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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.
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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".
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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."
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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
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