Around 146 light-years from Earth, a planet could be orbiting an alien star that is more similar to our home than almost any other celestial body known to date.
The candidate, HD 137010 b, was presented in a recent study in The Astrophysical Journal Letters. It orbits a sun-like star and is estimated to be only around six per cent larger than the Earth.
What is particularly remarkable is that it takes 355 days to orbit its star, just ten days less than an Earth year. Its central star is also very similar to our sun, but is around 1,000 degrees cooler and significantly less luminous.
This lower radiation output has consequences. The planet receives less than a third of the energy that the Earth receives from the sun. The calculated surface temperature is correspondingly frosty: minus 68 to minus 70 degrees Celsius, values more reminiscent of Mars than of a habitable planet.
Ice world on the edge of the habitable zone
This puts HD 137010 b at the outer edge of the so-called habitable zone, the area around a star in which liquid water would, in principle, be possible. In its current orbit, however, the surface is probably completely iced over. However, more life-friendly conditions are not ruled out entirely.
Climate models show that if the planet had a dense atmosphere with high carbon dioxide levels, a strong greenhouse effect could significantly warm the surface. Under such conditions, even liquid water would be conceivable, at least temporarily.
As the researchers emphasise, HD 137010 b is therefore not necessarily a dead ice world. Such an atmosphere could even be detected with future telescopes. The prerequisite, however, is that the planet is first clearly confirmed.
Discovered by amateur researchers
The first indications of HD 137010 b do not come from a large observatory, but from the citizen science project "Planet Hunters."
Volunteers search the data from space telescopes for overlooked planetary signals. This was based on archive data from the Kepler space telescope (Mission K2), which monitored the brightness of more than 500,000 stars between 2009 and 2018.
In the case of HD 137010, Kepler recorded only a single transit in 2017: the star dimmed slightly for about 10 hours. Normally, several such transits are necessary to detect a planet beyond doubt. However, the researchers were able to draw conclusions about the size and orbital period from the duration and depth of this single signal.
The international team led by Alexander Venner, then at the University of Southern Queensland and now at the Max Planck Institute for Astronomy in Heidelberg, concluded that there was practically only one planet compatible with the observed signal. Nevertheless, HD 137010 b has so far only been officially recognised as a planet candidate.
Great hope among thousands of exoplanets
More than 6,000 exoplanets are now known. However, most of them are gas giants or extremely hot worlds orbiting their stars closely. Earth-like planets in sun-like stars are rare, and usually so far away that they can hardly be studied in detail.
So what makes HD 137010 b so special?
The planet candidate is exceptionally close on a cosmic scale and is therefore much easier to study than many other Earth-like worlds.
Given its relatively short distance and bright star, it would be an ideal candidate for future atmospheric analyses. The search would focus on gases such as oxygen or methane, which are considered possible indicators of biological processes.
Before this is possible, however, further transits must be observed, which is a challenge, because with an orbital period of almost a year, such events are rare.
And even if it turns out that the planet is life-friendly, it remains out of reach for a visit. It would take tens of thousands, if not hundreds of thousands of years, to get there, according to scientists.
