Astronomers Discover a Nearby Multiplanet System With Two Earth-Sized Planets

Astronomers from MIT and other institutions have identified a new multiplanet system in our galactic neighborhood. It's about 10 parsecs away from Earth, or roughly 33 light-years, making it one of the nearest multiplanet systems known to mankind.

A tiny and cold M-dwarf star dubbed HD 260655 is at the center of the system, and astronomers have discovered that it is home to at least two terrestrial, Earth-sized planets. The rocky worlds' orbits are quite close together, exposing them to temperatures too high to support liquid surface water. As a result, they're not likely to be livable.

Scientists are enthusiastic about this system, though, since the near closeness and brilliance of its star will allow them to examine the parameters of the planets and any traces of any atmosphere they may have.

“Both planets in this system are each considered among the best targets for atmospheric study because of the brightness of their star,” says Michelle Kunimoto, a postdoc at MIT's Kavli Institute for Astrophysics and Space Research and one of the discovery's main investigators. “Is there a volatile-rich atmosphere around these planets? And are there signs of water or carbon-based species? These planets are fantastic test beds for those explorations.” 

On June 15, 2022, during the American Astronomical Society conference in Pasadena, California, the team will present their discovery. Katharine Hesse, George Ricker, Sara Seager, Avi Shporer, Roland Vanderspek, and Joel Villaseor are among the members of the MIT team, which also includes colleagues from throughout the world.

Data power

The new planetary system was discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), an MIT-led project that monitors the closest and brightest stars for periodic dips in brightness that might indicate a passing planet.

Kunimoto, a member of the TESS scientific team at MIT, was watching the satellite's incoming data in October 2021 when she spotted a pair of periodic dips in brightness, or transits, from the star HD 260655.

She put the signals through the mission's science inspection process, and they were quickly categorized as two TESS Objects of Interest, or TOIs, which are probable planets. The Science Processing Operations Center (SPOC), the official TESS planet search pipeline situated at NASA Ames, discovered the same signals independently. Scientists usually seek to confirm that the objects are truly planets using other telescopes.

The classification and subsequent confirmation of new planets might take several years. With the use of archive data, the procedure for HD 260655 was greatly reduced.

The Keck observatory domes atop Mauna Kea 

Soon after Kunimoto discovered the two possible planets around HD 260655, Shporer checked to determine whether the star had been seen before by other telescopes. HD 260655 was discovered by chance in a survey of stars conducted by the High Resolution Echelle Spectrometer (HIRES), which is part of the Keck Observatory in Hawaii. The researchers were able to acquire the survey's publicly available data since HIRES has been watching the star, along with a slew of other stars, since 1998.

HD 260655 was also identified as part of a separate scan conducted by CARMENES, an instrument based at the Calar Alto Observatory in Spain. Because these data were confidential, the team reached out to members of both HIRES and CARMENES in order to pool their resources.

“These negotiations are sometimes quite delicate,” Shporer notes. “Luckily, the teams agreed to work together. This human interaction is almost as important in getting the data [as the actual observations].”

Planetary pull

After the end, in roughly six months, this collective effort verified the presence of two planets near HD 260655.

The researchers checked the star's HIRES and CARMENES data to ensure that the signals from TESS were definitely from two circling planets. Both surveys assess a star's radial velocity, which is also known as its gravitational wobble.

“Every planet orbiting a star is going to have a little gravitational pull on its star,” says Kunimoto. “What we’re looking for is any slight movement of that star that could indicate a planetary-mass object is tugging on it.” 

The researchers discovered statistically strong indicators that the signals recorded by TESS were definitely two orbiting planets from both sets of archival data. 

“Then we knew we had something very exciting,” Shporer adds.

The scientists then examined the TESS data more thoroughly to determine the parameters of both planets, such as their orbital period and size. The inner planet, named HD 260655b, circles the star every 2.8 days and is roughly 1.2 times the size of Earth, according to the researchers. HD 260655c, the second outer planet, circles every 5.7 days and is 1.5 times the size of Earth.

The researchers were able to compute the planets' mass using radial-velocity data from HIRES and CARMENES, which is directly proportional to the amplitude with which each planet tugs on its star. They discovered that the inner planet is around twice as huge as Earth, while the outer planet is roughly three times as massive. The density of each planet was calculated based on its size and mass. The inner, smaller planet has a somewhat higher density than Earth, but the outer, bigger planet has a lower density. Both planets are most likely terrestrial or rocky in composition, depending on their density.

Based on their brief orbits, the researchers estimate that the surface temperature of the inner planet is 710 kelvins (818 degrees Fahrenheit), whereas the outer planet is roughly 560 kelvins (548 degrees Fahrenheit).

“We consider that range outside the habitable zone, too hot for liquid water to exist on the surface,” Kunimoto argues.

“But there might be more planets in the system,” Shporer says. “There are many multiplanet systems hosting five or six planets, especially around small stars like this one. Hopefully, we will find more, and one might be in the habitable zone. That’s optimistic thinking.”