more than a hundred extrasolar planets, and still no earth in sight?

Revisiting the solar system: 400 years after Galieo

Astronomy/Space/Planètes extra-solaires

last questions on current events

MORE THAN A HUNDRED EXTRASOLAR PLANETS,
and still no Earth in sight?

Although the number of extrasolar planets discovered has just passed the one hundred mark, not one comparable to the Earth has yet been found. And competition between the different research teams is fierce…

It has been only seven years since the first planet located outside our Solar System was discovered. Since Swiss astronomer Michel Mayor's achievement, many other scientific teams have taken up this new sport: exoplanet hunting. Today, more than 100 extrasolar planets have been located...

Despite the impressive score, none of these planets can be compared to the Earth: all are "gas giants". However, although they resemble Jupiter or Saturn in mass and composition, there is a difference: their extreme proximity to the star around which they revolve, often at a distance of less than an astronomical unit* (against 5.2 AU for Jupiter). This characteristic means that they turn around their star very rapidly, in just a few days (against 12 years for Jupiter), in often highly-eccentric orbits. In short, these planetary systems are apparently very different to ours and no "Earth" could exist there. Although…

* Astronomical unit (AU): 1 AU = the average distance between the Earth and the Sun, i.e. 149,597,870 kms

A discovery that caused a stir

Artist’s view of the exoplanet orbiting 55 Cancri

Artist’s view of the exoplanet orbiting 55 Cancri
© NASA/Lynette Cook

On the 13th June 2002 came a dramatic turn of events. In a media operation deftly orchestrated by NASA, an American team led by Geoffrey Marcy and Paul Butler announced the discovery of thirteen new exoplanets. An impressive catch!

However, it was not the number that the Americans focused on, but one particular planet. They announced that a giant orbiting the star 55 Cancri was the first exoplanet truly comparable to our Jupiter…

Pourquoi découvre-t-on aujourd'hui autant d'exoplanètes ?
Réponse de Jean Schneider, planétologue

A planetary system similar to ours?

In reality, the similarity is limited to a single point: for the first time, the distance that separates the planet from its star (5.9 AU) approximates the distance from Jupiter to the Sun (5.2 AU). According to the Americans, this characteristic makes 66 Cancri the first system that could hold an "Earth".

It is an accepted fact that giant planets form far from their star. However, all those discovered so far have been very close. This proximity suggests that the planets concerned have migrated. During such a migration, they would have swept up everything in their path, including any smaller planets.

Comparison between the 55 Cancri System and our Solar System

Comparison between the 55 Cancri System and our Solar System

"The Americans claim that the system they have discovered is the first one compatible with the presence of an Earth", notes Jean Schneider, planetologist at the Paris-Meudon observatory. "But rather dishonestly, they kept quiet about the fact that 55 Cancri also has two other giant planets in very short orbits. So if there had been an Earth, it would have been swept up."

Helvetia strikes back

Wrong-footed, Michel Mayor and his team at the Geneva Observatory decided that they would also reveal the results of their latest research campaign. A week after their American competitors, they announced the discovery of twelve new extrasolar planets… including one that promises to be even more remarkable than the Americans'. It revolves far enough from its star (3.6 AU) and no other giant planet has been found in a shorter orbit in the system.

So this could be the first system truly able to hold an "Earth": a telluric planet of small size located at a the right distance from its star to make its temperature compatible with the development of life. Unfortunately, it is as yet impossible to say whether such a planet is present.

* a planet whose surface is solid

Michel Mayor : “ Cette annonce est un effet médiatique de la NASA. Cela n'a rien de très sérieux “ : Michel Mayor est astronome à l'Observatoire de Genève

Michel Mayor : “ Cette annonce est un effet médiatique de la NASA. Cela n'a rien de très sérieux “

Methods that have reached their limit?

The radial velocity method... : Cette courbe correspond à la variation de vitesse d'une étoile au cours du temps mesurée à l'aide d'un spectrographe. Elle permet d'en déduire la présence d'une ou plusieurs planètes. (Sans planète, la courbe aurait été plane)

zoom

The radial velocity method...

No instrument in the world - whether spatial or terrestrial - can produce images of extrasolar planets today. Their size is too small and their luminosity too weak, so astronomers have to fall back on indirect search methods.

The first of these - known as the radial velocity method - is based on the fact that a planet perturbs the movement of the star around which it orbits. Since the spectral analysis of a star enables its speed to be measured over time, it is possible to detect certain anomalies and deduce that one or more planets are present. Almost all exoplanets have been discovered by this method.

Another technique is know as the "transit method". This involves measuring the drop in luminosity of a star when a planet passes in front of it. So far, this technique has primarily enabled confirmation of the existence of planets previously detected by the radial velocity method. However, recently (at the end of June), a Polish team announced the probable discovery of thirteen new planets using the transit method. Other studies must be made to confirm these results.

"The smallest exoplanet detected today has a mass equivalent to 36 times that of the Earth", explains Jean Schneider. "But current instruments and procedures have not outlived their usefulness. I think they will allow us to detect planets as small as 10 Earth masses in the months to come." The particularity of this mass is that it marks the boundary between gas and telluric planets…

zoom

The transit method...

zoom

Corot

New instruments in view

When detecting exoplanets, the main obstacle is our own atmosphere, which acts as a moving filter. To overcome this problem, planetologists are banking heavily on future space observatories whose launches are scheduled between 2005 and 2008: Europe's Corot and Eddington and the USA's Kepler, three instruments that will apply the transit method and theoretically be able to detect Earth-sized planets.

Better, in around 2015, Darwin (Europe) and Terrestrial Planet Finder (United States) may finally allow us to see such planets! Their secret? Interferometry, a process that involves combining images from several telescopes to obtain the resolution of a much larger instrument. So objective "Earth" is not so far off. All we need is a little patience…

Olivier Boulanger

On line since 02/08/02

SEE ALSO