The tip of the iceberg

Along with the recognition that stars in the sky are anologs of our own Sun, came the hypothesis that planets could orbit these distant Suns. The large mass and luminosity ratio between a planet and its parent star has, until about 15 years ago, prevented us from testing this hypothesis. Thanks to recent technological advances, we are now able to detect some of these planets. Although Extrasolar Terrestrial Planets (planets similar to ours) are still out of reach of our most precise instruments, almost 200 extrasolar planets have now (Jan 2006) been discovered. Due to our current technological limitations, these planets are only the "tip of the Iceberg", and it is likely that most stars do have planets (there are about 100 billion stars in our galaxy). As astronomers sharpen their planet-finding tools in the next few years, we can expect the number of known exoplanets to grow exponentially. Most exiting will be the discovery and study of planets similar to the Earth, with the hope of detecting biological activity or signs of life.

Our solar system: not a "universal" model

The inner part of our solar system is inhabited by small rocky planets (Mercury, Venus, Earth, Mars), while giant gaseous planets (Jupiter, Saturn, Uranus, Neptune) are further out. Prior to 1995 (discovery of 51 Peg b planet, see below), this seemed to be the way planetary systems had to be: during the formation of planets, the central star "pushes away" gas outward, leaving only "rocky" materials in the inner part of the solar system - while outer planets can grow to a much larger size by accreting gas.

In fact, if all stars had planetary systems similar to ours (small rocky planets in the inner part, large gaseous giants further out), it is likely that no exoplanet would have been detected yet around Sun-like stars. The history of exoplanet discoveries is therefore one of unexpected surprises, as exoplanets showed up were many astronomers thought they could not exist. Thanks to new exoplanet discoveries, we now begin to understand why the simple model outlined above is incomplete, and a better picture of planetary formation is slowly emerging.

Selected milestones in the chronology of exoplanet discoveries

The first discovery of an extrasolar planet (a planet outside our solar system) was made in 1992 by timing observations of the pulsar PSR1257+12. Small deviations from the otherwise extremely regular arrival time of its radio pulses (synchronized with its rotation) have been used to infer the presence of 3 planets, of which 2 are about 4 Earth masses, and one is 6 Jupiter masses. A pulsar is a dead star, and these planets are very unlikely places for life to exist; however, this discovery confirmed that planets exist outside our solar system.

In 1995, the first exoplanet orbiting a main sequence star (a hydrogen-burning star star, like the Sun), 51 Peg, was discovered by precise radial velocity measurements: as the planet orbits the star, it induces a small periodic motion of the star (both the planet and the star circle around the center of mass of the system, which is slighlty offset from the star center). The radial component of this periodic motion can be measured as a shift of the entire stellar spectra towards the blue when the star approaches and towards the red as the star recedes (Doppler shift effect). Almost all of the currently known exoplanets have been identified by this radial velocity measurement technique. Most of the planets discovered by radial velocity are several hundred times more massive than the Earth (similar to Jupiter, which is 317 times as massive as the Earth), but the least massive is only 7.5 Earth masses and was recently (2005) discovered around a nearby low mass star.

Many of these planets orbit their parent star in a few days: due to their large mass and close proximity to the star, they are dubbed "hot Jupiters". 51 Peg b, the first planet to be discovered around a Sun-type star and the first planet identified by radial velocity measurement, is representative of hot Jupiters: it is about half the mass of Jupiter, its orbital period is 4.2 days and it is 7 times closer to its parent star than Mercury is to our Sun.

In 1999, a planetary system with 3 massive planets was discovered by radial velocity around the star Upsilon And. The planets masses range from 0.69 to 3.75 times the mass of Jupiter, and their distances to the star range from 0.059 to 2.53 times the Earth-Sun distance. We now (Jan 2006) know of 17 main sequence stars with multiple planets thanks to this radial velocity technique.

A particularly interesting planet is HD 209458 b, first discovered by radial velocity measurement. In late 1999, it was seen transiting its parent star: each 3.5 days, it passes in front of its star, thus blocking a small but measurable fraction of its light as seen from Earth. The amount by which the star luminosity decreases during the transit yields the planet diameter: 1.32 times the Jupiter diameter for this 0.69 Jupiter mass planet. In 2001, spectroscopic observations revealed an extended atmosphere of sodium around the planet. Oxygen and Carbon were also detected in this extended atmosphere in 2003. Last year (2005), thermal infrared emission from the hot surface of the planet (the distance to its star is only 4.5% the Sun-Earth separation) was discovered as the overall infrared luminosity of the star+planet system drops each time the planet passes behind the star.

Direct imaging of exoplanets

The techniques mentioned above (pulsar timing, radial velocity, transit) are indirect: the effect of the planet on the starlight, rather the planet light itself, are detected. There is considerable interest in being able to directly image an extrasolar planet, as its light can then be analysed spectroscopically to characterize its atmosphere and/or surface (temperature, composition, weather, biological activity ?).

Current technology does not yet allow us to directly image planets similar to the ones in our solar system. However, if a planet is massive (more massive than Jupiter), very young (less than a tenth of the age of our solar system), orbits a star less luminous than our Sun, and is more distant from its parent star than our Jupiter is from the Sun, then, thanks to direct thermal emission from the planet, direct imaging in the near-infrared is possible. Three such candidate planets have been reported in the last 2 years (2004 and 2005): 2M1207 b, GQ Lup b and AB Pic b. Due to uncertainty in their mass (derived from their apparent luminosity, and therefore model-dependent), their classification as planets rather than low mass stars is still debated. As more observations are performed in the next few years, more of these candidates will be discovered, and the planetary nature of many of them will be confirmed.

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