Astronomers have discovered 2 super-Earths orbiting a nearby star

Two new worlds of likely rocky mineral goodness have just been found orbiting a star close to our own cosmic neighborhood.

The two newly discovered exoplanets are known as super-Earths – larger than Earth but smaller than an ice giant – orbiting a cool red dwarf star called HD 260655, which is just 33 light-years away.

While the worlds are unlikely to be habitable, given our current understanding of life, the star and its exoplanets are among the closest multi-world systems to Earth.

This makes it an excellent target for follow-up research to try to understand what exoplanets are made of and assess their atmospheres — an effort that will help our search for extraterrestrial life, even if the two worlds are unable to host it themselves.

“Both planets in this system are considered among the best targets for atmospheric study because of the brightness of their star,” says astronomer Michelle Kunimoto of MIT’s Kavli Institute for Astrophysics and Space Research.

“Is there a volatile-rich atmosphere around these planets? And are there signs of water or carbon-based species? These planets are fantastic test beds for these explorations.”

To date, more than 5,000 exoplanets have been confirmed in the Milky Way, and astrobiologists are keenly interested in finding terrestrial or rocky worlds like Earth, Venus and Mars.

We have a sample size of exactly one world known to host life – ours – so finding planets similar to Earth in size and composition is one of the main criteria in the search for life in other parts of the galaxy.

Rocky exoplanets, however, are relatively small in size and mass, which makes them more difficult to detect; most exoplanets we’ve been able to measure to date tend to fall into the giant category. Rocky worlds – and better yet, nearby rocky worlds – are in high demand.

The two worlds orbiting HD 260655 – called HD 260655 b and HD 260655 c – were discovered because they pass between us and their star during their orbit. The faint dips in starlight due to these exoplanetary transits were recorded by the TESS exoplanet-hunting telescope, designed to detect exactly these phenomena.

When Kunimoto detected these transit dips in the TESS data, the next step was to verify that the star had appeared in previous surveys – and it did.

The high-resolution Echelle spectrometer on the Keck telescope (now known as ANDES) had publicly available data since 1998. Another spectrometer, CARMENES at the Calar Alto Observatory in Spain, also recorded the star.

This makes a huge difference to the science of exoplanets: spectrographic data can reveal whether or not a star is moving there.

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

Between the TESS data and the HIRES and CARMENES data, the team was able to confirm that two exoplanets were orbiting HD 260655. Additionally, with both datasets, the team was able to compile a comprehensive profile of the two exoplanets.

The transit data provides a physical size, based on how much light is blocked from the star; and spectral data reveal the exoplanet’s mass, based on how much the star moves. Both sets of data can be used to calculate the exoplanet’s orbit.

The inner exoplanet, HD 260655 b, is about 1.2 times the size of Earth and twice the mass of Earth, and orbits the star every 2.8 days. The outer world, HD 260655 c, is 1.5 times the size and three times the mass of Earth, and has an orbit of 5.7 days.

At these sizes and masses, their densities suggest that the two exoplanets are likely rocky worlds.

Unfortunately, although the star is cooler and dimmer than the Sun, the planets’ proximity to HD 260655 means the worlds would be too hot for life as we know it. HD 260655 b has an average temperature of 435 degrees Celsius (816 Fahrenheit), and HD 260655 c is a milder but still sizzling 284 degrees Celsius (543 degrees Fahrenheit).

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

However, both exoplanets may still have atmospheres, which should be ready for probing by the newly deployed James Webb Space Telescope, which includes observing exoplanet atmospheres among its mission objectives.

Furthermore, there may even be additional exoplanets orbiting the star that we have yet to discover.

“There are many multiplanetary systems that host five or six planets, especially around small stars like this one,” says astrophysicist Avi Shporer of MIT’s Kavli Institute for Astrophysics and Space Research.

“We expect to find more, and one could be in the habitable zone. That’s optimistic thinking.”

The team presented their findings at the 240th meeting of the American Astronomical Society.

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