An exoplanetary drama: a planet collapses on its star
2/9/10

Several hundred planets gravitating around a star other than our sun, have already been detected over 20 years of research. But the recently discovered WASP-18b stands out from the crowd: it is the first one to be observed that is in the process of collapsing on its star “just before” it disintegration. A publication in Nature paints the portrait of this relatively unexciting planetary system. (1)

In fact, WASP-18b is very special. It is the second exoplanet, among those detected, to have such an enormous mass. Consider it more from this point of view: it is ten times bigger than our own colossal Jupiter, i.e. three thousand times bigger than the earth. “We’re really at the limit with failed stars, the brown dwarfs”, explains the astrophysicist, Michaël Gillon, who participated in its discovery. It orbits around its star in less than a day, which in no way compares with our small planet which calmly completes its tour of the sun in one year. Its star is also bigger and shinier than our sun. All these ingredients make this a unique system.

planetary syst

When a planet gravitates around its star, tidal effects cause a transfer of orbital energy from the planet to its star, whose own rotation accelerates, disrupting the orbit of its planet in turn. “In reality, it is the relationship between the period of the planet’s rotation and the star’s orbital period that determines whether the planet is moving away or moving closer to its star”, points out Michaël Gillon from the department of astrophysics, geophysics and oceanography at the University of Liège. “A similar case is that of the earth-moon system: because the earth’s period of rotation (24 hours) is less than the moon’s orbital period (27 days), the moon moves just under 4 metres away from the earth every century. In the case of WASP-18b, the opposite is true: its orbital period is 0.9 days while its star completes a full rotation in 5.6 days. This is why the planet moves closer to its star, implying increasing tensions. After a time, the hydrostatic balance will be broken: the planet will lose its spherical form and finally disintegrate. Its entire gaseous envelope will be torn away and gradually fall onto its star.” Such is the fate of this planet, which will disappear in several hundred million years, well before its young star. “This scale in time, which is very short in astronomical terms, means that statistically, we are unbelievably lucky to see this exoplanet just when it’s falling on its star.”

Albeit tragic, this planet will be the subject of regular observations in the future that will constrain the planetary models and, in particular, the tidal parameter describing the energy exchanges between a planet and its star: “we don’t know whether this parameter has a universal value. If there is an incidental value in the WASP-18b system identical to the value measured for our sun, our instruments should observe the planet moving closer in 10 or 20 years: the period of its transit should have reduced by some ten seconds in ten years. It’s observable…”, continues Michaël Gillon.

(1) Hellier C., Anderson D. R., Collier Cameron A., Gillon M., Hebb L., Maxted P. F. L., Queloz D., Smalley B., Triaud A. H. M. J., West R. G., Wislon D. M., Bentley S. J., Enoch R., Horne K ., Irwin J., Lister T. A., Mayor M., Parley N., Pepe F., Pollaco D., Segransan D., Udry S., Wheatley P. J., 2009, «An orbital period of 0.94 days for the hot-jupiter planet WASP-18b», Nature. 2009-03-02173B

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