Science fiction stories from the last century that take place on alien planets should not hide this from us: just thirty years ago, no one had any idea whether there were alien planetary systems similar to ours and what they would look like.

But if one does not know the foreign, it is natural to assume that it must resemble the familiar.

But this often-used strategy works poorly for planets orbiting distant suns—as was demonstrated as early as 1995, when the first exoplanet orbiting a main-sequence star was discovered.

This planet, dubbed 51 Pegasi b, surprised scientists as a gas planet that takes only about four days to orbit its host star.

This demonstrated: The order of our solar system, in which there are rocky planets on the inside and gas planets on the outside,

Sibylle Anderl

Editor in the feuilleton, responsible for the "Nature and Science" department.

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Since then, the assumption that our own solar system cannot claim to be a model in the cosmos has been substantiated.

More than 5000 exoplanets are now known, and yet we are far from a complete understanding of planet formation and a comprehensive characterization of the existing systems.

This is also due to the fact that the observation of distant planets is very challenging, as they are massively outshined by their suns.

It is favorable here when a planet passes in front of its star and temporarily darkens it: Not only its existence can then be derived from this darkening, but also its size and, in principle, even its chemical composition from the light shining through its atmosphere.

This works particularly well when the difference in size between planet and star is comparatively small and when the planet eclipses the star in the shortest possible time intervals, i.e. moves closely around the star.

Attractive observation target for exoplanet researchers

These two points make red dwarfs, the smallest stars still capable of hydrogen fusion, a particularly attractive observation target for exoplanet researchers.

Planets orbiting these cool dwarfs are not only easy to observe, but are also temperate enough to support liquid water, a key attribute for expecting habitable conditions there.

But if you want to know whether these distant worlds actually have all the preconditions for the emergence of life, you have to determine additional information about their composition beyond the expected solar radiation.

The Spanish astronomers Rafael Luque and Enric Pallé have now taken on this.

They have compiled observations for 34 exoplanets orbiting red dwarf stars which, based on their dynamically determined masses, also allow the density of these celestial bodies to be determined in addition to the radius.

The result published in the current issue of the journal "Science": The planets examined fall into three different groups.

The first group includes rocky planets with Earth-like composition.

The second has a density equivalent to a 50/50 mixture of water ice and rock with no gas to speak of.

And finally there are gas planets with larger masses and lower densities.

The result was astonishing for the researchers,

Migration of planets not uncommon

The newly discovered water-rich planets probably originated in the outer regions of their solar systems, since it is too dry within the so-called ice line near the central star due to the high temperatures there.

From there, however, they must have moved close to their home stars over time - at least that is the scenario preferred by the authors of the study.

Such migration of planets does not seem to be uncommon and already explained at 51 Pegasi b why a gas giant can be observed so close to its star.

However, there are still many unanswered questions and ambiguities regarding the formation of planets.

As is so often the case, hopes rest on future observations and in particular on the analysis of the planetary atmospheres, which should further help to refine the existing models.