A pair of ‘super Earth’ planets orbiting ‘just within the habitable zone’ of a nearby red dwarf star half the size of the Sun have been discovered by astronomers.
The star these potentially rocky world’s orbit is called Gliese 887 – it is 11 light years away from the Earth and one of the brightest red dwarf stars in the night sky.
As the planets are on the ‘inner edge’ of the habitable zone – that is the region where liquid water can remain on the surface – they may be too hot to sustain life.
The new planets were found using a high-precision, planet-finding instrument on the European Southern Observatory’s 3.6-metre telescope at La Silla in Chile.
Astronomers from Australia, who discovered the planets, were working with the international RedDots team that hunts for terrestrial planets around red dwarf stars.
Called GJ 887b and GJ 887c, the planets ‘could be rocky worlds’ like Earth but are very likely to be ‘tidally locked’ to their star meaning one side will always face the star and be in perpetual daytime while the other always night.
As the planets are on the ‘inner edge’ of the habitable zone – that is the region where liquid water can remain on the surface – they may be too hot to sustain life. Artists impression
The discovery was made by an international team, including those from the University of Hertfordshire, Open University and Queen Mary University of London.
The scientists used a technique known as Doppler wobble, which enabled them to find the planets by measuring their gravitational interaction with GJ 887.
As a planet orbits, it causes its parent star to wobble by a tiny degree and astronomers can see the signature of this effect in the light emitted by the star.
The researchers believe both GJ 887b and GJ 887c lie close to the inner edge of habitable zone and may be too hot to maintain liquid water on their surfaces.
They estimate the surface temperature of GJ 887c, which takes 21.8 days to circle its host star, to be around 70C (158F).
GJ 887b, meanwhile, is much closer to the star, with an orbital period of 9.3 days.
As this is a red dwarf with a lower surface temperature the habitable zone is closer to the star – which is why with a 9.3 day orbit , GJ 887b is still ‘just’ in that area.
In comparison, Mercury, which is well outside the habitable zone of the Sun has an orbit of 88 days and the Earth – which is inside it – has a 365 day orbit.
The researchers say GJ 887 is less active than other red dwarfs, so the newly discovered worlds may be spared from strong stellar winds – outflowing material from the star which can erode a planet’s atmosphere.
Hence, the team are keen to learn more about this super-Earth system.
Dr John Barnes, an astrophysicist at the Open University and one of the authors on the study published in the journal Science, said: ‘Close orbiting planets like these have a high chance of being tidally locked to their host star.’
‘This means that as the planet orbits its star, the same hemisphere always faces the star,’ the lead author explained.
‘For the planets orbiting GJ 887, half of the planets would be in perpetual daytime and the other half in perpetual night time.
‘So, it could be much cooler on some parts of the planets than others.’
The researchers also detected an unconfirmed signal, raising hopes of a third planet with a more ‘temperate’ orbit, but they are ‘cautious’ about the finding.
‘If the signal is a planet, it would orbit every 51 days,’ said Barnes, adding that we have already seen signals with similar periods we know are coming from the star.
‘This is why we are currently unable to say that the third signal is actually a planet.’
If subsequent observations do confirm it as a planet, it would lie just within the habitable zone of this system
Writing in a related comment piece, Melvyn Davies, professor of astronomy at Lund University in Sweden – who was not involved in the research, said if a third planet is detected in the habitable zone this would become a major study target.
RedDots discovered two more interesting facts about GJ 887, which turn out to be good news not only for the newly discovered planets but also for astronomers.
The first is that the red dwarf has very few starspots, unlike our Sun.
If the star was as active as our Sun, it is likely that a strong stellar wind – outflowing material which can erode a planet’s atmosphere – would simply sweep away the planets’ atmospheres.
The new planets were found using a high-precision, planet-finding instrument on the European Southern Observatory’s 3.6-metre telescope at La Silla in Chile
This means that the newly discovered planets may retain their atmospheres, or have thicker atmospheres than the Earth, and potentially host life, the team wrote.
The other interesting feature the team discovered is that the brightness of GJ 887 is almost constant.
‘In the era of space-based exoplanet-hunting telescopes like NASA’s Kepler and TESS, this result shows that astronomy from the ground continues to play a crucial role in our understanding of planets in our local neighbourhood,’ said Dr Simon O’Toole, a co-author on the study from Macquarie University.
It will be relatively easy to detect the atmospheres of the super-Earth system due to the stable and relatively inactive host star.
This makes it a prime target for the James Webb Space Telescope, a successor to the Hubble Telescope that is due to launch in the next year.
Dr Sandra Jeffers, from the University of Göttingen and lead author of the study, said the planets provide the best possibilities for the study of signs of alien life.
‘The exciting thing about these planets are that they orbit a star so close to the Sun, and so very bright,’ said UNSW-based planet hunter Prof Chris Tinney.
‘We now know of thousands planets of Super-Earth-mass, or smaller. But most of those planets orbit distant and faint stars,’ he said.
‘Planets orbiting nearby stars are key for searches with future telescopes for both explanatory atmospheres, and eventually evidence for life.’
‘If someone had to live around a red dwarf, they would want to choose a quieter star like GJ 887,” writes Davies in his related Perspective article.
The findings were published in the journal Science.
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