Signal from space confirms existence of Earth-like planet that could host life

HD 20794 d is in an area around a star where temperatures could allow liquid water to form

An artist's impression of the HD 20794 d exoplanet. Photo: Nasa

A distant signal from space has helped astronomers to confirm the existence of a planet outside our solar system that could be capable of supporting life.

The exoplanet, called HD 20794 d, orbits a star similar to our Sun. It is 20 light-years away, which is so close in cosmic terms that future, more advanced telescopes could be used to capture direct images of the stellar discovery.

It was first detected two years ago by Michael Cretignier, a researcher at the University of Oxford, but its existence has now been confirmed using data from advanced planet-hunting space instruments in Chile.

HD 20794 d is one of the most exciting exoplanets discovered so far and offers astronomers an opportunity to study a world that may share some similarities with Earth.

“For me, it was naturally a huge joy when we could confirm the planet's existence,” Mr Cretignier said. “It was also a relief, since the original signal was at the edge of the spectrograph’s detection limit, so it was hard to be completely convinced at that time if the signal was real or not. Excitingly, its proximity to us means there is hope for future space missions to obtain an image of it.”

Findings from the research have been published in the Astronomy & Astrophysics journal.

Mystery signal

The exoplanet was first detected because of tiny changes in the light spectrum of its host star, which were seen using archived data from the High Accuracy Radial velocity Planet Searcher (Harps) spectrograph and the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (Espresso) instrument.

These shifts suggested something was pulling on the star with its gravity. But the signal was so faint that there that was still doubt about what was causing it. It could have been caused by the star itself, or an error in the instruments.

Mr Cretignier and an international team of researchers used more than 20 years of data from the two instruments to verify the signal and confirm the Earth-like exoplanet.

Earth similarities

What makes HD 20794 d so special is its location in the habitable zone, the region around a star where temperatures are just right for liquid water.

But unlike Earth, the exoplanet does not have a near-circular orbit. Instead, its path around the star is elliptical, meaning its distance from the star changes over the course of its year.

Jonathan Ward, a fellow of the UK’s Royal Astronomical Society, who was not involved in the study, explained how an elliptical orbit could affect the likelihood that the exoplanet can host life.

“For liquid water to exist, a planet’s temperature has to stay in the right range. If the orbit is too elliptical, the planet might swing too close to its star, making it too hot, or drift too far away, making it too cold,” he told The National. “It’s not clear yet how that would affect the potential for life to form and evolve.”

Strange Earth-like planets that lie beyond our solar system

The landmark discovery of Trappist-1 set off a wave of excitement across the space community. It was the first time multiple Earth-sized planets were found in a habitable zone and were orbiting the same star – Trappist-1. Courtesy: Nasa

All seven planets orbiting Trappist-1 are believed to be rocky and have potential for water on the surface. Courtesy: Nasa

Located in a habitable zone, Kepler-22b was labelled by Nasa as ‘super Earth’. Researchers believe the planet, which is larger than Earth, could be another ocean world. However, because of the planet’s extreme tilt, the north and south poles would experience sunlight and darkness for half a year each. Courtesy: Nasa

Discovered in 2020, Toi 700 d is only 20 per cent larger than Earth and orbits an M Dwarf star – a cool and stable type of star that do not release killer stellar flares, or radiation. Located 100 million light years away from Earth, one year on the planet is 37.4 days. Courtesy: Nasa

Discovered in 2011, KEPLER-20e is an Earth-size planet that orbits a sun-like star outside of the solar system. It is too close to its star to be in the habitable zone, so it cannot hold liquid water. However, the discovery was still groundbreaking because it was the first time small exoplanets were found to be orbiting around a star that was similar to the Sun. Courtesy: Nasa

Kepler-20f was discovered along with Kepler-20e. A year on the planet is19.6 days. Courtesy: Nasa

Another factor is the exoplanet’s size, but the researchers have to wait until more advanced telescopes have been developed to determine how big the celestial body is.

"At the moment the best we can say is that its minimum mass is six Earth-mass and there is 75 per cent of chance that the planet is smaller than 10 Earth-mass," Mr Cretignier told The National. "Direct imaging data will allow us to reconstruct the 3D orbit of the planet, leaving this fundamental degeneracy and providing the real mass of the exoplanet."

Telescope projects, including the Extremely Large Telescope (ELT) and the Large Interferometer For Exoplanets (Life), could revolutionise the study of HD 20794 d. The ELT could take images of the planet directly, revealing its true mass and atmospheric composition, while Life would analyse its atmosphere for water vapour and potential biosignatures, offering deeper insights into its habitability.

Searching for water

Mr Cretignier said that, if these telescopes are developed, he would prioritise the search for water molecules in the exoplanet's atmosphere. "After all, it is well-accepted that searching for life is useless if there is no water to sustain this life," he said.

If there are water molecules, another indicator of life could be biosignatures. These are chemical markers, such as oxygen or methane, in a planet's atmosphere that could indicate the presence of life.

"So far as I know, there are no perfect biosignatures at the moment, even if some species are often mentioned," Mr Cretignier added. "For instance, we know that life began on Earth well before leaving an active or detectable trace in the Earth's atmosphere.

"Similarly, some geophysical phenomena can mimic the signatures of life or thick clouds/haze may hide them so we should be very careful here."