Earth-sized planet discovered close to one of many lightest purple dwarfs
Astronomers have found another Earth-sized planet. It is about 31 light-years away and orbits the habitable zone of a red dwarf star. It’s probably tidally dependent, which can be a problem with red dwarf stars. But the team that found it are optimistic about its potential habitability.
The prospect of finding Earth-like planets increases the prospect of finding life elsewhere. But they are difficult to spot. Of the approximately 5,200 exoplanets known to us, only a tiny minority can be described as Earth-like. NASA calls them terrestrial planets, and they range from half the mass of the Earth to twice the mass of the Earth. However, the designation only refers to their size and composition.
An Earth-sized planet is not Earth-like unless its star behaves well. And that was a problem for planets orbiting red dwarfs. Red dwarfs are notorious for intense UV flaring. This can destroy the atmosphere of any planet in its habitable zone, Earth-like or not.
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A team of astronomers from the Max Planck Institute for Astronomy think they have found a planet orbiting a red dwarf that could be an exception to the rule. Though the planet is tidally locked, researchers believe it may remain habitable beyond its dayside. They also think the planet is a good candidate to look for biosignatures.
The researchers reported their findings in an article 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 Planet and Star Formation department at MPIA.
Low-mass planets like Earth are difficult to spot, especially near 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 consists of two spectrographs on the 3.5 meter telescope at Calar Alto Observatory in Spain. CARMENES surveyed hundreds of low-mass red dwarf stars, looking for low-mass planets in their habitable zones. CARMENES uses radial velocity to detect the tiny changes that small planets make in the small red dwarfs that orbit them.
One of the stars found by CARMENES is called Wolf 1069. It has only 17% the mass of the Sun and only 18% the radius of the Sun. It hosts a single planet called Wolf 1069b. “As we analyzed the data from the star Wolf 1069, we discovered a clear, low-amplitude signal from what appears to be a planet about the mass of Earth. It orbits the star within 15.6 days at a distance one-fifteenth the distance between Earth and the Sun,” said lead author Kossakowski.
Because 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 orbited our sun at the same distance as the red dwarf, it would be fried to a crisp. So, even though Wolf 1069 b is so close to its star, the planet gets less energy than Earth from the Sun, only about 65%. “As a result, the so-called habitable zone shifts inwards,” says Kossakowski.
This figure compares three exoplanet systems of red dwarf stars that host Earth-sized planets. The green rings mark the individual habitable zones. Image source: Graphics department of the MPIA/J. niece
“Wolf 1069 b, at a distance of 0.0672 ± 0.0014 AU from the star, lies 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 their paper.
This research figure shows planets around M dwarf stars. The star’s temperature is on the y-axis and solar irradiance is on the x-axis. The optimistic and conservative HZ regions for a one-Earth-mass planet are shaded 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 gray-filled ones are transiting exoplanets, and the size of the circle indicates the planet’s radius. Wolf 1069b compares to our neighbor Proxima Centauri b and to other rocky, Earth-sized exoplanets like Kepler 1649 c. Image source: Kossakowski et al. 2023
CARMENES is excellent at finding low-mass planets around low-mass stars because it uses radial velocity instead of the transit method. “The CARMENES survey thus provides a comprehensive overview of planetary systems around nearby northern M dwarfs,” explains the CARMENES website. “By entering the realm of Earth-like planets, it provides a treasure trove for follow-up studies examining their habitability.”
Wolf 1069 b does not pass the star from our point of view, so astronomers might never have found it without CARMENES. “The CARMENES instrument was built precisely for the purpose of making it easier to discover as many potentially habitable worlds as possible,” says co-author of the study Jonas Kemmer from Heidelberg University.
Habitable just means the surface could withstand the presence of liquid water. For this, however, an appropriate atmosphere would have to be present. The astronomers say that if Wolf 1069 b had an Earth-like atmosphere, the temperature on the day side could be as high as 13 degrees Celsius. With that, liquid water could exist in a large region of the planet. The researchers used computer modeling to show that the planet can sustain water across a variety of atmospheric types.
