Most researchers are certain that there must be extraterrestrial life somewhere out there in the universe. For statistical reasons alone, this is more than likely, after all, there are an estimated 100 billion galaxies in space with 70 trillion (a 7 with 22 zeros) visible stars, as Australian astronomer Simon Driver has calculated. These in turn are orbited by an even larger number of planets.
Since the two Swiss researchers Michel Mayor and Didier Queloz presented evidence of the first exoplanet in 1995, astronomers have been using ever better methods to search for planets that orbit in the "habitable" zone around their central star. At a distance where the temperatures are not too hot but not too cold either, where there is liquid water and carbon and a life-friendly atmosphere exists, in short: where life could develop.
The number of exoplanets discovered has now exploded, to 5,569 (as of January 5, 2024). And there are more and more of them almost every day, also thanks to the James Webb Space Telescope, which has been providing images and data in previously unknown resolution since July 2022. It's high time to bring order to this planetary zoo. Because we humans understand the strange best when we compare it with the familiar, the researchers have classified the exoplanets according to their similarity to the planets in our solar system.
Hot Jupiters In our solar system, Jupiter is by far the largest planet. It's so big that it could fit all the other planets from Mercury to Neptune. Jupiter's orbit is quite far from the sun, so it is cold there. When researchers first discovered a gas giant that orbited much closer to its star than the previously known gas planets, they were surprised.
It is so hot there that some of their atmosphere evaporates. Exoplanet researchers therefore call them “Hot Jupiters”. 51 Pegasi b was the first planet in this category and the first exoplanet ever, discovered in 1995. It orbits its star in just four days and is around 1000 degrees hot.
Ice Giants The counterpart to hot Jupiters are the ice giants. These are not found in our solar system either. They are much further from their star than our outermost planet Neptune. This means they are extremely cold and some of the gases they are made of freeze.
Mini-Neptunes are among the most common exoplanets that astronomers have identified to date. Mini-Neptunes are just over twice the size of our Earth, but are much lighter than our Earth because they consist mostly of gases. GJ 1214b was the first planet of this type, discovered in 2009. It orbits a small, red star that is around 42 light-years away from Earth. While diamonds are a rare commodity on Earth because they can only be formed at very hot temperatures and high pressures inside the Earth, on mini-Neptunes they probably rain from the sky. Scientists from several German research institutes recently came to this conclusion after studying in the laboratory the atmospheric conditions under which diamonds can form. Until then, it had been assumed that gems only rained on gas giants.
Super-Earths are the most promising candidates in the search for extraterrestrial life. The inner four of the eight planets in our solar system (Mercury, Venus, Earth, Mars) are all rocky planets, meaning they have a solid surface. Earth is the largest among them and yet it is rather small compared to many other rocky planets in the universe. Super-Earths are planets with a solid surface and a weight that can be many times greater than that of our Earth.
The heaviest super-Earth discovered to date is 18 times heavier than our Earth. Rocky planets are so interesting because they could have liquid water, which in turn would be the prerequisite for life in the forms we know. Around 35 planets have now been identified that could be similar to Earth. It sounds like a little, but it's quite a lot, says the Austrian astrophysicist Lisa Kaltenegger. With current telescopes, Earth-sized planets are very difficult to discover, if at all. This mainly has to do with their comparatively small size and the common methods for detecting them.
One method for finding exoplanets is the so-called dimming method. Telescopes measure how much a star darkens when a celestial body passes by it. Compared to the Sun, the Earth is so small that it only shades a tiny fraction of the sunlight. With current telescopes we would not be able to detect them even from great distances.
Things look different when the size ratio between star and rocky planet is somewhat more balanced. The known super-Earths therefore mostly orbit red suns, which are significantly smaller than our own yellow sun. In order to find smaller earths, even more powerful telescopes and longer observation times are needed than before.
In 2009, researchers discovered a possible super-Earth in the habitable zone for the first time: Gliese 581 d is located in the constellation Libra and, at just 20 light-years away, is quite close to us by astronomical standards. The planet receives less starlight than Mars does from the sun, but with enough greenhouse gases in the atmosphere it would be quite hospitable to life.
In 2013, the Kepler space telescope detected two candidates on which life could potentially exist: Kepler-62e and Kepler-62f. Both are only about one and a half times as large as Earth and travel in the habitable zone around their star.
Around a dozen rocky planets have now been identified that are smaller than two Earth radii and orbit around their star in the life-friendly zone. Among them is one, Proxima Centauri b, just four light-years away from us. Unfortunately, it is impossible to explore its atmosphere for traces of possible living creatures. To do this, as seen from Earth, it would have to pass in front of its star Proxima Centauri - which is not the case. Proxima Centauri b would still be an interesting target for an unmanned mission, says Kaltenegger.
Lava Worlds Some rocky planets fly so close to their star that their surface is made of molten rock. They are therefore called lava planets. The first of its kind was identified in 2009. It is called CoRoT-Tb and is located in the constellation Unicorn. One and a half times the size of the Earth, CoRoT-Tb orbits its star every 20 hours, so a year on it is shorter than an Earth day. Seas of glowing lava swell on its surface, which is more than 1,000 degrees hot.
Steppenwolf Planets They are the most bizarre of all the planets. Normally planets orbit in fixed orbits around one star, and in rare cases around two stars. But this dogma was no longer tenable when the first planet appeared that flew through space all alone without a star. What made him (and several others who have since discovered) do this can only be guessed at. However, it is very likely that they were thrown out of their original orbit by a collision with another celestial body and catapulted into space. The Austrian astrophysicist Lisa Kaltenegger calls them the “Steppenwolf planets”.
And further? With the James Webb Space Telescope, which was launched in 2021, exoplanets can be observed much more precisely than with all previous measuring devices. Whether researchers will actually find a second Earth remains an open question. What is certain, however, is that the colorful zoo from foreign worlds is likely to expand to include many exciting objects.