Earth-size planet discovered around cool red dwarf star shares its name with a biscuit

Astronomers have discovered an Earth-size planet orbiting an ultracool red dwarf star similar in size to Jupiter. The red dwarf, located some 55 light-years away, is 100 times less bright than the sun and exhibits half the temperature of our star.

This new extrasolar planet, or exoplanet, is named SPECULOOS-3 and represents just the second time astronomers have discovered a planetary system around a red dwarf star, the first being the Trappist-1 system.

The Earth-size world was discovered by the Search for habitable Planets Eclipsing Ultra-cool Stars (SPECULOOS) project located at the Paranal Observatory in the Atacama Desert of northern Chile. The aim of the mission (which is named after a popular Belgian biscuit called "Speculoos") is to use infrared observations to detect rocky exoplanets around nearby ultracool stars like red dwarfs and so-called "failed stars," or brown dwarfs.

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"SPECULOOS-3 b orbits a red dwarf star. The small size of these stars makes it easier to detect small Earth-size planets around them, and above all to observe any planetary atmospheres," Émeline Bolmont, team member and assistant professor in the astronomy department at the UNIGE, said in a statement. "Numerous studies show that life could develop on planets orbiting red dwarfs. They are, therefore, ideal candidates for us."

While SPECULOOS-3 b is practically the same size as our planet, there are other differences that marks it out as radically different from Earth. This includes the fact a full orbit of its star, and thus a year on SPECULOOS-3 b, lasts just 17 hours. This exoplanet is also likely "tidally locked" to its star, meaning it has an eternal dayside and an everlasting nightside.

"We believe that the planet rotates synchronously, so that the same side, called the day side, always faces the star, just like the moon does for the Earth," SPECULOOS team leader Michaël Gillon said in a statement. "On the other hand, the night side hand, would be locked in endless darkness."

SPECULOOS-3 b and habitability

Red dwarfs, or "M-dwarfs," are believed to account for 70% of the stars in the Milky Way galaxy. Because they are thousands of degrees cooler than the sun, they burn through their fuel supplies needed for nuclear fusion relatively slowly. This means red dwarfs have exceptionally long lifetimes — at least a hundred times longer than the estimated 10-billion-year lifetime of the sun.

The planet's star, SPECULOOS, is actually part of an even cooler subclass of red dwarfs that astronomers predict will be the last stars alight in an eventually cold and dark cosmos; these orbs will likely burn for more than 100 billion years. This means life may have more time to develop around red dwarfs than it does in systems around stars that live fast and die young.

Yet, red dwarfs aren't very well understood because of their low luminosity. Additionally, it is rare to discover planets around these small, cool stars. So, although red dwarf exoplanets might technically be the most common worlds in our galaxy, we don't actually know much about them. SPECULOOS aims to change that.

That fact that ultracool dwarf stars are scattered across the sky means scientists must observe the objects one at a time and over a period of weeks in order to detect possible transiting planets. That requires a dedicated network of professional telescopes like those that comprise SPECULOOS, and the discovery of SPECULOOS-3 b marks a step in the right direction toward the goal of understanding red dwarf worlds.

However, though red-dwarf-orbiting planets are touted as ideal locations for life to take hold, SPECULOOS-3 b isn't habitable, to be clear. The planet is bombarded by about 16 times more radiation than Earth receives from the sun, making liquid water unable to exist on its surface.

When it comes to hunting for life on a planet around a red dwarf, the seven Earth-like planets of the Trappist-1 system, many of which are in the habitable zone of their star, are still better targets. But this doesn't make SPECULOOS-3 b uninteresting. Far from it.

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SPECULOOS-3 b will be the ideal target for the James Webb Space Telescope (JWST), when it comes to testing if planets that exist in close proximity to their stars can hold on to an atmosphere, the team believes.

"The JWST should be able to determine whether the planet has been able to retain an atmosphere despite its proximity to its star," Bolmont said. "If we find one on this highly irradiated planet, it gives us good hope that one also exists on the planets in TRAPPIST-1's habitable zone."

The team's research was published on Wednesday (May 15) in the journal Nature Astronomy.