Megan's Blog

In the news this month... Hubble spots a planet-eating star

Most of the extra-solar planets discovered so far are in the class known as "hot Jupiters", large gas giants orbiting close to their parent stars, since many of the search techniques used are most sensitive to this type of planet. Usually these planets are located close enough to their parent star that they orbit in just a few days, but a team have now discovered one that is orbiting so close to its parent star that it is actually being disrupted.

The planet, known as WASP-12b, is located in the constellation of Auriga and was discovered in the Wide-Area Search for Planets survey, or WASP, operated by a consortium of eight academic institutions. WASP consists of two robotic observatories, one located at La Palma in the Canary Islands, the other at the South African Astronomical Observatory at Sutherland in South Africa, both scanning the sky for the tiny dimming effects caused when a planet transits in front of a star. This particular planet orbits its parent star, a yellow dwarf known as WASP-12, in just 1.1 Earth days, and shows evidence of an atmosphere which extends far further from the planet than would be expected for a body of this size.

Previous observations have shown that at least one other exoplanet displays evidence of such an extended atmosphere, and two different mechanisms have been suggested: either heating from the parent star, or an interaction with the stellar wind. This new planet was first discovered by the WASP survey in 2008, and was predicted to be physically distorted by its proximity to the host star. These new observations, made with the Cosmic Origins Spectrograph on the Hubble Space Telescope and published in the Astrophysical Journal during May, have verified the prediction. WASP-12b is so close to the star that the tidal forces exerted on it have heated and deformed it far from the normal almost-spherical planetary shape, so far in fact that the internal heating has caused the atmosphere to expand far enough that it is being dragged off onto the surface of the star.

Absorption from elements such as sodium, magnesium, aluminium and tin was expected in the atmosphere of the star, and the increase in absorption during the transit allowed the astronomers to calculate how common these elements are in the planet's atmosphere. The research, led by Luca Fossati at the Open University in the UK, examined the ultra violet spectrum of the planet's atmosphere and found a much greater abundance of heavy elements than expected from models of planetary atmospheres. The suggested reason for this unexpected result is that the high amount of incident radiation due to the close proximity of the star, together with tidal effects, cause a large amount of mixing within the atmosphere, pulling heavy elements higher in the atmosphere than they would normally be found. The heating also causes the atmosphere to expand, overflowing what is known as the Roche limit, the point beyond which particles escape the gravitational pull of the planet and are lost to the surrounding space.

From the evidence provided by their ultra-violet observations, the researchers conclude that the planet is probably undergoing photo-evaporation by its host star, and the material lost from the atmosphere is forming a diffuse ring around the star along the planet's orbit. While few examples of such systems are currently known, further observations and detailed modelling will help to determine exactly what is going on in these peculiar atmospheres.

---

This blog post is a news story from the Jodcast, aired in the June 2010 edition.

Fossati, L., Haswell, C., Froning, C., Hebb, L., Holmes, S., Kolb, U., Helling, C., Carter, A., Wheatley, P., Cameron, A., Loeillet, B., Pollacco, D., Street, R., Stempels, H., Simpson, E., Udry, S., Joshi, Y., West, R., Skillen, I., & Wilson, D. (2010). METALS IN THE EXOSPHERE OF THE HIGHLY IRRADIATED PLANET WASP-12b The Astrophysical Journal, 714 (2) DOI: 10.1088/2041-8205/714/2/L222