An impressive 5000 exoplanets and counting have been detected thus far. On the hunt for an Earth analog, out of these thousands of planets, only ∼50 have been found to sit in their stellar host’s so-called habitable zone (HZ). Only 20 of these are considered to be Earth-sized.
A team of astronomers led by MPIA scientist Diana Kossakowski has discovered an Earth-mass exoplanet orbiting in the habitable zone of the red dwarf star Wolf 1069. Although the rotation of this planet, named Wolf 1069 b, is probably tidally locked to its path around the parent star, the team is optimistic it may provide stable habitable conditions across a wide area of its dayside.
The likelihood that Wolf 1069 b may have kept most of its atmosphere increases in the absence of any visible star activity or strong UV radiation. Therefore, the planet is one of a few promising targets to look for biosignatures and signs of habitability.
MPIA’s Diana Kossakowski. She is the main author of the underlying paper and said, “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.”
Wolf 1069 b receives only around 65% of the incident radiant power that the Earth receives from the Sun, despite being so close to it. Wolf 1069 emits substantially less energy and has a colder surface temperature than the Sun, giving it an orange appearance. Reduced heating power is the outcome of these characteristics.
Wolf 1069 b is the sixth nearest Earth-mass planet in its host star’s habitable zone, at a distance of 31 light-years. It is one of a select few illustrious targets to look for biosignatures because of its promising chances of being habitable, along with Proxima Centauri b and TRAPPIST-1 e.
Kossakowski said, “As a result, the so-called habitable zone is shifted inwards. Therefore, planets around red dwarf stars such as Wolf 1069 can be habitable even though they are much closer than the Earth is to the Sun.”
Co-author Jonas Kemmer from Heidelberg University adds, “The CARMENES instrument was built to make it easier to discover as many potentially habitable worlds as possible.”
Like Earth, Wolf 1069 b may have a natural greenhouse effect, which might assist in raising the average temperature over the 250 Kelvin (-23 °C) figure. This value assumes a simple bare rocky planet. In fact, according to the scientists’ calculations, if it had an atmosphere similar to that of Earth, the mean temperature could increase as high as 286 Kelvin (+ 13 °C), maintaining liquid water across a substantial area on the planet’s side that faces the star.
The team concludes that the planet may sustain moderate temperatures and surface liquid water under various atmospheric conditions and surface types based on computer simulations utilizing advanced climate models.
What’s more, Wolf 1069 appears benign. There is no evidence of harmful star activity based on the observations. Even still, it’s too soon to have excessive optimism.
MPIA scientist Remo Burn, a team member of the study, said, “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. The simulations also reveal a stage of violent encounters with planetary embryos during the construction of the planetary system, leading to occasional catastrophic impacts.”
Wolf 1069 b is one of those uncommonly isolated planets. Astronomers rule out new planets with orbital periods of fewer than 10 days and at least one Earth mass based on their measurements. It’s less than the 15.6 days they calculated for Wolf 1069 b. They are unable to exclude planets with larger orbits, though.
Although Wolf 1069 b is an intriguing candidate to constrain its habitability conditions further, it shares one distinctive characteristic with almost all planets in the habitable zones of red dwarf stars. Most likely, it is tidally locked to the host star’s orbit. In other words, a rotation around an object’s axis equals a complete revolution. It always has a day on the side that faces the star, whereas it always has darkness on the other hemisphere because that side always faces the star.