Exoplanets

Earth-Sized Planet Found At One of the Lightest Red Dwarfs

Astronomers have found another Earth-sized planet. It’s about 31 light-years away and orbits in the habitable zone of a red dwarf star. It’s probably tidally locked, which can be a problem around red dwarf stars. But the team that found it is optimistic about its potential habitability.

The prospect of finding Earth-like planets raises the prospect of finding life elsewhere. But they’re difficult to spot. Of the approximately 5,200 exoplanets we know of, only a tiny minority can be described as Earth-like. NASA calls them terrestrial planets, and they range from half of Earth’s mass to twice Earth’s mass. But the designation only refers to their size and their composition.

An Earth-sized planet isn’t Earth-like unless its star behaves well. And that’s been a problem for planets orbiting red dwarfs. Red dwarfs are notorious for violent UV flaring. That can strip away the atmosphere of any planet in its habitable zone, Earth-like or not.

A team of astronomers from the Max Planck Institute for Astronomy think they’ve found a planet orbiting a red dwarf that might be an exception to the rule. Even though the planet is tidally locked, researchers think it could remain habitable across its dayside. They also think the planet is a good candidate to search for biosignatures on.

The researchers reported their findings in a paper titled “The CARMENES search for exoplanets around M dwarfs. Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star.” The paper is published in the journal Astronomy and Astrophysics, and the lead author is Diana Kossakowski. Kossakowski is from the Department of Planet and Star Formation at the MPIA.

Low-mass planets like Earth are difficult to spot, especially around large stars. CARMENES stands for Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Échelle Spectrographs. The instrument is a pair of spectrographs on the 3.5-meter telescope at the Calar Alto Observatory in Spain. CARMENES looked at hundreds of low-mass red dwarf stars and searched for low-mass planets in their habitable zones. CARMENES uses radial velocity to detect the tiny changes small planets induce in the small red dwarfs they orbit.

One of the stars found by CARMENES is called Wolf 1069. It’s only 17% as massive as the Sun and only 18% the radius of the Sun. It hosts a single planet called Wolf 1069b. “When we analyzed the data of the star Wolf 1069, we discovered a clear, low-amplitude signal of what appears to be a planet of roughly Earth mass. It orbits the star within 15.6 days at a distance equivalent to one-fifteenth of the separation between the Earth and the Sun,” said lead author Kossakowski.

Since Wolf 1069 is so much smaller and less energetic than a star like our Sun, its habitable zone is much closer. If Wolf 1069 b were orbiting our Sun at the same distance it orbits the red dwarf, it would be fried to a crisp. So even though Wolf 1069 b is so close to its star, the planet receives less energy than Earth does from the Sun, only about 65%. “As a result, the so-called habitable zone is shifted inwards,” said Kossakowski.

This illustration compares three exoplanet systems of red dwarf stars hosting Earth-mass planets. The green rings indicate the individual habitable zones. Image Credit: MPIA graphics department/J. Neidel

“Wolf 1069 b, with a distance of 0.0672 ± 0.0014 au to the star, sits comfortably within the conservative HZ limits, namely, 0.056 au to 0.111 au, given the runaway-greenhouse and maximum greenhouse limits, respectively,” the authors write in their paper.

This figure from the research shows planets around M-dwarf stars. The star’s temperature is on the y-axis, and insolation is on the x-axis. The optimistic and conservative HZ regions for a one Earth-mass planet are shaded with light and dark green, respectively. Only the planets in either the conservative or optimistic HZ of each planetary system are shown. White-filled planets are non-transiting planets, and grey-filled are transiting exoplanets, and the size of the circle indicates planet radius. Wolf 1069b compares with our neighbour, Proxima Centauri b and with other rocky, Earth-sized exoplanets like Kepler 1649 c. Image Credit: Kossakowski et al. 2023.

CARMENES excels at finding low-mass planets around low-mass stars because it uses radial velocity rather than the transit method. “The CARMENES survey is thus providing a comprehensive overview of planetary systems around nearby northern M dwarfs,” the CARMENES website explains. “By reaching into the realm of Earth-like planets, it provides a treasure trove for follow-up studies probing their habitability.”

Wolf 1069 b doesn’t transit the star from our point of view, so without CARMENES, astronomers may never have found it. “The CARMENES instrument was built for the very purpose of making it easier to discover as many potentially habitable worlds as possible,” said study co-author Jonas Kemmer from Heidelberg University.

