The smaller and lighter an exoplanet is, the more difficult it is to track it using general methods. Astronomers have now discovered one of the lightest and smallest known rocky exoplanets about 31 light years away. The planet GJ 367b is the size of Mars and only half the mass of Earth. From the mass-to-volume ratio, the researchers concluded that the exoplanet’s iron core should occupy most of its interior – similar to Mercury’s, the core makes up 80 percent of its diameter. However, GJ 367b is not friendly to life: because it orbits near its star, a red dwarf, its surface temperature reaches 1,500 degrees.
Since the discovery of the first exoplanets in the 1990s, astronomers have tracked thousands of other worlds around exotic stars. Some are revealed by the effect of gravity on their star, others make themselves visible by the fact that they pass right in front of their star and thus darken periodically. In general, large exoplanets close to their stars are easier to find using this method than smaller planets. Rocky planets the size of Earth or even smaller, which are of great interest to astronomy, are difficult to detect, because they only exert a low gravitational effect on their stars and with bright stars, transits are often difficult and difficult to see in light curves.
Telltale wanders around the red dwarves
But red dwarfs – a very common type of star, especially in our near cosmic environment, provide a better chance of tracking such a small object to a planet. These relatively cool stars glow faintly at less than 60 percent of the Sun’s radius. “Transit signals for planets orbiting M dwarf stars are relatively larger than for planets of the same size around a Sun-like star,” explained Christine Lamm of the Technical University of Berlin and colleagues. This opens up the opportunity to identify even very small planets orbiting near their stars. Astronomers have discovered such a sub-Earth candidate with the help of the Transiting Exoplanet Survey (TESS) just 31 light-years away.
Observations suggest that there may be a planet around the nearby red dwarf GJ 367 with a size of just 0.75 Earth radii.
To investigate these suspicions and learn more about these potential exoplanets, Lamm and his team evaluated more TESS data and also observed planetary candidates and their stars using the HARPS spectrometer from the Southern European Observatory’s La Silla Observatory in Chile. “We run several tests to make sure this candidate is not a false positive,” they explained. But new observations confirm that the glow of red dwarfs regularly darkens every 0.32 days. From the duration and shape of the descent of this light curve, astronomers concluded that this shadow must have come from a planet.
Small, dense and very hot
As the analysis shows, there is a planet around the red dwarf with a diameter of about 9,000 km – barely bigger than Mars. With only about 0.55 Earth masses, GJ 367b is also one of the lightest exoplanets known to date. “From precise radius and mass determinations, we conclude that GJ 367b must be a rocky planet,” Lamm said. “It belongs to the terrestrial sub-earth and takes us one step further in our search for a second earth.” In terms of climate, GJ 367b is not exactly like Earth. It orbits its star in a very close orbit and takes only 0.32 days for one orbit – so a year in it only takes about eight hours. Although its central star is relatively cold and dim, the planet receives 576 times more radiation from its star than Earth. As a result, the temperature on the daytime side is almost 1500 °C. “Hot enough to melt or vaporize metallic iron and silicates,” the astronomers explained.
From the mass and size of GJ 367b, the researchers were also able to determine the planet’s density. This is 8.1 grams per cubic centimeter, which is much higher than other exoplanets of the same size. “The higher density indicates that the planet is dominated by an iron core,” explains Lamm’s colleague Szilard Sismadia. According to the team’s calculations, this mineral core could occupy 85 percent of the planet’s radius. “In this respect it resembles Mercury, which distinguishes iron and nickel disproportionately from all other celestial bodies in the solar system.” The circle has not yet been described.
Cowell: Christine Lamm (TU Berlin) et al, Science, doi: 10.1126/sains. aay3253
–
