Astronomers have found a new multiplanetary system just 33 light-years from our own planet.
The system, now the closest known to Earth, has two terrestrial planets orbiting a cool M-dwarf star, called HD 260655.
The inner planet, HD 260655b, orbits the star every 2.8 days and is about 1.2 times larger than Earth, but slightly denser, while the outer planet, HD 260655c, orbits every 5.7 days. days and is 1.5 times larger than Earth. but it is less dense.
These planets, unfortunately, are not habitable; the planets orbit their star at a very close distance, exposing them to temperatures too high to keep liquid water on their surfaces. Based on their short orbits, the surface of the inner planet is estimated to be 436 degrees Celsius, while the outer planet is around 286 degrees Celsius.
“We consider this band outside the habitable zone too hot for liquid water to exist on the surface,” says Michelle Kunimoto, a postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research and one of the lead scientists on the discovery.
But there may be more planets to be discovered, as many multiplanetary systems have five or six planets and this is especially common around small stars. It’s possible that someone is in the habitable zone, although MIT scientist Avi Shporer said such a possibility was “optimistic thinking.”
Until then, HD 260655’s proximity and brightness means that scientists can closely examine the planets’ properties and study their atmospheres.
“Both planets in this system are considered among the best targets for atmospheric study because of the brightness of their star,” says Michelle Kunimoto, a postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research and one of the lead scientists on the discovery.
“Is there a volatile-rich atmosphere around these planets? And are there signs of water-based or carbon-based species? These planets are fantastic test beds for these explorations.”
The system was first detected by NASA’s Transiting Exoplanet Survey Satellite (TESS), which detected periodic dips in light from HD 260655 – indicative of a planet passing in front of the star.
HD 260655 was also in a star survey by the High Resolution Echelle Spectrometer (HIRES), an instrument that operates as part of the Keck Observatory in Hawaii.
HIRES has been monitoring the star since 1998, so the researchers had access to publicly available data from the survey, which reduced the time needed to confirm the system.
Scientists used data from other research to measure the star’s radial velocity — the motion it experiences from gravity when another planet is close to it. “Every planet that orbits a star will have a small gravitational pull on its star,” said Michelle Kunimoto.
“What we’re looking for is any slight motion from this star that might indicate that a planetary-mass object is pulling on it.”
Scientists have now discovered more than 5,000 exoplanets orbiting distant suns. “It’s not just a number,” said Jessie Christiansen, scientific lead at the Exoplanet Archive and a scientist at NASA’s Institute for Exoplanet Science at Caltech.
“Each one of them is a new world, a whole new planet. I get excited about each one because we don’t know anything about them.”