The system, which is only 33 light years away from Earth, appears to include two rocky, Earth-sized planets.
Astronomers from MIT and other institutions have identified a new multiplanet system within our galactic neighbourhood that is only 10 parsecs, or approximately 33 light-years, away from Earth, making it one of the nearest multiplanet systems known to humankind.
A small and cool M-dwarf star dubbed HD 260655 is at the centre of the system, and astronomers have discovered that it is home to at least two terrestrial, Earth-sized planets. The rocky worlds are unlikely to be habitable due to their close orbits, which expose them to temperatures too high to maintain liquid surface water.
Scientists are enthusiastic about this system, though, because 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.
"Because of the brightness of their star, both planets in this system are considered among the greatest prospects for atmosphere investigation," says Michelle Kunimoto, a postdoc at MIT's Kavli Institute for Astrophysics and Space Research and one of the discovery's main investigators. "Do these planets have a volatile-rich atmosphere?" Is there any evidence of water or carbon-based life? For those explorations, these planets are wonderful test beds."
The researchers will report its findings today at the American Astronomical Society meeting in Pasadena, California. Katharine Hesse, George Ricker, Sara Seager, Avi Shporer, Roland Vanderspek, and Joel Villaseor are among the MIT team members, as are collaborators from throughout the world. .
The power of data
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 light that could indicate a passing planet.
Kunimoto, a member of the TESS science team at MIT, was watching the satellite's incoming data in October 2021 when she spotted a pair of periodic dips in starlight, or transits, from the star HD 260655.
She ran the signals through the mission's science inspection pipeline, and they were quickly categorised 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 can take several years. With the use of archive data, the process for HD 260655 was greatly shortened.
Soon after Kunimoto discovered the two possible planets around HD 260655, Shporer checked to determine if 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 conversations can be pretty touchy at times," Shporer observes. "Fortunately, the teams decided to collaborate. This human connection is almost as crucial as the actual observations in gathering data."
The pull of the planets
In 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 will have a small gravitational pull on its star," says Kunimoto. "Any tiny movement of that star that could imply a planetary-mass object is tugging on it is what we're looking for."
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 were on to something special," Shporer recalls.
The scientists then examined the TESS data more closely to determine the parameters of both planets, such as their orbital period and size. The inner planet, named HD 260655b, orbits the star every 2.8 days and is about 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 calculate the planets' mass using radial-velocity data from HIRES and CARMENES, which is directly related to the amplitude with which each planet tugs on its star. They discovered that the inner planet is roughly twice as massive as Earth, and 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, larger 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 also estimate that the surface temperature of the inner planet is 710 kelvins (818 degrees Fahrenheit), whereas the outer planet is roughly 560 K. (548 F).
"That range is outside the habitable zone," Kunimoto argues, "too hot for liquid water to exist on the surface."
"However, the system could have more planets," Shporer adds. "Many multiplanet systems, especially around tiny stars like this one, include five or six planets." We're hoping to find more, and one of them might be habitable. That's a positive outlook."
NASA, the Max-Planck-Gesellschaft, the Consejo Superior de Investigaciones Cientificas, the Ministerio de Economia y Competitividad, and the European Regional Development Fund all contributed to this study.
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