Usually, low -mass red dwarf stars are more or more circled by smaller rock planets – the mass of their common clouds is not enough for larger gas giants. But now astronomers have discovered a dwarf star around 240 light years away, which is only 0.2 of the sun masses, but has a gas giant as a companion. The exoplanet toi-6894b is slightly larger than the Saturn and around half as heavy as this. His little mother star is the lowest dwarf star with such a gas giant, as the team reports. It is still unclear how this gas planet can have arisen because some scenarios are plausible, but none can completely explain the formation of the gas giant. The astronomers hope to be able to solve this puzzle through upcoming observations with the James Webb telescope.
Three quarters of all stars in our Milky Way are red dwarfs – cool, light -shaped stars with a significantly lower mass than our sun. They are considered to be particularly promising candidates for planetary systems with earth -like worlds. According to common theory, such dwarf stars mainly create smaller rock planets similar to our earth, because planets are created in the same cloud of dust and gas, which also produced its central star. The amount of material in such a accretion disc therefore determines how big the star can be and how much is left for its planets. If the material is only enough for a dwarf star, common theory forms only small rocks in his orbit. Examples of such red dwarfs with smaller planets are, for example, the 40-year-old star Trappist-1 with its seven earth-like planets or our neighboring star Proxima Centauri. In recent years, however, astronomers have discovered a few red dwarfs that, despite their small mass, are or even circled by one or even more gas giants. But whether these are only extreme outliers, how these planets came about and how often such gas giants around dwarf stars occur, has so far been unclear.
Powerful star with a large gas planet
Now Astronomers around Edward Bryant from the University of Warwick have discovered another of these extreme pairings in Great Britain. For your study, you had evaluated data from the Transiting Exoplanet Survey Satellite (Tess) of NASA for more than 91,000 low red dwarfs. The space telescope is designed to record stars from stars and thus track down possible transit events – passages of exoplanets in front of your star. The astronomers found it in the approximately 238 light years away toi-6894. The light curve of this red dwarf showed a regular, clear shading every 3.3 days – an indication of a larger, the star -surrounding planet. On top of that, Bryant and his team targeted this star with several earth -based telescopes, including the very large telescope of the European Southern Sternharte in Chile.
The new observations confirmed that the red dwarf toi-6894 is circled by a planet. The exoplanet toi-6894b is around 53 earth masses and has a good mass of the Saturn, but is slightly larger than this. “According to our analyzes, TOI-6894B is a planet with a very low density,” explain the astronomers. It is therefore a gas giant. What is special about this planet is which star he circles around. Because the red dwarf toi-6894 only has 0.2 solar masses and only 20 percent of the size of our sun. “The Gas giant TOI-6894B circles the muffled star with such a large gas planet,” explains Bryant. “We did not expect planets like TOI-6894B to be created by stars with such a small mass.” Because in the formation of planetary formation through the so -called core acretion, a gas giant must first put on enough dust and chunks to form a massive solid core. Its gravity then attracts large gas amounts from which the thick gas cover of these planets forms. “In the case of low -mass stars, however, the main hurdle lies in the limited amounts of solid material in their protoplanetary disc,” explain the astronomers.
How did TOI-6894B come about?
Gas giants around very low-mass stars such as TOI-6894 are therefore difficult to explain by this scenario of planet formation. One possibility would be that TOI-6894B did not accumulate enough core mass in order to quickly attract large gas amounts. “Instead, the planet could have arisen from an intermediary core acceleration process, in which a protoplanet accumulates gas steadily without the core being massive enough for uncontrolled gas ability,” explains Bryant. With this scenario, the planet is slowly growing up by pulling heavy elements and gas out of the accretion disc on the same time. Such an intermediary origin is discussed for smaller gas giants with less mass than Saturn. “However, both classic core acretion and the sub-natural formation require a sufficient mass of heavy elements in the protoplanetary disc,” the astronomers write. According to models, the red dwarf toi-6894 should therefore have at least 120 earth masses of solid material in its primeval cloud in order to form the twelve earth mass of its gas planet. It is unclear whether that was the case.
Alternatively, the Gas giant TOI-6894B could also have been created by the gravitative collapse of gas and dust in part of the accretion disc. This planet formation is accepted for some mass pond gas giants, and Jupiter in our solar system could also have arisen in this way. “Simulations suggest that this mechanism also represents a plausible educational path for TOI-6894B,” write Bryant and his team. However, none of the theories discussed could completely explain the formation of this exoplanet. “This system represents a new challenge for models of planet formation and offers a very interesting goal for successor observations,” explains co-author Andrés Jordán from the Millennium Institute for Astrophysics in Santiago. The astronomer has already applied for observation time at the James-Webb-Wirraum Telescope to examine TOI-6894B and its gas cover with the high-resolution infrared spectroscopes of the telescope. The composition of the atmosphere can provide more information on how gas cover and core are composed and thus allow conclusions to be drawn about its formation.
Source: Edward Bryant (University of Warwick, UK) et al., Nature Astronomy, DOI: 10.1038/S41550-02552-4
