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Post Info TOPIC: WASP-18b
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NASA's Chandra X-ray Observatory Finds Planet That Makes Star Act Deceptively Old

A planet may be causing the star it orbits to act much older than it actually is, according to new data from NASA's Chandra X-ray Observatory. This discovery shows how a massive planet can affect the behaviour of its parent star.
The star, WASP-18, and its planet, WASP-18b, are located about 330 light-years from Earth. WASP-18b has a mass about 10 times that of Jupiter and completes one orbit around its star in less than 23 hours, placing WASP-18b in the "hot Jupiter" category of exoplanets, or planets outside our solar system.
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Title: No X-rays from WASP-18. Implications for its age, activity, and the influence of its massive hot Jupiter
Author: Ignazio Pillitteri, Scott J. Wolk, Salvatore Sciortino, Victoria Antoci

About 20% out of the >1000 known exoplanets are Jupiter analogs orbiting very close to their parent stars. It is still under debate to what detectable level such hot Jupiters possibly affect the activity of the host stars through tidal or magnetic star-planet interaction. In this paper we report on an 87 ks Chandra observation of the hot Jupiter hosting star WASP-18. This system is composed of an F6 type star and a hot Jupiter of mass 10.4MJup orbiting in less than 20 hr around the parent star. On the basis of an isochrone fitting, WASP-18 is thought to be 600 Myr old and within the range of uncertainty of 0.5-2 Gyr. The star is not detected in X-rays down to a luminosity limit of 4 x 10^26 erg/s, more than two orders of magnitude lower than expected for a star of this age and mass. This value proves an unusual lack of activity for a star with estimated age around 600 Myr. We argue that the massive planet can play a crucial role in disrupting the stellar magnetic dynamo created within its thin convective layers. Another additional 212 X-ray sources are detected in the Chandra image. We list them and briefly discuss their nature.

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Blobrana


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Title: On the heat redistribution of the hot transiting exoplanet WASP-18b
Authors: Nicolas Iro, Pierre Maxted

The energy deposition and redistribution in hot Jupiter atmospheres is not well understood currently, but is a major factor for their evolution and survival. We present a time dependent radiative transfer model for the atmosphere of WASP-18b which is a massive (10 MJup) hot Jupiter (Teq ~ 2400 K) exoplanet orbiting an F6V star with an orbital period of only 0.94 days. Our model includes a simplified parametrisation of the day-to-night energy redistribution by a modulation of the stellar heating mimicking a solid body rotation of the atmosphere. We present the cases with either no rotation at all with respect to the synchronously rotating reference frame or a fast differential rotation. The results of the model are compared to previous observations of secondary eclipses of Nymeyer et al. (2011) with the Spitzer Space Telescope. Their observed planetary flux suggests that the efficiency of heat distribution from the day-side to the night-side of the planet is extremely inefficient. Our results are consistent with the fact that such large day-side fluxes can be obtained only if there is no rotation of the atmosphere. Additionally, we infer light curves of the planet for a full orbit in the two Warm Spitzer bandpassses for the two cases of rotation and discuss the observational differences.

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Title: Spitzer 3.6 micron and 4.5 micron full-orbit lightcurves of WASP-18
Authors: P.F.L. Maxted (1), D.R. Anderson (1), A.P. Doyle (1), M. Gillon (2), J. Harrington (3), N. Iro (1), E. Jehin (2), D. Lafrenière (4), B. Smalley (1), J. Southworth (1) ((1) Keele University, (2) Université de Liège, (3) University of Central Florida, (4) Université de Montréal)

