Artistic representation of a so-called super-earth, Gliese 876 d Photo: Trent Schindler in the public domain
Read on In search of a possible second Earth, astronomers are constantly discovering new planets outside our solar system thanks to better and better space telescopes: 450 of these so-called exoplanets have already been discovered, and since 1995 knowledge of alien planets has exploded. However, the ultimate goal of exoplanet research remains the answer to the old question, "Are we alone?" Explains astronomy professor Sarah Seager of the Massachusetts Institute of Technology (MIT). Since the mid-1990s, the scientist has been working on extrasolar planets and is today regarded as a pioneer in this field. "COROT" and "Kepler" are currently the stars among the space telescopes. Since the end of 2006, COROT has been on the road for the French space agency CNES to find new planets in the depths of the universe. The US space agency Nasa has launched Kepler into space in March 2009. Both have their sights set on tens of thousands of sun-like stars and are looking for periodic brightness variations that suggest planets.

It is becoming increasingly clear that the variety of manifestations of exoplanets is even more fantastic than in the wildest sci-fi scenarios. Around the neighboring stars of our sun circle the strangest exotics. Whether fluffy ultra-dense gas balls, lava-covered super-earths with clouds of vaporized rock, or pure water worlds: "Exoplanets seem to be capable of anything the laws of chemistry and physics allow, " Seager speculates in a July report - Edition of the science magazine "bild der wissenschaft".

In the past, astronomers have been able to determine the diameter, calculate the density, and draw conclusions about the internal composition. In the meantime, what has been considered impossible ten years ago has even been successful: Researchers have light from several dozen hot Jupiter - gas gasses heated to 1, 000 to 2, 000 degrees Celsius, which orbit their suns at a very small distance - and collected by some more distant planets.

The largest attraction among the exoplanets is currently the rock ball Corot-7 b. It has about the same density as the earth and probably consists mostly of silicate rock. The most exotic is their atmosphere of vaporized rock. "Instead of a cloud of water from which it rains, rock clouds form, from which pebbles fall, " Bruce Fegley of the University of Washington speculates in "Bild der wissenschaft." display

Corot-7 b is one of about 20 previously known super-earths. The mass of these planets is about two to ten times the mass of the earth. Giant rock or cannonball planets made of pure iron could be underneath. The coat of some could consist of pure carbon. "Then there should be a diamond layer under its surface, " speculates Sara Seager. "But probably these planets would be hard to identify because their density resembles that of Earth-like silicate planets." Plenty of water - in the form of ice in special high-pressure versions - is believed to exist on the surface of GJ 1214 b, the second super-Earth, whose density was determined.

The more than 60 known hot Jupiter prove to be extremely complex and diverse. Especially the birds of paradise among the gas balls, Wasp-12 b and Kepler-7 b, have an extremely low density, such as cork or polystyrene. Such lightweight giant balls are unlikely to exist on popular planetary models. "Pumping up these planets is one of the biggest puzzles in exoplanet research, " admits Sara Seager.

The many unexpected constellations in the foreign solar systems have relativized the previous theories on the formation of planetary systems. "The architecture of our solar system is not omnipresent, and probably not even common, " explains Seager. "Near a young star, there is not enough material to form a giant planet. So the hot Jupiter must have formed further out and eventually moved in. "Also, many planetary orbits are not properly in a plane perpendicular to the axis of rotation of the star. Exoplanets have lanes with all possible angles of inclination. They run around to strongly elliptical, as it was previously known only by comets. According to the latest findings, the orbit of almost a quarter of all hot Jupiter runs counter to the direction of rotation of their star.

To find a sister planet of the earth and life in the universe is still future music - but quite realistic. And who knows: Maybe even within the next ten years there will be successes to celebrate - even before 2020, the NASA planned super-telescope "Terrestrial Planet Finder" at the start. "A terrestrial planet is the holy grail of exoplanet research, " enthuses Sara Seager. "Humanity will always look for a second home."

By ddp correspondent Gunnar Henze

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