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 and it is 1.5 times larger than Earth. but it is less dense.
These planets, unfortunately, are not habitable; the planets orbit their star at too close a distance, exposing them to temperatures too high to support liquid water on their surfaces. Based on their short orbits, the inner planet’s surface is estimated to be 436 degrees Celsius, while the outer planet is around 286 degrees Celsius.
“We consider that range to be outside the habitable zone, too hot for liquid water to exist on the surface,” says Michelle Kunimoto, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research and one of the lead scientists on the discovery.
But there could be more planets to discover, as many multiplanetary systems have five or six planets, and this is especially common around small stars. It’s possible that one could be 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 scientists can closely examine the properties of planets and study their atmospheres.
“Both planets in this system are considered among the best targets for atmospheric study due to the brightness of their star,” says Michelle Kunimoto, a postdoc 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 or carbon species? These planets are fantastic testbeds for such explorations.”
The system was first discovered 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 had been monitoring the star since 1998, so the researchers had access to publicly available data from the survey, which shortened the time it took to confirm the system.
The scientists used data from other studies to measure the star’s radial velocity, the motion it experiences due to gravity when another planet is nearby. “Each planet orbiting a star will have a small gravitational pull on its star,” said Michelle Kunimoto.
“What we’re looking for is any slight movement of that star that might indicate 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, lead scientist for the Exoplanet Archive and a scientist at NASA’s Exoplanet Science Institute at Caltech.
“Each of them is a new world, a new planet. I get emotional with each one because we don’t know anything about them.”