A year lasts only 16 hours: astronomers have discovered an extrasolar gas giant that orbits its star closer than any other. The “hot Jupiter” is only 2.6 million kilometers away from its star and only needs 0.6 days for one orbit. This proximity has consequences: TOI-2109b is not only the second hottest planet ever discovered – its orbit is also shrinking faster than any other known exoplanet due to the stellar attraction.
Hot Jupiter are the extremists among the exoplanets: These gas giants orbit their mother stars so closely that conditions are hellish on them. At surface temperatures of up to 4,000 degrees, even metals such as iron or titanium evaporate and molecules are torn into their atomic constituents. Some of these planets are inflated and bulged by the gravitational pull of their star, and the orbit of some of these gas giants is also becoming shorter and shorter due to the star’s gravity.
Astronomers working with Ian Wong from the Massachusetts Institute of Technology (MIT) have now discovered a particularly hellish representative of these extremists. When observing a sun-like star around 860 light-years away with the TESS space telescope, they noticed periodic fluctuations in the brightness of the star – evidence of the transit of a planet in front of the star. Additional observations with earth-based telescopes confirmed this: The star TOI-2109 has a planet.
“But we noticed that we were dealing with something very exciting and relatively rare,” says co-author Avi Shporer from MIT. The light curves suggested that there a gas giant about 1.3 times the size of Jupiter and five times Jupiter’s mass orbits its star very closely – it was therefore a hot Jupiter.
Shortest orbit and second highest temperature
What is unusual, however, is how closely the gas giant orbits its parent star: it only takes 0.6 days to complete one orbit – it only takes around 16 hours a year. “TOI-2109b has the shortest orbital time of all gas giants discovered so far,” report the astronomers. No other exoplanet of this size comes within 2.6 million kilometers of its star. This corresponds to just 0.017 astronomical units and thus one hundredth of the distance between the earth and the sun.
This has consequences: Because the nearby star is around 50 percent larger and more massive than our sun, the TOI-2109b receives a correspondingly large amount of radiation and heat. The astronomers determined that the temperature on the day side of the bound rotating planet is around 3,650 Kelvin – around 3,350 degrees Celsius. This makes it the second hottest exoplanet ever discovered, after KELT-9b.
“Ultra-hot Jupiters like TOI-2109b form the most extreme subclass of exoplanets,” explains Wong. “We are only just beginning to understand the unique chemical and physical processes that take place in their atmospheres. Because there is nothing like it in our own solar system. “
Planet spirals closer and closer to the star
The close proximity to the star has other consequences, however: The researchers suspect that the star’s strong gravitational pull is already clearly deforming the planet and its gas envelope bulges, especially along the equator. Without its high density, TOI-2109b might have been torn apart by the extreme forces long ago. “The large mass of the planet ensures that, despite its extremely narrow orbit, it is not directly threatened by tidal disruption,” explain Wong and his team.
In return, TOI-2109b threatens a different danger: The enormous pull of its star draws the planet closer and closer and thus shortens its orbit by up to 740 milliseconds per year. That doesn’t sound like much, but it is the highest rate of orbital decay that has ever been measured in an exoplanet, as the astronomers report. Even the gas giant WASP-12b, discovered in 2020, is spiraling more slowly towards its star.
“In our lifetime we will not see TOI-2109b fall into its star,” says Wong. “But in ten million years the planet could be gone.”
Still a mystery
Astronomers hope that further observations will reveal even more about the features and fate of the unusual exoplanet TOI-2109b. This opportunity could be offered by the powerful optics of the Hubble space telescope, but also by future space telescopes such as the James Webb Space Telescope, which will be launched in December 2021.
These observations may then also provide further clues as to how the hot Jupiter got so close to their stars in the first place. Because according to common assumption, large gas planets tend to form in the outer areas of the protoplanetary disks. “From the very beginning of exoplanet research, the hot Jupiter were eccentric,” explains Shporer. Because it is still completely unclear how these massive and large planets got into their current orbits. (The Astronomical Journal, 2021; doi: 10.3847 / 1538-3881 / ac26bd)
Quelle: Massachusetts Institute of Technology