Astronomers have found at least 45 planets orbiting distant stars that possess Earth-like qualities including a similar atmosphere and life sustaining liquid water.
Experts from Luleå University of Technology, Sweden, created a new way to determine the habitability of these distant worlds by studying their atmosphere.
The team found a way to use information on ‘atmospheric species’, that is chemicals in the atmosphere, and how fast they escape into space to determine how close an exoplanet is in terms of temperature and composition to the Earth.
They then used their new model on the 55 potentially habitable worlds listed in the existing exoplanet catalogue – which focuses on habitable zone and the host star.
Only 17 of the 55 planets in the catalogue met the criteria defined by the Swedish researchers for being ‘Earth-like’ but the team found another 28 planets on the wider exoplanet list that also met the criteria – bringing the total of Earth-like worlds to 45.
The TRAPPIST-1 system contains one planet on the new list of 45 potentially Earth-like worlds – in this system it is known as TRAPPIST-1 d – the fourth from the star
This graphic shows the list of all potentially Earth-like worlds as determined by the Swedish team. The planets in blue were already on the Exoplanet Catalogue list of habitable worlds
Advanced ground and space based missions including the current CHEOPS and future James Webb telescope can now use this research to focus their searches.
Detecting habitable exoplanets is a difficult challenge as we can’t just ‘send a probe’ the vast distances between stars, the team explained.
The nearest exoplanet with a potentially habitable atmosphere is Proxima b around the star Proxima Centauri 4.22 light-years – or 25 trillion miles away.
NASA’s Juno probe reached speeds of 165,000 miles per hour as it approached Jupiter – at those speeds it would take 17,157 years to reach Proxima b.
This is an artists impression of Kepler-1649 c orbiting its host star. It is a planet on the list of potential Earth-like worlds that wasn’t already in the exoplanet catalogue habitability list
Currently, determining an exoplanet’s capability to host life relies on the restricted low-resolution spatial and spectral information of their atmospheres.
To create a ‘short list’ of habitable worlds, the team used the ‘kinetic theory of gases’ – how the gases move about the atmosphere – and a list of possible chemical retained in the atmospheres of currently known exoplanets.
‘We conclude that, based on our current knowledge of the detected exoplanets, 45 of them are good candidates for habitability studies,’ the authors wrote.
‘These exoplanets could have Earth-like atmospheres and should be able to maintain stable liquid water.’
As part of the study the team used the atmosphere of planets in our own solar system – where we know the true makeup – as a base of reference.
They then listed the exoplanets with hydrogen, oxygen, dinitrogen and carbon dioxide atmospheres in their list of Earth-like candidates.
‘We also propose a conservative list of 45 exoplanets with favourable conditions such as temperature and ability to retain the essential life-related gases in their atmospheres for further habitability studies,’ the team wrote.
The team also recommend that the current definition of a habitable zone around a star should be revisited when considering whether a planet could host life.
Ross 128 b is a terrestrial Earth-mass planet 11 light-years away. Swedish researchers say this wasn’t on the exoplanet habitability catalogue but is likely to have an Earth-like atmospheree
They say the capacity of the planet to host an Earth-like atmosphere to support the stability of liquid water should be added to the required conditions for habitability.
‘The discrimination of exoplanets based on the gas [chemicals] that they can retain in their atmospheres will help to determine the most probable candidates for potential habitability,’ the team explained.
These future missions can then look ‘for further atmospheric composition studies and for photochemical models.’
One of the major developments made by the Swedish team is reducing the amount of information required to asses the habitability of a distant world.
‘We present an atmospheric model capable of estimating the plausible composition of exoplanet atmospheres using the readily available or estimable parameters and with minimal assumptions,’ they wrote.
Teegarden’s Star b and c. Two planets orbiting an ultracool red dwarf 13 light years away. The planet is on both exoplanet Earth-like lists
‘Our model is designed for low-mass, low-irradiated exoplanets. These exoplanets have atmospheric evolution driven by classical thermal escape,’ the team added.
The researchers say data from future observations, including actual temperature profiles, albedo and elemental abundances could help them further refine their own model and get better at painting a picture of their true atmosphere.
The team plan to keep updating their list of potentially Earth-like worlds by studying new exoplanets as they are discovered.
The study has been published in the journal Proceedings of the Royal Society A.
Scientists study the atmosphere of distant exoplanets using enormous space satellites like Hubble
Distant stars and their orbiting planets often have conditions unlike anything we see in our atmosphere.
To understand these new world’s, and what they are made of, scientists need to be able to detect what their atmospheres consist of.
They often do this by using a telescope similar to Nasa’s Hubble Telescope.
These enormous satellites scan the sky and lock on to exoplanets that Nasa think may be of interest.
Here, the sensors on board perform different forms of analysis.
One of the most important and useful is called absorption spectroscopy.
This form of analysis measures the light that is coming out of a planet’s atmosphere.
Every gas absorbs a slightly different wavelength of light, and when this happens a black line appears on a complete spectrum.
These lines correspond to a very specific molecule, which indicates it’s presence on the planet.
They are often called Fraunhofer lines after the German astronomer and physicist that first discovered them in 1814.
By combining all the different wavelengths of lights, scientists can determine all the chemicals that make up the atmosphere of a planet.
The key is that what is missing, provides the clues to find out what is present.
It is vitally important that this is done by space telescopes, as the atmosphere of Earth would then interfere.
Absorption from chemicals in our atmosphere would skew the sample, which is why it is important to study the light before it has had chance to reach Earth.
This is often used to look for helium, sodium and even oxygen in alien atmospheres.
This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium