Information sheet n° 23 :

the detection of the extrasolar planets

J. Schneider, Paris observatory

The Venus passage gives the opportunity to see how the transits make it possible to detect extrasolar planets, i.e. planets orbiting around stars other than the Sun, and, beyond, to give a progress report on these new worlds.

Why be interested in the extrasolar planets ?

It is natural to wonder whether life exists elsewhere than on Earth. Already Epicure raised the question into 300 before JC (in its Posthumous letter to Herodote; moreover it answered it without hesitating positively).

The life can, a priori, take forms extremely different from ours. In this unbounded range one cannot know in which direction to seek a priori, which does not help us much. To make research more effective, one starts by searching forms of life sharing with us the characteristics that one thinks if not universal at least rather widespread: to be founded on the organic chemistry, to develop in an environment, if not aqueous at least sufficiently wet and finally to have at its disposal a source of permanent luminous energy. The environments the most favorable for such conditions are solid planet surfaces (or liquids) located at good distance of their star. They are called "habitables" planets.

In other words, it is necessary to first detect other planetary systems. This first condition answers besides also "planetologic" questions of nature such as: how much are there planetary systems in the galaxy? how are their characteristics distributed (number of planets, masses, orbits etc), how those are correlated with the type of star?

How to detect the other planetary systems?

The most natural method would be to make an image of the planetary system where the planets seem points beside their star. It is also the most promising method because it will be possible to see the colors of planets, to deduce the characteristics of their atmosphere and their ground and to detect indices of a possible life there. Unfortunately it is as most difficult to implement because, the star being a billion times more brilliant than the planet, dazzles the observer.

One should then start with indirect approaches, the direct imagery not possible before approximately 2007, the instruments of observation being in preparation.

Indirect methods:

Disturbance of the trajectory of the star by the planet

When the planet orbits around the star, it turns in fact around the centre of gravity of the pair star-planet, just as the star.

This movement of the star, which one wants to detect, is periodic, on an orbit smaller than the one of the planet with a radius "a" given by the law of the barycentre:

a = A (m/M)


    A is the distance star-planet
m is the mass of the planet
M is the mass of star

It results from this some periodic variations on three measurable quantities by the observer:

- "radial " speed V of the star (component the speed of star in the direction star-observer). The amplitude of this variation is of 13 m/s for a planet similar to Jupiter, located, like Jupiter, at 5 astronomical units (au) from its star.
- distance star-observer (measured by the variation of the dates of arrival of periodic signals emitted by the star, as in the case of pulsars for example)
- position of star on the sky ("astrometrical " method) since the star turns around the centre of gravity of the pair star-planet.


Disturbance of the luminosity of the star (transits)

A planet can slightly transitorily darken a star around which it turns (transit). It is the transposition to stars of the Venus passage in front of the Sun. It is nevertheless quite different: whereas one can see in the case of Venus an image of planet in front of the solar disc, in the case of a extrasolar planet the star is seen like a point and all that one can observe is a very small drop of the glare of the star at the time of the passage. The relative drop of luminosity is proportional to the surface of planet.

This drop of luminosity is 1% for a giant planet like Jupiter and 0,01% for a planet of the size of the Earth.

A disadvantage of this method comes from the weak geometric probability (p = R/A where R is the radius of the star) so that the orbit of planet is correctly directed to produce a transit. This probability is around 0,5% for a planet located at one au from its star; in other words, if all the stars have a planet at one au, it is necessary to follow 200 of them to see a transit. If 10% of stars have a planet at one au, in order to detect 10, 20000 stars should be followed photometrically.

Assessment of the discoveries

The method of the radial speeds made possible the detection of (at the date of November 2003) 103 planetary systems, of which 13 are multiple, giving in all 118 planets. These detections indicate that at least ~7% of stars are accompanied by at least one planet. Extrapolated to all the galaxy, this proportion means that this one contains at least 7 billion planets. The masses of detected planets go from 36 terrestrial masses to 13 Jupiter masses. The lower limit comes from the limitation of the instrumental precision. The higher limit is fixed by the definition adopted for a planet: beyond 13 Jupiter masses a body has a source of own energy due to a central thermonuclear activity: it is thus a star). The distances from planets detected with their stars go from 0,02 UA to 4,8 UA. Curiously, half of the detected orbits are rather elliptic (strong eccentricity) contrary to what happens in the solar system where all the planets (except Mercure and Pluto) have quasi-circulars orbits . This phenomenon is not yet understood.

Among 118 planets detected by radial speed, one of it (HD 209458 b) also produces (every 4 days) a transit. That was observed by the space telescope Hubble. Spectroscopic observations during transits by this planet already provided indications on the chemical composition of its atmosphere. It is necessary to add also the planet OGLE-TR-56 B which was detected by the method of the transits (and confirmed later on by radial velocity measurements). There is in this case a transit every 30 hours, which implies, if the interpretation of the observations is correct[1], a very tight orbit (5 times the radius of the Sun).

All these planets are giant planets, not being able to contain a form of life similar to the terrestrial life.


Future space missions for the search of planets.

During the next years, most of the observatories will continue or develop planet systematic research programmes by radial speeds, transits, astrometry or direct imagery. In parallel a profitable harvest is awaited from several telescopes already put into orbit, in orbit and operational, or in the way of construction or under study:

–  the Space Telescope Hubble observed, in June 2003, 50 000 stars with an aim of seeking planetary transits. A hundred planets is awaited from this; the results of the analysis are awaited for 2004.

– the satellite CoRoT [2], will follow 60 000 stars during two years and an half to seek planetary transits. Its launching by the CNES (French Space Agency) is scheduled for June 2006. One expects that it finds the first "big Earthes" (approximately twice the radius of the Earth) in the galaxy, some being likely to lodge a form of life.

-  theThe American satellite Kepler (launching envisaged in 2008) must detect by the same method a few hundreds of planets whose size will be similar to the one of Mars.

-  the U.S. satellite SIM (Space Interferometric Mission), whose launching is programmed for 2009, should detect by astrometry at least a few tens of planets, primarily giant. Much more ambitious, European project GAIA (launching envisaged in 2011) will follow with the same goal a billion stars. A harvest of a few tens of thousands of planets is awaited.

–  the U.S. satellite JWST (with a European contribution for the planet detection) will endeavour to seek giant "hot" planets by imagery. With a little chance it could find a few tens of them.

–  finally, two parallel projects (Darwin for Europe, TPF for the USA) are being studied to detect livable planets by imagery. In the second time a spectroscopic study of these planets will seek there, around 2020 signs of biological activity, like the presence of oxygen or the colors, characteristic of a vegetation. In the interval, NASA studies precursors to TPF with smaller ambitions.


We are living at a period when a question, unresolved from the antiquity, will find an answer: near 2020, a great change will take place in tne history of astronomy. It may be important for Humanity, too.

To know more:
- Benest Daniel & Froeschlé Claude (Eds.)
Invitation aux planètes.
ESKA 1999
-Mayor Michel & Frei Pierre-Yves
Les nouveaux mondes dans le cosmos
Le Seuil, 2001
-The encyclopedia of the extrasolar planets on the Web:

[1]It is necessary to be sure that the transits are not artefact due to a binary star with its own eclipses.

[2] CoRoT means ''Convection Rotation and planetary Transits''  since this satellite will also study the rotation of the stars..