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One Thousand and One Planets

22 octobre 2013

Much time has massed since the eighties when astronomers asked themselves (as did the ancient Greeks) if the Sun is the only star surrounded by planets. 25 years later we have found about 1000 planets, and we are still counting, have been detected in about 800 planetary systems. The growth of this population of new worlds can be tracked in the catalog of the Extrasolar Planets Encyclopaedia, which is updated daily by the Paris Observatory :
see http://exoplanet.eu . The catalogue furnishes an opportunity to remind oneself about old discoveries and to anticipate future ones.

Some facts.

At the turn of the ’80s-’90s, a few astronomers began to search, courageously and in fact successfully, to look for exoplanets. But we had to wait until the XXth century to confirm the
first exoplanets : this was because they are practically invisible, swamped in the glow of their parent stars. Fortunately they can be detected by applying various other "tricks",
exploiting their effect on the luminosity or the motion of their central star. An important, but difficult, step was taken when the direct light of the planets (emitted by the planet or reflected stellar light) was finally observed. These meagre photons are the precious Holy Grail that exoplanet hunters look for, since they carry vital information about the temperature, color and chemical composition of planets.

Planet or tiny cool star ?

Until Galileo Galilei’s first observations, the Earth had been the abode of humans, the stars the abode of gods, while the planets were their messangers. But astronomers then installed
the Earth among the planets, and the Sun among stars. These two families of objects were considered to be very different. Stars are sufficiently massive to trigger thermonuclear reactions, which explains their light. Moreover, they are the producgt of the collapse of interstellar gas. The formation of planets in a solar system is a more complex process, involving phenomena which take place in a circumstellar disc. Firstly, dust grains agglomerate to form solid planets like the Earth. After that, the largest solid planets accrete gas to form the atmospheres of giant planets like Jupiter. With the discovery of of exoplanets, the distinction between tiny stars and planets has become less clear.
We now know that certain massive planets can have nuclear reactions in their cores,
while the mass of certain stars formed by the collapse of interstellar matter is too low to trigger thermonuclear reactions. In the 10 - 30 Jupiter mass range, the populations are mixed. There also exist isolated objects in interstellar space which could be true planets expelled from a planetary system or tiny stars formed in situ : we cannot at present decide between the two possibilities.

1000 and one planets, of all kinds

We now know that a large fraction of the stars in the Galaxy is accompanied by planets.
Most of the planets discovered so far are close to their parent star, perhaps because these are the ones which are easier to detect. But in the very diverse planetary community, several are almost earth-sized, and a few are at a distance from their star which
which would allow liquid water to exist on the planetary surface. It is clear that an underlying motivation in the hunt for exoplanets is the search for life outside the Solar System.
That is why the discovery of a planet around a solar-type star in 1995 is generally considered to be a landmark discovery, in its way more important than the discovery in 1992 of planets around a pulsar.
Today, we are still not able to detect planetary systems identical to ours. However, such a discovery would be fascinating, since the environment of such a system could be sufficiently stable to allow the development of life, in contrast to systems in which the planetary orbits are unstable.

The future

Exoplanetary science would appear to have a great future. Several large
instruments and telescopes are being completed :

  • the ESPRESSO spectrograph at the VLT will start, in 2016, to make very
    accurate measurement of stellar "wobles,
  • the ELT-MICADO and ELT-PCS will be installed at the upcoming 39 m ELT
    telescope,
  • the ESA space mission Gaia (to be launched on 20th of November 2013)
    will discover thousands of planets in the years
    following the launch,
  • the ESA mission Cheops (to be launched in 2017) will observe up to
    several hundred planets,
  • the NASA mission TESS (to be launched in 2017-2018)
  • starting in 2018 the NASA/ESA space mission JWST will observe and
    presumably discover several tens of planets.

Other space missions (Echo, Plato) are still under study by ESA for a possible launch around 2020. Finally a large community of over 600 astronomers has recently proposed to ESA that exoplanets should be the scientific theme of a large mission to be launched
around 2030.

Contact

  • F. Roques
    Observatoire de Paris - LESIA - CNRS
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