Astronomers from MIT and elsewhere have discovered a new multiplanet system in our galactic environment that is only 10 parsecs, or about 33 light-years, from Earth, making it one of the most well-known multiplanet systems of ours.
At the heart of the system is a small and cool M dwarf star, called HD 260655, and astronomers have discovered at least two Earth-sized terrestrial planets. The rocky worlds are likely uninhabitable, as their orbits are relatively tight, exposing the planets to temperatures too high to sustain liquid surface water.
Nevertheless, scientists are excited about this system, as the proximity and brightness of its star will give them a better understanding of the planets’ properties and signs of any atmospheres they might contain.
“Both planets in this system are each considered to be among the best targets for atmospheric study because of their star’s brightness,” said Michelle Kunimoto, a postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research and one of the lead scientists at MIT. the discovery. “Is there a volatile-rich atmosphere around these planets? And are there any signs of water- or carbon-based species? These planets are fantastic test beds for those explorations.”
The team will present its discovery today (June 15) at the American Astronomical Society meeting in Pasadena, California. Team members at MIT include Katharine Hesse, George Ricker, Sara Seager, Avi Shporer, Roland Vanderspek and Joel Villaseñor, along with collaborators from institutions around the world.
The new planetary system was initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission designed to observe the nearest and brightest stars and detect periodic drops in light that could signal a passing planet.
In October 2021, Kunimoto, a member of the TESS science team at MIT, was monitoring the satellite’s incoming data when she noticed some periodic dips in starlight, or transits, from the star HD 260655.
She led the detections through the mission’s scientific inspection pipeline, and the signals were soon classified as two TESS Objects of Interest, or TOIs – objects marked as potential planets. The same signals were also found independently by the Science Processing Operations Center (SPOC), the official TESS planet search pipeline at NASA Ames. Scientists usually plan to follow other telescopes to confirm that the objects are indeed planets.
Classifying and then confirming new planets can often take several years. For HD 260655, that process was significantly shortened using archive data.
Shortly after Kunimoto identified the two potential planets around HD 260655, Shporer looked to see if the star had been previously observed by other telescopes. Coincidentally, HD 260655 was listed in a star chart created by the High Resolution Echelle 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 the researchers had access to the study’s publicly available data.
HD 260655 was also listed as part of another independent study by CARMENES, an instrument operating as part of the Calar Alto Observatory in Spain. Since this data was private, the team reached out to members of both HIRES and CARMENES with the aim of combining their data power.
“These negotiations are sometimes very delicate,” Shporer noted. “Luckily, the teams agreed to work together. This human interaction is almost as important in getting the data [as the actual observations]†
Ultimately, this joint effort quickly confirmed the presence of two planets around HD 260655 in about six months.
To confirm that TESS’s signals did indeed come from two orbiting planets, the researchers looked at both the star’s HIRES and CARMENES data. Both studies measure a star’s gravitational fluctuations, also known as its radial velocity.
“Any planet orbiting a star will exert a little gravitational pull on its star,” explains Kunimoto. “What we’re looking for is any little movement of that star that could indicate that an object of planetary mass is pulling on it.”
From both sets of archive data, the researchers found statistically significant signs that the signals detected by TESS were indeed two planets orbiting Earth.
“That’s when we knew we had something really exciting,” Shporer says.
The team then looked more closely at the TESS data to determine the properties of both planets, including their orbital periods and sizes. They determined that the inner planet, called HD 260655b, orbits the star every 2.8 days and is about 1.2 times the size of Earth. The second outermost planet, HD 260655c, orbits every 5.7 days and is 1.5 times the size of Earth.
From the radial velocity data from HIRES and CARMENES, the researchers were able to calculate the planets’ masses, which are 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 has 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 likely terrestrial or rocky in composition.
The researchers also estimate, based on their short orbits, that the inner planet’s surface is a blazing 710 kelvin (818 degrees Fahrenheit), while the outer planet is about 560 K (548 F).
“We consider that range outside the habitable zone, too warm for liquid water to exist at the surface,” Kunimoto says.
“But there could be more planets in the system,” Shporer adds. “There are many multiplanet systems with five or six planets, especially around small stars like this one. Hopefully we’ll find more, and maybe one is in the habitable zone. That’s optimistic thinking.”
Two rocky exoplanets discovered around nearby star
Provided by the Massachusetts Institute of Technology
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