We present new lightcurves of the massive hot Jupiter system WASP-18 obtained with the Spitzer spacecraft covering the entire orbit at 3.6 micron and 4.5 micron. These lightcurves are used to measure the amplitude, shape and phase of the thermal phase effect for WASP-18b. We find that our results for the thermal phase effect are limited to an accuracy of about 0.01% by systematic noise sources of unknown origin. At this level of accuracy we find that the thermal phase effect has a peak-to-peak amplitude approximately equal to the secondary eclipse depth, has a sinusoidal shape and that the maximum brightness occurs at the same phase as mid-occultation to within about 5 degrees at 3.6 micron and to within about 10 degrees at 4.5 micron. The shape and amplitude of the thermal phase curve imply very low levels of heat redistribution within the atmosphere of the planet. We also perform a separate analysis to determine the system geometry by fitting a lightcurve model to the data covering the occultation and the transit. The secondary eclipse depths we measure at 3.6 micron and 4.5 micron are in good agreement with previous measurements and imply a very low albedo for WASP-18b. The parameters of the system (masses, radii, etc.) derived from our analysis are in also good agreement with those from previous studies, but with improved precision. We use new high-resolution imaging and published limits on the rate of change of the mean radial velocity to check for the presence of any faint companion stars that may affect our results. We find that there is unlikely to be any significant contribution to the flux at Spitzer wavelengths from a stellar companion to WASP-18. We find that there is no evidence for variations in the times of eclipse from a linear ephemeris greater than about 100 seconds over 3 years.

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Giant alien planet pulls surface of its star up and down.

Astronomers believe they have spotted a distant star's surface rising and falling in response to the gravity of an orbiting planet, just as the Moon tugs Earth's seas up and down.
It was known that the planet, which orbits the star WASP 18 in the constellation Phoenix, would induce huge tides in its star because the planet is ten times the mass of Jupiter, and lies so close to WASP 18 that it orbits it in less than a day.
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Title: Spitzer Secondary Eclipses of WASP-18b
Authors: Sarah Nymeyer, Joseph Harrington, Ryan A. Hardy, Kevin B. Stevenson, Christopher J. Campo, Nikku Madhusudhan, Andrew Collier-Cameron, Jasmina Blecic, William C. Bowman, Christopher B. T. Britt, Patricio Cubillos, Coel Hellier, Michael Gillon, Pierre F. L. Maxted, Leslie Hebb, Peter J. Wheatley, Don Pollacco, David Anderson

The transiting exoplanet WASP-18b was discovered in 2008 by the Wide Angle Search for Planets (WASP) project. The Spitzer Exoplanet Target of Opportunity Program observed secondary eclipses of WASP-18b using Spitzer's Infrared Array Camera (IR\ AC) in the 3.6-micron and 5.8-micron bands on 2008 December 20, and in the 4.5-micron and 8.0-micron bands on 2008 December 24. We report eclipse depths of \math{0.31±0.02, 0.38±0.03, 0.41±0.02, 0.43±0.03\%}, and brightness temperatures of 2920 ±90, 3150 ±130, 3040 ±130 and 2960 ±130 K, respectively. WASP-18b is one of the hottest planets yet discovered - as hot as an M-class star. The planet's pressure-temperature profile features a thermal inversion. The observations also require WASP-18b to have near-zero albedo and almost no redistribution of energy from the day-side to the night side of the planet.

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The planet WASP-18b has maybe 1 million years to live, said planet discoverer Coel Hellier, a professor of astrophysics at Keele University in England. Hellier's report on the suicidal planet is in the Aug. 27 issue of the journal Nature.
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Blobrana


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The gas planet named WASP-18b, which is 10 times the mass of Jupiter and 330 light-years from Earth, is perplexing astronomers because of its ability to resist crashing into the star, WASP-18, it orbits in one Earth day.

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New Exoplanet Shouldn't Exist
A report in the journal Nature cites the discovery of a new planet, WASP-18b, which challenges assumptions about tidal interactions--it's too close and orbiting too fast not to have collided with its star, according to current knowledge

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Astrophysicists puzzle over planet that's too close to its sun
Scientists have discovered a planet that shouldn't exist. The finding, they say, could alter our understanding of orbital dynamics, a field considered pretty well settled since the time of astronomer Johannes Kepler 400 years ago.
The planet is known as a "hot Jupiter," a gas giant orbiting the star Wasp-18, about 330 light-years from Earth. The planet, Wasp-18b, is so close to the star that it completes a full orbit (its "year") in less than an Earth day, according to the research, which was published in the journal Nature.

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