Astronomers at MIT and elsewhere have discovered a new multi-planetary system in our galactic neighborhood that is only 10 parsecs or about 33 light-years from Earth, making it one of the closest multi-planetary systems known to us.
At the center of the system is a small and solid M-dwarf named HD 260655, and astronomers have found that it contains at least two terrestrial, Earth-sized planets. Rocky worlds are unlikely to be habitable because their orbits are quite narrow and the planets are too high to hold liquid surface water.
Nevertheless, scientists welcome this system because the proximity and brightness of its star will allow them to take a closer look at the properties of the planets and the signs of any possible atmosphere within them.
“Both planets in this system are considered the best targets for atmospheric research because of the star’s brightness,” said Michelle Kunimoto, a PhD student at MIT’s Kavli Institute for Astrophysics and Space Research and one of the leading scientists in the discovery. “Is there a volatile atmosphere around these planets? And are there signs of water- or carbon-based species? These planets are fantastic test sites for those explorations.
The team will present their discovery today at a meeting of the American Astronomical Society in Pasadena, California. MIT team members include Katharine Hesse, George Ricker, Sara Seager, Avi Shporer, Roland Vanderspek and Joel Villaseñor, as well as collaborators from institutions around the world.
The power of data
The new planetary system was originally identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), a MIT-led mission to monitor the nearest and brightest stars and detect periodic light drops that could signal a passing planet.
2021 In October, Kunimoto, a member of the MIT TESS science team, was observing satellite data when he observed a couple of periodic star light drops or transits from the star HD 260655.
It conducted detections through the mission’s science verification pipeline, and the signals were soon classified as two TESS objects of interest or TOIs marked as potential planets. The same signals were also independently found by the Center for Science Processing Operations (SPOC), NASA’s official Ameso TESS planet search pipeline. Scientists usually plan to track other telescopes to confirm that the objects are indeed planets.
The process of classifying and subsequently approving new planets can often take several years. In the case of HD 260655, this process was significantly shortened by using archival data.
Shortly after Kunimoto identified two potential planets around HD 260655, Shporer looked to see if the star had previously been spotted by other telescopes. If lucky, HD 260655 was included in a stellar survey conducted by the High Resolution Echelle Spectrometer (HIRES), a device operating as part of the Keck Observatory in Hawaii. HIRES has been observing the star along with many other stars since 1998, and researchers have had access to publicly available survey data.
HD 260655 was also included in another independent study carried out by CARMENES, an instrument operating as part of the Calar Alto Observatory in Spain. As this data was private, the team contacted HIRES and CARMENES members to pool their data power.
“These negotiations are sometimes quite delicate,” Shporer said. “Fortunately, the teams have agreed to work together. This human interaction is almost as important to obtaining data [as the actual observations].
Eventually, these collaborative efforts quickly confirmed the presence of two planets around HD 260655 in about six months.
To confirm that the TESS signals were indeed from two orbiting planets, the researchers reviewed data from both the HIRES and CARMENES stars. Both studies measure the gravitational oscillation of a star, also known as its radial velocity.
“Every planet orbiting a star will have its own gravitational pull,” explains Kunimoto. “We are looking for any small movement of that star that could mean that it is attracted to an object of mass on the planet.
From both archival datasets, the researchers found statistically significant indications that the signals detected by TESS were in fact two orbiting planets.
“Then we knew we had something very interesting,” says Shporer.
The team then took a closer look at the TESS data to determine the properties of both planets, including their orbital period and size. They found that an inner planet called HD 260655b orbits the star every 2.8 days and is about 1.2 times larger than Earth. The second outer planet, HD 260655c, orbits every 5.7 days and is 1.5 times larger than Earth.
Based on HIRES and CARMENES radial velocity data, scientists have been able to calculate the mass of the planets, which is directly related to the amplitude with which each planet pulls its star. They found that the inner planet is about twice as massive as Earth, and the outer planet is about three Earth masses. Based on their size and mass, the team estimated the density of each planet. The inner, smaller planet is slightly denser than Earth, and the outer, larger planet is slightly less dense. Both planets, depending on their density, are likely to be terrestrial or rocky.
The researchers also estimated, based on short orbits, that the surface of the inner planet is 710 Kelvin (818 degrees Fahrenheit) and that the temperature of the outer planet is about 560 K (548 F).
“We think this range outside the living area is too hot for liquid water to exist on the surface,” says Kunimoto.
“But there may be more planets in the system,” Shporer adds. “There are many multi-planetary systems with five or six planets, especially around small stars like this. Hopefully we will find more and one may be in a residential area. This is optimistic thinking.
This study was supported in part by NASA, the Max-Planck-Gesellschaft, the Consejo Superior de Investigaciones Científicas, the Ministerio de Economía y Competitividad and the European Regional Development Fund.
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