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Pathway to the Stars

In December 2006, the Corot satellite (1) will be placed in orbit by a “modernized” Soyuz launch vehicle. The French space agency is now ready to examine hundreds of thousands of stars in an effort to detect new, previously undreamt-of worlds...

After twelve years of patience comes the reward: The Corot mission, led by the French national centre for space research (CNES),2 with major input from French laboratories,3 is finally on its way. Annie Baglin, mission leader and researcher at LESIA4 from the Paris Observatory, states, “Corot will be pointed towards the stars with the purpose of detecting the slightest variations in their light.” After years of waiting, a few technical obstacles, and some serious threats of budget cuts, the satellite is finally ready. At the end of August, in Cannes, Alcatel Alenia Space carried out the final touches, fine-tuning its two key capabilities, each focused on a different area of astronomy–the hunt for extrasolar planets and stellar seismology.


But why track the flickering of the stars at all? Actually, these celestial micro-eclipses tell us a great deal about the lives of the stars: For example, a diminution in a star's brightness repeated over time means that a planet is passing periodically in front of it. And that signals the presence of an extrasolar planet–one more to add to the list of those discovered since 1995, when the first one was discovered by Michel Mayor. Ten years on, it seems that the formation of planets around stars is a constant feature of our galaxy. As for variations in stellar luminosity, these are symptoms of internal upheavals: Stellar quakes, which resemble our earthquakes, provide clues about a star's inner structure.



© D. Ducros/CNES 2005

When a planet (the black circle here) transits across its star (in the background), the latter's brightness diminishes very slightly. This allows Corot to detect exoplanets with a size of between 2 and 10 terrestrial masses.


Corot will concentrate on the two regions of the sky in its view, referred to colloquially as “Corot's eyes.” Located where the equator intersects with the galactic plane, that is to say, in the direction of the Milky Way, Corot will be closely observing a limited number of these celestial lights and examining them in the greatest detail: Not even the smallest blink will go undetected. Baglin adds, “We have already drawn up the program for the first two years of observation.” Every time the satellite is re-positioned and angled in a different direction, its field will take the shape of a square in the sky where each side measures 5.5 times the apparent diameter of the full moon. In this tiny portion of the firmament, Corot will focus on approximately ten bright stars and 12,000 dimmer ones on a precisely defined star field. Each half year, it will take two different positions, one for 150 days, producing very long continuous observations of the same stars, and a shorter period lasting 20 to 30 days, to complement the sample. This translates into a total of around 150,000 stars for the nominal life of the satellite, which is three years... But the final figure may be as high as–or even higher than–200,000, given that the team is counting on Corot's lifespan to actually be five or six years... But exactly what kind of information do the scientists expect from this 200,000-star mission? Hopes are high and wide-ranging for this groundbreaking satellite.


To start with, hundreds upon hundreds of planets could be detected! It is the lessening in the brightness of the dimmest stars that will indicate the presence of a planet. Baglin explains, “when the Earth passes in front of the Sun, the latter's brightness diminishes by 0.01% over a period of three hours. Corot is not able to detect that small a diminution in luminosity, but a drop of 0.05% can be detected, and that lets us spot planets as small as 1.5 terrestrial radius or larger,  that orbit their star in a period of a few days to two or three months.”

As for the brightest stars, their light varies because they vibrate. And the way they vibrate–for example, the frequency, high or low, of the vibrations–enables us to find out more about their inner structure: “A giant star resonates like a barrel drum, a dwarf more like a hunting horn.” All in all, there are several hundred different vibrations for each star. Barrel drum, trumpet, and clarinet, Corot's harvest promises to be rich, giving us more to marvel at as we contemplate the sheer diversity of the cosmos.


Azar Khalatbari

Notes :

1. Corot stands for COnvection, ROtation & planetary Transits:
View web site and View web site
2. Centre national d'études spatiales.
3. The major French laboratories involved in the project are the Paris Observatory, the Orsay Institute of Space Astrophysics, the Marseille Astrophysics Laboratory, and the Midi-Pyrénées Observatory.
4. French Laboratory for Space Research and Instrumentation in Astrophysics (Observatoire de Paris / CNRS / Universités Paris-VI et VII.)

Contacts :

Annie Baglin
Paris Observatory.


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