A planet as hot as the sun

The Kepler space telescope has in its sights two small rocky planets which were detected in another solar system. Could they be copies of our Earth orbiting their sun? Not really, because the planets show a surface temperature of more than 6000°C and their star, which is already in a late stage of evolution, has ejected half of it its mass into interstellar space. The story was published in the journal Nature.

Launched in March 2009 from Cape Canaveral, the American space telescope Kepler is opening up new horizons to the science of astrophysics. While its main objective is to detect exoplanets around stars in our galaxy, its results go far beyond this brief. An article which has just appeared in Nature (1) supplies further proof of this, as explained by Valérie Van Grootel, co-author of the publication and researcher with the Stellar Astrophysics and Asteroseismology division of the department of Astrophysics, Geophysics and Oceanography at the University of Liège.

There are several techniques used for revealing exoplanets. Kepler uses the transit method. This method is indirect: the telescope constantly observes the luminous flux from stars situated in a small region of the Cygnus constellation in the northern hemisphere. When a planet passes in front of one of these stars, the apparent luminosity of the star drops before returning to its original level when the transit has finished.

Some geometrical configurations of the star-planet-observer system do not, however, allow for the viewing of transits. Yet even in these unfavorable cases, the exoplanet can be detected by means of photometry. Indeed, just as the quantity of light that the moon sends us depends on its position relative to the sun and ourselves, as observers, the light received from an exoplanet varies according to its orbit: when the planet is almost in front of the stellar disc, it appears to us as completely dark, while when it is almost behind the stellar disc it is fully lit up. This phenomenon leads to a modulation in the apparent luminosity of the star (with the planet), similar to that produced by a transit. If the effect is sufficiently strong, it is detectable. This is how Kepler has just brought to light some very particular exoplanets around an equally particular sun.

The detection of an exoplanet is one thing but characterizing it is another: is it large like Jupiter or small like our Earth? Does it gravitate near its star or to the outer edges of the planetary system? Is it gaseous or rocky? Characterizing an exoplanet necessitates a minimal knowledge of its star. There is a kind of degeneration to be drawn between the parameters of the star and those of the planet. Asteroseismology is the only doorway to an understanding of the physics at work in the cores of stars (see article entitled “Journey to the heart of the stars”). This science is therefore the indispensable tool for characterizing exoplanets. What does it involve? The majority of stars vibrate just like a heart beats. These vibrations inside the star manifest themselves on the surface by variations in luminosity which, when interpreted, constitute a veritable mine of information about the nuclear reactions that occur at the heart of the star, its chemical composition, its mass, etc.

(1) S. Charpinet, G. Fontaine, P. Brassard, E.M. Green, V. Van Grootel, S.K. Randall, R. Silvotti, A.S. Baran, R.H. Ostensen, S.D. Kawaler & J.H. Telting, A compact system of small planets around a former red-giant star; Nature, December 2011

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