Scientists discover multi-planetary system just 33 light-years away

(ORDO NEWS) — Astronomers at MIT and other universities have discovered a new multi-planet system in our galactic neighborhood that is only 10 parsecs, or about 33 light-years, from Earth, making it one of the closest known multi-planet systems to our own.

At the center of the system is the small, cool M-dwarf star HD 260655, and astronomers have found that it hosts at least two Earth-sized planets. Rocky worlds are most likely uninhabitable because their orbits are relatively tight, which exposes the planets to temperatures too high to sustain liquid water on the surface.

However, scientists are excited about this system because the proximity and brightness of its star will allow them to get a closer look at the properties of the planets and signs of their atmosphere.

“Both planets in this system are considered among the best objects to study the atmosphere because of the brightness of their star,” says Michelle Kunimoto, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Studies and one of the leading scientists.

“Do these planets have atmospheres rich in volatiles? And are there signs of water or carbon species? These planets are a fantastic testing ground for such research.”

The team will present their discovery today at a meeting of the American Astronomical Society in Pasadena, California. The MIT team includes Katherine Hesse, George Reeker, Sara Seeger, Avi Sporer, Roland Vanderspeck, and Joel Villasen, as well as staff from institutes around the world.

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

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

She ran the detected signals through the satellite’s scientific verification system, and soon the signals were classified as two TESS objects of interest, or TOIs – objects marked as potential planets.

The same signals were detected by the Science Operations Data Processing Center (SPOC), the official TESS planet search engine located at NASA Ames. Scientists usually plan to conduct follow-up studies with other telescopes to confirm that these objects are indeed planets.

The process of classifying and then confirming new planets can often take several years. In the case of HD 260655, this process has been significantly reduced thanks to archival data.

Shortly after Kunimoto identified two potential planets around HD 260655, Sporer tried to find out if the star had been seen before by other telescopes. Luckily, HD 260655 was included in the survey of stars by the High Resolution Echelle Spectrometer (HIRES), an instrument operated by the Keck Observatory in Hawaii.

HIRES has been observing this star, as well as many other stars, since 1998, and the researchers have been able to access publicly available survey data.

HD 260655 was also included in another independent survey, CARMENES, an instrument operated by the Calar Alto Observatory in Spain. Because this data was private, the team reached out to members of HIRES and CARMENES to merge their data.

“Such negotiations are sometimes very delicate,” says Sporer. “Fortunately, the teams agreed to work together. This human interaction is almost as important to getting the data [as the observations themselves].”

As a result, the joint 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 the HIRES and CARMENES data on this star. Both studies measure the star’s gravitational wobble, also known as its radial velocity.

“Each planet orbiting a star will have a slight gravitational pull on its star,” explains Kunimoto. “We are looking for any slight movement of the star that could indicate that an object with a planetary mass is being pulled towards it.”

Based on both sets of archival data, the researchers found statistically significant indications that the signals detected by TESS were indeed two orbiting planets.

“That’s when we realized we had something very interesting,” Sporer says.

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 determined that the inner planet, dubbed HD 260655b, orbits the star every 2.8 days and is about 1.2 times the size of Earth. The second outer planet, HD 260655c, orbits every 5.7 days and is 1.5 times the size of Earth.

Based on the radial velocity data obtained with 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 its star. They found that the inner planet is about twice as massive as the Earth, while the outer one 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, while the outer, larger planet is slightly less dense. Both planets, based on their density, are most likely terrestrial or rocky in composition.

The researchers also estimated, based on their short orbits, that the surface of the inner planet has a temperature of 710 kelvins (818 degrees Fahrenheit) and that of the outer planet is about 560 K (548 degrees Fahrenheit).

“We think this range is outside the habitable zone, too hot for liquid water to exist on the surface,” says Kunimoto.

“But there could be more planets in the system,” Sporer adds. “There are a lot of multi-planet systems with five or six planets, especially around small stars like this one. Hopefully we’ll find more and one of them could end up in the habitable zone. That’s optimistic thinking.”

This study was supported in part by NASA, the Max Planck Society, the Higher Council for Scientific Research, the Ministry of Economics and Competition and the European Regional Development Fund.

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