So far, only around 20 potentially life-friendly exoplanets the size of Earth are known - now another one has been added. Around the red dwarf star Wolf 1069, 31 light-years away, astronomers have discovered a planet about the size and mass of Earth orbiting its star in the habitable zone. The rocky planet receives about 65 percent of the amount of radiation that Earth receives from the sun. However, because the exoplanet always faces the same side of its star, this could allow for habitable conditions on its day side. So there could be liquid water there, and maybe even an ocean, the team reports.
Thanks to better and better telescopes, astronomers have discovered countless extrasolar planets in recent years. Many of them also orbit stars in our cosmic neighborhood, including our nearest neighboring star Proxima Centauri or the star Trappist-1, which is about 40 light-years away and is orbited by seven rocky planets. But among the 5,000 or so known exoplanets, fewer than 100 are about the size of the Earth and lighter than two Earth masses. Only about 20 of them are in the so-called habitable zone - the area of a planetary system in which a mild climate and water in liquid form on the planet's surface are possible. However, this is less due to the fact that such "Earth Twins" are rare per se, but to the limited possibilities to track down such rather small and light planets. Because of their small size, they only leave a weak signal when they transit in front of their star, and their gravity also causes their star to "wobble" only slightly, making detection difficult.
Earth-sized planet orbiting a nearby red dwarf
However, one chance to find more such Earth-like, potentially habitable exoplanets is to look for them among particularly low-mass red dwarf stars. Because with these, the gravitational influences of lighter planets can also be read more easily from the light spectrum using the so-called radial velocity method. This is exactly what Diana Kossakowski from the Max Planck Institute for Astronomy (MPIA) in Heidelberg and her colleagues have achieved as part of the CARMENES program. For their study, they had targeted the star Wolf 1069, 31 light-years away, with the Calar Alto Observatory in Spain for four years. This telescope has dual high-resolution visible and near-infrared spectrographs, making it particularly useful for revealing the subtle shifts in the stellar spectrum left behind by a planet orbiting the star.
The astronomers found what they were looking for: "As we evaluated the data from the star Wolf 1069, we discovered a clear, low-amplitude signal that indicates a planet about the mass of the Earth," reports Kossakowski. The team determined that the exoplanet, dubbed Wolf 1069 b, is about the same size as Earth and has around 1.2 times the mass of Earth. It orbits its star within 15.6 days at a distance that is one-fifteenth the distance between Earth and the Sun. This means that the planet is comparatively close to its star, but because the red dwarf is significantly dimmer and cooler than the sun, Wolf 1069 b only receives around 65 percent of the sun's radiation on earth. That's enough to place the planet in its star's conservative habitable zone, astronomers say.
Liquid water possible on the day side
"This planet could therefore have the key features of a livable world," state Kossakowski and her colleagues. At a distance of 31 light years, Wolf 1069 b is the sixth closest rocky planet in the habitable zone of its host star in our cosmic neighborhood. According to the analysis, Wolf 1069 b orbits its star in a locked rotation, always facing the same side. This could cause the day side to heat up even more. If this planet has an atmosphere that stores additional heat, mean temperatures on the day side of Wolf 1069 b could be around 13 degrees. That would be enough for liquid water and maybe even an entire ocean, the team explains. In addition, unlike other red dwarfs in our area, Wolf 1069 is a rather quiet star characterized by few bursts of radiation. This increases the quality of life on his planet.
"Wolf 1069b is thus a promising addition to the nearby habitable planets that would be worth searching for biosignatures," the astronomers state. However, it is difficult to find out more about Wolf 1069 b for the time being: Because the planet does not pass directly in front of its star from our point of view, no transits can be observed. As a result, light spectra are also missing, from which one could, for example, read the composition of its atmosphere - if it has one.
Source: Diana Kossakowski (Max Planck Institute for Astronomy, Heidelberg) et al., Astronomy & Astrophysics, doi: 10.1051/0004-6361/202245322