Astronomers discover nearby multiplanetary system

Astronomers at MIT and beyond have discovered a new multiplanetary system in our galactic neighborhood that lies just 10 parsecs, or about 33 light-years, from Earth, making it one of the oldest known multiplanetary systems. close to ours.

At the system’s core is a small, cool M dwarf star, named HD 260655, and astronomers have found that it hosts at least two Earth-sized terrestrial planets. Rocky worlds are probably not habitable, as their orbits are relatively narrow, exposing the planets to temperatures too high to keep surface water liquid.

Still, scientists are excited about this system because its star’s proximity and brightness will allow them to take a closer look at the properties of planets and signs of any atmospheres they might contain.

“The two planets in this system are each considered among the best targets for atmospheric study due to their star’s brightness,” says Michelle Kunimoto, postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research. and one of the leading scientists in the discovery. “Is there a volatile-rich atmosphere around these planets? And are there signs of water- or carbon-based species? These planets are fantastic testbeds for these explorations. .

The team will present their discovery today at the American Astronomical Society meeting in Pasadena, California. Members of the MIT team include Katharine Hesse, George Ricker, Sara Seager, Avi Shporer, Roland Vanderspek, and Joel Villaseñor, as well as collaborators from institutions around the world.

Data power

The new planetary system was initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission that is designed to observe the nearest, brightest stars and detect periodic dips in light that could signal the passage of a planet.

In October 2021, Kunimoto, a member of MIT’s TESS science team, was monitoring incoming data from the satellite when she noticed a pair of periodic starlight dips, or transits, of the star HD 260655.

She ran the detections through the mission’s science inspection pipeline, and the signals were quickly classified as two TESS Objects of Interest, or TOI – objects flagged as potential planets. The same signals were also found independently by the Science Processing Operations Center (SPOC), the official TESS planet-finding pipeline based at NASA Ames. Scientists usually plan to follow up with other telescopes to confirm the objects are indeed planets.

The process of classification and subsequent confirmation of new planets can often take several years. For HD 260655, this process has been significantly shortened using archival data.

Shortly after Kunimoto identified the two potential planets around HD 260655, Shporer investigated whether the star had already been observed by other telescopes. Luckily, HD 260655 was listed in a survey of stars taken by the High Resolution Scale Spectrometer (HIRES), an instrument that operates as part of the Keck Observatory in Hawaii. HIRES had been monitoring the star, along with a host of other stars, since 1998, and researchers were able to access publicly available data from the survey.

HD 260655 has also been listed in another independent survey by CARMENES, an instrument that operates as part of the Calar Alto Observatory in Spain. As this data was private, the team reached out to members of HIRES and CARMENES in an effort to combine their data power.

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

Planetary pull

In the end, this collaborative effort quickly confirmed the presence of two planets around HD 260655 in about six months.

To confirm that the signals from TESS were indeed coming from two orbiting planets, the researchers looked at HIRES and CARMENES data from the star. Both readings measure a star’s gravitational wobble, also known as radial velocity.

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

From the two archival datasets, the researchers found statistically significant signs that the signals detected by TESS were indeed two orbiting planets.

“Then we knew we had something really exciting,” Shporer says.

The team then took a closer look at the TESS data to determine the properties of the two planets, including their orbital period and size. They determined that the inner planet, dubbed 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.

Using radial velocity data from HIRES and CARMENES, the researchers were able to calculate the mass of the planets, which is directly related to the amplitude with which each planet pulls on its star. They found that the inner planet is about twice as massive as Earth, while the outer planet is about three Earth masses. From their size and mass, the team estimated the density of each planet. The smaller inner planet is slightly denser than Earth, while the larger outer planet is a little less dense. Both planets, depending on their density, are probably terrestrial or rocky in composition.

The researchers also estimate, based on their short orbits, that the inner planet’s surface is at 710 Kelvin (818 degrees Fahrenheit), while the outer planet is around 560 K (548 F).

“We consider this beach outside the habitable zone, too warm for liquid water to exist on the surface,” Kunimoto says.

“But there could be more planets in the system,” adds Shporer. “There are many multiplanetary systems hosting five or six planets, especially around small stars like this. Hopefully we’ll find more, and we might be in the habitable zone. This is optimistic thinking.

This research 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.