But the atmosphere allows for more than water to exist in liquid form. If it’s there, it can play another crucial role in habitability. It could help protect the planet from radiation and high-energy particles that would otherwise render the planet sterile. Red dwarfs are notorious for intense UV flaring that can destroy the atmosphere.
This figure shows the projected temperature over the dayside of Wolf 1069 b in Kelvin when it has an Earth-like atmosphere. Image source: Kossakowski et al. 2023
Proxima Centauri is the closest star system to Earth, and it was big news when astronomers discovered exoplanets there. Proxima Centauri b was discovered in 2016. It is only slightly more massive than Earth and orbits the red dwarf in its habitable zone. But Proxima Centauri is a flare, and that could mean the habitable zone isn’t habitable at all.
Wolf 1069 appears to lack the kind of strong flare that other red dwarfs exhibit. If that’s the case, then its habitable zone might actually be habitable. But flares can be intermittent, and astronomers may not have seen any. So there are good reasons to dampen optimism.
On the other hand, red dwarfs do not flare up at the same rate throughout their lives. When young they are more energetic and likely make it very difficult for neighboring planets to hold an atmosphere. Depending on how early Wolf 1069 developed an atmosphere, it may retain it to this day if the star’s flare is a thing of the past. It’s even possible that the small planet has a magnetic field that could shield it.
Artist’s rendering of a massive stellar flare erupting on our neighboring star, Proxima Centauri. Credit: NRAO/S. Dagnello.
Wherever astronomers find exoplanets, they find them in groups. But this system seems to be different. Astronomers have found no evidence of siblings for Wolf 1069 b. Some computer models show that low-mass stars can end up being orbited by only a single planet, which seems to be the case here. “Our computer simulations show that about 5% of all evolving planetary systems around low-mass stars like Wolf 1069 end up with a single detectable planet,” explained MPIA scientist Remo Burn, a team member on the study. The team cannot completely rule out another planet in the system. But if you are there, it would have to be in a wide orbit well outside the habitable zone.
The simulations also explain that Wolf 1069 b may still have one of life’s most valuable assets: a molten core.
“The simulations also show a phase of violent encounters with planetary embryos during the construction of the planetary system, occasionally resulting in catastrophic impacts,” Burn added. Impacts generate a tremendous amount of heat, which could mean that Wolf 1069, like Earth, went through a magma-ocean phase. If so, then the planet’s core should still be hot and liquid today, made of dense iron and nickel. That could create a dynamo that creates a protective global magnetic field similar to Earth’s.
Wolf 1069 b is just 31 light-years away, making it the sixth-closest planet to Earth mass in its star’s habitable zone. Its proximity, as well as its potential habitability, make it a strong candidate for more detailed follow-up studies. It is in the same class as Proxima Centauri b and TRAPPIST-1 e. All three are suitable candidates in the search for biosignatures. However, since Proxima Centauri b and Wolf 1069 b were both found using the radial velocity method, they are not targets for standard spectroscopy. They do not move between us and their star, so there is no opportunity for the JWST or any other telescope to study their atmospheres. Astronomers must wait for their opportunity.
“We’ll probably have to wait another ten years for that,” Kossakowski points out. “Although it is crucial that we develop our facilities, given that most of the nearest potentially habitable worlds will only be discovered using the RV method.” ESO’s Extremely Large Telescope (ELT) may be able to detect the Characterize conditions of these planets using reflected light, but its first light is still a few years away, hopefully in 2028. Until then, Kossakowski and her team look forward to finding other exciting candidates like Wolf 1069 b.
“In conclusion,” the researchers write, “Wolf 1069 b is a remarkable discovery that will allow further exploration of the habitability of Earth-mass planets around M dwarfs, as well as a case study to test theories of planet formation.”