The hunt for planets

Direct observation of extrasolar planets is not yet possible. But this could soon change

"So the only one is the sky, the immeasurable space. … In it are visible numerous stars, celestial bodies, globes, suns and earths … And countless others must reasonably be amed to exist. … We see only the suns, which are the coarser, even the roughest bodies, but not their earth bodies or planets, which, because their masses are much smaller, are invisible to us. … According to this, there is not one single world, one single earth, one single sun, but as many worlds as we see luminous sparks above us." this was not written by a contemporary astrophysicist as a plea for the existence of inhabitable worlds beyond our own solar system, but by giordano bruno in 1584 in his work de l’infinito, universo e mondi (on the infinite, the universe and the worlds).

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The exoplanet hd 209458b could look like this or similar, image: esa/alfred vidal-madjar (institut d’astrophysique de paris, cnrs)

The ideas of giordano bruno, with his abandonment of the geocentric view of the world, frightened the church so much that on 17. February 1600 burned as heretics on the campo de’ fiori in rome. The vatican has not yet rehabilitated the philosopher.

In the meantime, one may openly claim that the earth is not the center of the universe, no one seriously disputes this fact any more. And our planet is demonstrably not the only one in space. In the last ten years already 135 generally recognized exoplanets were discovered, whereby still around the definition of the term "exoplanet" there is a controversy "planet" is argued. Even the ninth planet of our solar system is meanwhile disputed, many astronomers call pluto for a long time a kuiper-gurtel object (heibe times on quaoar).

Conventionally, a planet is a celestial object that is too small to ignite nuclear fusion, is roughly spherical in shape and orbits its star directly. In addition to these minimum requirements, there are often sophisticated additional elements such as minimum coarseness, surface activity, or atmosphere, in order to z. B. To exclude asteroids and comets (cf. Planet definition). Meanwhile there are new proposals of short definitions like: "a planet is a spherical object, which cannot sustain fusion, in orbit around a star or brown dwarf." (gibor basri). There is no agreement yet, the discussion continues.

How to discover exoplanets?

The first extrasolar planet was discovered in 1995 using the so-called radial velocity method (in the hunt for extrasolar planets). The gravity of a planet causes its parent star to wobble a bit, that is, the star changes its radial velocity and these shifts can be measured and determined. Indirectly, the orbit of the satellite orbiting around it can be determined. However, this method does not allow the mass of the invisible object to be determined precisely; moreover, it must have a minimum size.

That is why so far mainly rough planets, similar to jupiter, have been tracked. So no candidates for life. However, astronomers are increasingly succeeding in refining the radial velocity method to find even smaller objects (cf. We are ready to discover earthly worlds!).

The second method is the observation of a transit, which succeeded for the first time in 1999 (cf. Detection of planetary transits across a sun-like star). If a planet passes in front of its star in such a way that it obscures part of the light as seen from us, then the drop in brightness can be measured during this small eclipse. In this method, the diameter of the satellite can be measured and its density determined.

Direct view of an exoplanet

So far only glimpses of the tumbling of the mother star or the shadows of extrasolar planets have been achieved. Earth’s sisters are too far away from us and too dark to be observed directly. Only brown dwarfs (astronomers determine for the first time fundamental properties of two astral birds of paradise) already showed up directly.

An up-close view of an exoplanet is, of course, what astronomers dream of. In the current ie of nature, adam burrows of the university of arizona summarizes the theoretical possibilities of a direct detection of extrasolar planets. If it was possible to image the light reflected from a planet, the spectrum could be analyzed and thus information about the chemical composition of the atmosphere could be obtained.

Die jagd auf planeten

Extrasolar planets are found all over the sky, image: harvard/korzennick

Already in the past burrows has intensively dealt with such theoretical analyses of the radiation of extrasolar planets (cf. Theoretical spectra and atmospheres of extrasolar giant planets and spectra and diagnostics for the direct detection of wide-separation extrasolar giant planets). A prerequisite would be an imaging technique that is capable of capturing extreme contrasts or. To filter accordingly. If it is possible to superimpose the light of a star, for example by interconnecting several telescopes, and to extinguish it by the resulting interference, the faint dimming of a planet could be observed directly. The over-radiating light of the central star has thus been blocked out.

This has not yet been achieved for planets, but the us astronomer is confident that it will soon be achieved through better and better instruments. Upcoming projects such as the large binacular telescope (lbt) on earth or corot in space open up promising perspectives.

Then a direct look at the chemical structure and evolutionary secrets of planets in other solar systems will be possible, as burrows writes: "over the next twenty years, there will be a growing crescendo of new results about extrasolar planets that will completely change our views about the nature of planetary systems."