GJ 367b is only 31 light years away (illustration)
Photo: SPP 1992 / Patricia Klein / REUTERS
Berlin researchers have tracked down one of the smallest planets that has so far been discovered outside of our solar system.
GJ 367b has to be imagined as a red-hot world that is slightly larger than Mars and only half as heavy as Earth.
The exoplanet rotates around its star at lightning speed - after eight hours the year is over.
Much is extraordinary about the planet, not just the surface temperature of 1500 degrees Celsius.
It consists almost entirely of iron.
"Our model of the internal structure shows that the iron core makes up around 86 percent of the planet," write Kristine Lam from the Institute for Planetary Research at the German Aerospace Center (DLR) in Berlin and her colleagues.
That is why its crust and most of its rock mantle has probably evaporated and only the core, which is mainly made of iron, remains, according to the scientists in the journal "Science".
At 31 light years from Earth, the unusual planet is comparatively close.
It orbits a red dwarf star, which is smaller, cooler and less luminous than our sun, and only needs 7.7 hours.
So one day on earth corresponds to three years on FY 367b.
The researchers call this "ultra-short-period".
Lam and her colleagues tracked GJ 367b with the US satellite observatory TESS.
The space telescope searches for periodic fluctuations in the brightness of stars, triggered by planets orbiting the star and regularly passing in front of it when viewed from Earth.
Like the planet closest to the Sun, GJ 367b is also exposed to strong radiation.
It is true that the red dwarf star, at 3800 degrees Celsius, is significantly cooler than our sun, which has a surface temperature of around 6000 degrees Celsius.
But due to the extreme proximity to its star, the radiation on the exoplanet is 500 times stronger than on Earth.
Red dwarf stars and the search for planets
With the help of further observations at the European Southern Observatory, the astronomers determined the mass of GJ 367b. Because with its gravitational pull a planet pulls at its central star and puts it in a slight movement. This movement can be detected by means of small changes in the spectrum of starlight - and researchers can then calculate the mass from the strength of the movement. From size and mass, the density of the planet finally results at 8.1 grams per cubic centimeter, which roughly corresponds to the density of iron.
Red dwarf stars are of particular importance to astronomers when searching for planets.
For one, it is the most common type of star in the Milky Way.
On the other hand, planets can be more easily detected there due to the lower brightness of the star.
On average, the researchers estimate, each of these stars orbits two to three planets - many of them in very narrow orbits similar to GJ 367b.
"So far we don't know anything about the formation of such extremely short-period planets," explains Lam.
"But by determining the exact properties of such planets, we get an insight into the formation and development of such systems."
fww / dpa