasolar Planets
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Title: Microlensing search for extrasolar planets
Authors: A. Cassan (ARI/ZAH Heidelberg, Germany), D. Kubas (ESO Vitacura, Chile)

Microlensing has recently proven to be a valuable tool to search for extrasolar planets of Neptune- to super-Earth-mass planets at orbits of few AU. Since planetary signals are of very short duration, an intense and continuous monitoring is required, which is achieved by PLANET : ''Probing Lensing Anomalies NETwork''. Up to now the detection number amounts to four, one of them being OGLE 2005-BLG-390Lb, an extrasolar planet of only ~5.5 M_earth orbiting its M-dwarf host star at ~2.6 AU. For non-planetary microlensing events observed from 1995 to 2006, we compute detection efficiency diagrams which can then be used to derive an estimate of the limit on the Galactic abundance of sub-Jupiter-mass planets, as well as relative abundance of Neptune-like planets.

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