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Reflexions, le site de vulgarisation de l'Université de Liège 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. © Université de Liège - http://reflexions.ulg.ac.be/ - 18 June 2017 -1- Reflexions, le site de vulgarisation de l'Université de Liège 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. Detecting exoplanets The bigger an exoplanet and/or the closer it is to its star, the greater the induced tidal effects. "We believe that if an exoplanet bigger than Jupiter were to venture too close to its star, it would disappear into it. This would explain why we detect so few enormous planets gravitating around their star: they would disappear too quickly to be statistically numerous. WASP-18b is the first of this sort to be observed so close to its star, in 15 years of research and among hundreds of detections." The majority of exoplanets that have been found up until now, are planets on tight orbits with a mass similar to Jupiter, or even bigger; however, they are further from their star. Several methods are used to uncover exoplanets. The first one to bear fruit was the radial speed method: when a body gravitates around a star, the latter's movement in the direction of the earth is modulated by an amplitude periodic signal, which increases with the mass of the body. This modulation can be shown by the spectroscopic monitoring of stars. Initially, in the 1970s, the radial speed method was used to determine the masses of stars' binary systems. In the 1990s, instrument accuracy was sufficient to begin exoplanetary research. Today, almost 300 of these planets have been detected this way. The first one dates from 1995. With a mass equivalent to half of that of Jupiter, it gravitates very closely around its sun in 4.5 days. These high-mass exoplanets that orbit close to their star are called "Hot Jupiters". © Université de Liège - http://reflexions.ulg.ac.be/ - 18 June 2017 -2- Reflexions, le site de vulgarisation de l'Université de Liège A second method has also allowed dozens of exoplanets to be discovered. The year 2000 inaugurated the transit method: when a planet passes in front of its star's disk, it causes a slight drop in the star's light intensity. Ambitious projects, based on this method, quickly appeared, including Kepler or CoRoT (Read : Voyage to the centre of the stars), which track planetary transits from space. For instance, the CoRoT satellite detected a planet whose mass is equivalent to only five times that of our planet, with a radius twice that of earth. It is the smallest planet ever detected outside our solar system. Up until now, planetary transits have led to the discovery of some sixty exoplanets. Today, exoplanetary transits are coveted by numerous projects. "For instance, there's the English consortium WASP (Wide Angle Search for Planets) which has a telescope in every hemisphere, one in La Palma (WASP-North) and one in South Africa (WASP-South)", explains Michaël Gillon. "Every instrument, equipped with eight mirrors, has a total field of 482 degrees squared, which is equivalent to sixty times that of the full moon! Each field continuously measures the fluxes from several hundreds of thousands of stars at the same time, in the hope of detecting a fall in flux in percent for some of them, which would possibly indicated the presence of a planet. By comparison, a binary system induces reductions in flux of up to 40%. Complete instrumental know-how had to be developed and is constantly being improved to track planetary transits." © Université de Liège - http://reflexions.ulg.ac.be/ - 18 June 2017 -3- Reflexions, le site de vulgarisation de l'Université de Liège In two years, among the hundreds of thousands of stars placed under high surveillance, WASP-South spotted 300 possible exoplanets using photometry. They then had to be confirmed (or not) with spectroscopy, mainly using Coralie, an instrument installed in La Silla on the Swiss Euler telescope. "Out of the 300 WASP-South possibilities, we've confirmed about 20, including WASP-18, which is the subject of our publication in Nature", continues Michaël Gillon. "The others turned out to be star binary systems, false-positives, atmospheric or systematic disturbances." In the WASP harvest, number 18 is not the only one to stand out. There is WASP-17b, which is also at the centre of a recent publication (2) though for quite another reason. It is the least-dense planet known, but that is not all. Since it is the same cloud of gas and dust that gives rise to a star and its trail of planets, we expect a planet to orbit its star in the same direction as its own rotation. This is the case for all the planets in the solar system. This was also the case for all the exoplanets discovered… before WASP-17b, which turns in the opposite direction… "We believe that a "planetary pinball machine" violently modified WASP-17b's orbit in the past. This implies the presence of either a stellar companion, or another planet in the system. We shall have to test these hypotheses in the future", explains Michaël Gillon. Read Liège astrophysicists in seventh heaven (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 (2) Anderson D. R., Hellier C., Gillon M., Triaud A. H. M. J., Smalley B., Hebb L., Collier Cameron A., Maxted P. F. L., Queloz D., West R. G., Bentley S. J., Enoch B., Horne K., Lister T. A., Mayor M., Parley N. R., Pepe F., Pollaco D., Ségransan D., Udry S., Wilson D. M., 2009, "WASP-17b: an ultra-low density planet in a probable retrograde orbit", A&A (accepted) - arXiv:0908.1553 © Université de Liège - http://reflexions.ulg.ac.be/ - 18 June 2017 -4-