Astronomers have discovered a super-Earth planet orbiting a distant star that could potentially host life.
The exoplanet, known as HD 20794 d, lies in the habitable zone of its star
This means it's not too close and not too far from the star, such that it could host liquid water on its surface, a key ingredient for life.
HD 20794 d is a super-Earth, an Earth-like planet that's much larger than our own.
It orbits a G-type star, much like our Sun and is located just 20 lightyears away: very close to our own Solar System, in cosmic terms.
Studying HD 20794
Because HD 20794 is so 'close' to Earth makes it easier to study and means astronomers have learnt a lot about the star and planet HD 20794 d.
"HD 20794, around which HD 20794 d orbits, is not an ordinary star," says Xavier Dumusque, Senior Lecturer and researcher in the Department of Astronomy at the UNIGE and co-author of the study.
"Its luminosity and proximity makes it an ideal candidate for future telescopes whose mission will be to observe the atmospheres of exoplanets directly."
The planet itself lies in an eccentric orbit, which means its journey around the host star is not perfectly circular.
As a result, it travels in and out of the habitable zone as it orbits.
And astronomers say the star's brightness and closeness make it a good candidate for observing the atmospheres of exoplanets with future telescopes.
Exoplanets and life beyond Earth
Astronomers estimate there is a planetary system around nearly every star in our Galaxy.
There are at least 100 billion stars in our Galaxy, and yet the number of confirmed exoplanets – planets beyond our Solar System – discovered so far totals about 7,000.
Clearly, there are many more to discover, and there's much more to learn about the chances for life existing beyond our Solar System.
Astronomers are looking for exoplanets that are easier to characterise or have interesting features.
HD 20794 d and the habitable zone
HD 20794 d is in the habitable zone of its star, but what does this mean, and why is it important?
The habitable zone is often referred to as the Goldilocks zone, because it's 'not too hot and not too cold', just the right distance from its star for liquid water to exist on its surface.
Liquid water is one of the conditions necessary for the development of life as we know it.
But where exactly the habitable zone exists around a star depends on the star itself: its size, mass and brightness.
Astronomers say that for stars like our Sun, or HD 20794, the habitable zone can stretch from 0.7 to 1.5 astronomical units (AU) from the star.
In the case of our Sun, this includes Earth and Mars.
Exoplanet HD 20794 d takes 647 days to orbit its star and follows an elliptical – egg-shaped – orbit.
This means the planet's orbit takes it between the inner edge of the star's habitable zone (0.75 AU) and outside it (2 AU).
Astronomers say this enables them to study the star and planet and adjust theoretical models, testing their understanding of what makes a planet habitable.
If there is water on HD 20794 d, it would pass from the ice to liquid during the course of its orbit.
Discovery and study
Dr Michael Cretignier from the University of Oxford in the UK first discovered the exoplanet candidate that would later be confirmed as HD 20794 d.
He was analysing archive data recorded by the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph at the La Silla Observatory in Chile.
Dr Cretignier spotted shifts in the of light emitted by the star, which is a sign it's being tugged by the gravitational pull of an orbiting planet.
An international team of astronomers then used measurements recorded over two decades by HARPS and its successor ESPRESSO, also in Chile.
"We worked on data analysis for years, gradually analysing and eliminating all possible sources of contamination," says Dr Cretignier.
While it's not yet known whether HD 20794 d does indeed harbour life, the fact that it's close to Earth and in its habitable zone, orbiting a Sun-like star, makes it ripe for study.
One more step closer to answering that pervading question: are we alone in the Universe?
What do you think? Are we alone in the Universe, and are distant stars the best places to look, or should we concentrate on our own Solar System? Let us know by emailing contactus@skyatnightmagazine.com.
Read the full paper at www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202451769
