The smaller and lighter an exoplanet, the more difficult it is to track down using common methods. Astronomers have now discovered one of the lightest and smallest known extrasolar rock planets around 31 light years away. The planet GJ 367b is about the size of Mars and only half the mass of the earth. From the ratio of mass and size, the researchers conclude that the iron core of this exoplanet must occupy most of its interior – similar to Mercury, the core makes up a good 80 percent of the diameter. However, GJ 367b is not life-friendly: Because it orbits very close to its star, a red dwarf, its surface temperature is a good 1500 degrees.
Since the discovery of the first extrasolar planet in the 1990s, astronomers have tracked down thousands of other worlds around alien stars. Some of them were betrayed by their gravitational effect on their star, others made themselves noticeable by the fact that they passed directly in front of their star and thereby periodically darkened it. In general, however, larger exoplanets close to their star are easier to find using these methods than smaller ones. The rocky planets of earth size or even smaller, which are particularly interesting for astronomy, are much more difficult to detect because they only exert low gravity effects on their stars and with bright stars the transit is often outshone and difficult to see in the light curve.
Telltale shadowing around a red dwarf
Red dwarfs – a type of star that is very common, especially in our close cosmic environment, offer better chances of tracking down such tiny planetary creatures. These relatively cool, fainterly glowing stars are less than 60 percent of the solar radius. “The transit signal of a planet orbiting such an M dwarf star is therefore relatively larger than that of a planet of the same size around a sun-like star,” explain Kristine Lam from the Technical University of Berlin and her colleagues. This opens up the opportunity to identify even very small planets that are orbiting close to their star. Astronomers discovered a candidate for such a sub-earth with the help of the Transiting Exoplanet Survey Satellite (TESS) just 31 light years away.
The observations suggested that there might be a planet around the nearby red dwarf GJ 367 with a size of only 0.75 Earth radii.
In order to investigate this suspicion and to find out more about this possible exoplanet, Lam and her team evaluated further TESS data and also observed the planet candidate and its star with the HARPS spectrograph of the La Silla Observatory of the European Southern Observatory in Chile. “We ran several tests to make sure this candidate wasn’t a false positive,” they explain. But the new observations confirmed: The light of the red dwarf is regularly darkened every 0.32 days. From the duration and shape of this depression in the light curve, the astronomers conclude that this shadow must have come from a planet.
Small, dense and quite hot
As the analyzes showed, there is a planet around the red dwarf with a diameter of around 9,000 kilometers – it is hardly larger than Mars. With only around 0.55 Earth masses, the GJ 367b baptized planet is also one of the lightest exoplanets known to date. “From the precise determination of the radius and mass, we conclude that GJ 367b must be a rocky planet,” says Lam. “It belongs to the terrestrial sub-earths and takes us one step further in the search for a second earth.” In terms of its climate, however, GJ 367b is not very earth-like. It orbits its star in a very close orbit and only needs 0.32 days for one orbit – a year on it therefore only takes around eight hours. Although its central star is relatively cool and faint, the planet receives 576 times as much radiation from its star as the earth. As a result, the temperatures on its day side are almost 1500 degrees Celsius. “It’s hot enough to melt or vaporize silicates and metallic iron,” explain the astronomers.
From the mass and size of GJ 367b, the researchers were also able to determine the density of the planet. It is 8.1 grams per cubic centimeter, which is significantly higher than that of other exoplanets of similar size. “The high density indicates that this planet is dominated by an iron core,” explains Lam’s colleague Szilard Csizmadia. According to the team’s calculations, this metal core could take up a good 85 percent of the planet’s radius. “In this it is similar to Mercury, whose disproportionately large iron and nickel core distinguishes it from all other terrestrial celestial bodies in the solar system.” circles is so far unexplained.
Source: Kristine Lam (TU Berlin) et al., Science, doi: 10.1126 / science.aay3253