HONOLULU — Three planets were recently discovered orbiting dangerously close to their host stars and nearing the end of their lives. At least one planet will be consumed by the star it orbits in less than one million years.
Before this discovery was made, there were no known planets that would be swallowed by their host star in such a short period of time. While one million years might seem like a long time, in astronomical terms, it is relatively short.
The discovery was announced at an American Astronomical Society press conference on Jan. 13, and a paper on the discovery will be published in the Astronomical Journal.
A team of astronomers made the discovery, including Daniel Huber, an astronomer at the University of Hawaii’s Institute for Astronomy.
The three giant gas planets were first detected by the NASA Transiting Exoplanet Survey Satellite Mission, which searches for planets outside of our solar system, including those where life might live, and originally launched in 2018.
“The remarkable thing about TESS is that it’s the first space-based telescope that is doing an all sky sweep, trying to find the nearest planets that are exciting,” said Huber.
Huber said it is most useful when TESS finds planets close to our solar system because it is easier to continue to study them. The three newly discovered planets — TOI-2337b, TOI-4329b and TOI-2669b — are located in the Milky Way about 2,000 light-years away from Earth.
After these three “exciting” planets were discovered by TESS, the researchers used the W. M. Keck Observatory on Mauna Kea to confirm the existence of the planets.
“We can make very precise ground-based observations that can help us to confirm that those planet-candidate signals that TESS is finding are actually real,” said Huber.
He said the telescope on Mauna Kea was also used to find the size and mass of the planets, which allows astronomers to determine the planets’ densities.
The three planets orbit their own individual stars and have very different densities, but what they have in common is that they all have fairly similar orbital periods that are short. It only takes these planets about 10 days to orbit their host stars; in our solar system, Mercury has the shortest orbit around the sun, taking about 80 days to complete a rotation.
The newly discovered planets are very large compared to the Earth — they are between 0.5-1.7 times the mass of Jupiter. The size of the planets ranges between being slightly smaller than Jupiter to more than 1.6 times the size of Jupiter. What is most interesting is that their densities range widely from being similar to cork to three times denser than water.
Currently, models show that planets spiral in toward their host stars as the stars evolve over time, especially in the last 10% of the star’s lifetime, but one aspect of these types of planets that is still not understood is related to atmospheric inflation. In other words, the radii of some of these planets are much larger than astronomers would expect, based on current models. Huber said this might be related to the amount of flux that the planets receive from being so close to their host stars, which heats the planets up. Still, another mystery remains since some of the newly discovered planets were inflated, while others were not.
“It’s still an open question,” said Huber.
Huber said learning about these other planets in different solar systems gives us a “window into the future of our sun. … These stars are sort of future versions of our sun.”
“In a few billion years from now, our sun will expand to become larger and become more luminous and these stars have already done that,” he added. “So they’re in this more advanced evolutionary phase.”
Over a long period of time, the stars in the study had transformed into red giant stars, which is when a star becomes cooler, expands and brightens. These types of stars are about four times as large as our sun.
The newly discovered planets that orbit these stars have experienced a change that the planets in our solar system might go through in a billion years. Eventually, the fate of these planets, as well as our own, will be that they will be swallowed by their host stars. This occurs as the host stars continue to swell, becoming 10 times or even 100 hundred times the size of our sun.
“At some point the star will become so large that it will engulf the planet,” said Huber. “The planet will be ingested into the interior of the star, and this is a process that is, of course, quite violent.”
One of the newly discovered planets — TOI-2337b — is orbiting so close to its host star that it will be consumed in less than 1 million years.
Huber said astronomers still hope to learn how often this swallowing of nearby planets happens. The researchers expect to find more stars like them in the near future with the use of TESS.
“We expect that we will find many more,” said Huber.
With more data, the astronomers will look for patterns that would inform their understanding of how planets interact with each other, inflate and orbit around stars.
“Perhaps the planets that are very inflated have some particular characteristics … and then we can start to piece together the puzzle,” said Huber.
The planets will also be observed by the recently launched James Webb Space Telescope. This will allow astronomers to look at the atmospheres of these planets, in particular whether they have water or carbon dioxide. If these molecules are observed, it would offer clues to whether stellar evolution changes the atmospheric chemistry of planets.