Habitable only means that the surface could sustain the presence of liquid water. But there would have to be an appropriate atmosphere for that to happen. The astronomers say that if Wolf 1069 b had an Earth-similar atmosphere, the temperature could be as high as 13 Celsius on the dayside. That would enable liquid water to exist over a large region of the planet. The researchers used computer modelling to show that the planet could sustain water over a wide variety of atmospheric types.

But the atmosphere does more than allow water to exist in liquid form. If it’s there, it can fulfill another crucial role in habitability. It could help protect the planet from radiation and high-energy particles that would otherwise make the planet sterile. Red dwarfs are notorious for intense UV flaring that can strip away atmospheres.

This figure shows the projected temperature across Wolf 1069 b’s dayside in Kelvin if it has an Earth-like atmosphere. Image Credit: Kossakowski et al. 2023.

Proxima Centauri is the nearest star system to Earth, and it was big news when astronomers detected exoplanets there. Proxima Centauri b was discovered in 2016. It’s just a little more massive than Earth and orbits the red dwarf in its habitable zone. But Proxima Centauri is a flare star, and that could mean the habitable zone is not habitable at all.

Wolf 1069 appears to lack the type of powerful flaring that other red dwarfs exhibit. If that’s the case, then its habitable zone could actually be habitable. But flaring can be intermittent, and astronomers may not have seen any yet. So there’s good reason to temper optimism.

On the other hand, red dwarfs don’t flare at the same rate throughout their lives. When they’re young, they’re more energetic and likely to make it very difficult for nearby planets to hold onto an atmosphere. Depending on how early Wolf 1069 developed an atmosphere, if it did, it may still retain it to this day if the star’s flaring is a thing of the past. It’s even possible that the little planet has a magnetic field that could help shield it.

Artist’s conception of a violent stellar flare erupting on our neighbouring star, Proxima Centauri. Credit: NRAO/S. Dagnello.

Wherever astronomers find exoplanets, they find them in groups. But this system appears to be different. Astronomers haven’t found evidence of any siblings for Wolf 1069 b. Some computer modelling shows that low-mass stars can end up with just a single planet orbiting them, which appears to be the case here. “Our computer simulations show that about 5% of all evolving planetary systems around low-mass stars, such as Wolf 1069, end up with a single detectable planet,” explained MPIA scientist Remo Burn, a team member of the study. The team can’t completely rule out another planet in the system. But if one’s there, it would have to be on a wide orbit well outside the habitable zone.

The simulations also explain that Wolf 1069 b could still have one of life’s most precious commodities: a molten core.

“The simulations also reveal a stage of violent encounters with planetary embryos during the construction of the planetary system, leading to occasional catastrophic impacts,” Burn added. Impacts create an enormous amount of heat which might mean that Wolf 1069 went through a magma ocean phase like Earth did. If it did, then the planetary core should still be hot and liquid today and composed of dense iron and nickel. That could create a dynamo that produces a protective global magnetic field similar to Earth’s.

Wolf 1069 b is only 31 light-years away, making it the sixth closest Earth-mass planet in its star’s habitable zone. Its proximity, as well as its potential habitability, make it a star candidate for more detailed follow-up study. It’s in the same class as Proxima Centauri b and TRAPPIST-1 e. All three are suitable candidates in the search for biosignatures. But since Proxima Centauri b and Wolf 1069 b were both found using the radial velocity method, they’re not targets for the usual spectroscopy. They don’t pass between us and their star, so there’s no opportunity for the JWST or any other telescope to examine their atmospheres. Astronomers will have to wait for their opportunity.

“We’ll probably have to wait another ten years for this,” Kossakowski points out. “Though it’s crucial we develop our facilities considering most of the closest potentially habitable worlds are detected via the RV method only.” The ESO’s Extremely Large Telescope (ELT) may be able to characterize the conditions of those planets using reflected light, but its first light is a few years away, hopefully in 2028. Until then, Kossakowski and her team look forward to finding more exciting candidates like Wolf 1069 b.

“To conclude,” the researchers write, “Wolf 1069 b is a noteworthy discovery that will allow further exploration into the habitability of Earth-mass planets around M dwarfs, as well as case study in testing planetary formation theories.”

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Evan Gough

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