After decades of searching, astronomers say they've finally detected compelling evidence of an alien world orbiting one of the closest and most iconic stars in our cosmic neighbourhood.
Barnard's star, a small red dwarf star in the constellation of Ophiuchus, is a favourite of astronomers and sci-fi fans.
Located less than six light years away, it's the closest single-star system to us — only the Alpha Centauri triple-star system is closer.
Its size and prime position make it the perfect place to look for alien worlds. But lots of astronomers have tried — and failed — to find anything.
In the 1960s, astronomer Peter van de Kamp claimed to find evidence of two Jupiter-sized planets orbiting the star. But his findings were later disproven; the bumps in his data were caused by telescope errors.
More than 50 years later, an international team of astronomers has found signs of a frozen 'super-Earth' around the star.
Dubbed Barnard's star b, the potential world is at least 3.2 times the mass of Earth and orbits the star every 233 days, they report today in the journal Nature.
With that orbit it would sit near its sun's so-called snow line — the point where ice or volatiles are frozen — so temperatures at the surface would be a chilly -170 degrees Celsius, they predict.
The team, led by Ignasi Ribas of the Institute of Space Studies of Catalonia, is more than 99 per cent confident their signal is correct.
"When we saw the signal the first thing we tried to do was to get rid of it. It's not there, let's try to kill it."
"But in 99.2 per cent of the cases this is a planetary signal.
"We'd like to be more convinced, we'd like this number to be 99.9 per cent, but this is what it is so far with the measurements," he said.
Barnard's star b is likely to be a frozen world.
The long hunt for alien planets
Thousands of alien planets have been discovered by the now-defunct Kepler telescope, but that just skims the surface of what is potentially out there.
Kepler and NASA's new TESS planet-hunting telescope can only find planets that orbit directly in front of their sun from Earth.
"That's only about 1 per cent of the planets that will have this property, which means we're missing 99 out of 100 planets," Dr Ribas said.
How to spot a planet
Three ways to spot a planet
- The transit method (used by Kepler and TESS) identifies dips in light as a planet passes directly in front of its sun
- The radial velocity technique (used in this study) which detects changes in light as an object moves closer or further away
- Astrometry (used by van der Kamp and the Gaia Space Telescope) which looks at the changing position of the star against the plane of the sky
To spot something around Barnard's star you need to use a technique called radial velocity.
This technique measures changes in light from the star as it moves towards or away from us, a phenomenon known as the Doppler effect.
"Using the Doppler effect we can measure to about one metre per second which is a typical walking speed of a person, or strolling speed.
"Then we can actually measure the star moving at this tiny little motion, this wobble caused by the planet," Dr Ribas explained.
This technique has been used before to look for planets around Barnard's star, without success.
"We have four times the data [than previous attempts]," Dr Ribas said.
"[We used] seven different instruments around the world that acquired a total of nearly 800 observations over 20 years.
The data included new observations from the Red Dots campaign, which is scouring the nearest red dwarf stars to us for planets.
"The tricky part here is stitching all the data sets into one," he said.
But this technique doesn't tell us exactly how big or what kind of planet this could be.
"It could be large rocky world or a small ice giant like a mini-Neptune," Dr Ribas said.
It will take even more data to confirm whether this signal really is caused by a planet and not an aberration such as a solar storm.
"One possible way of doing that is by going back to the 1960s claims and seeing if the star moves, and it if moves, by how much," Dr Ribas said.
"And if it does it could be related to this planet or to another planet," he said adding there are hints of another potential planet in the data.
Known as astrometry, this is the technique used by the European Space Agency's Gaia telescope, which has Barnard's star in its sights. Other telescopes such as the upcoming James Webb telescope may also help to fill in the blanks.
Pushing the boundaries of science
Astronomer Jonti Horner of the University of Southern Queensland said the finding pushes the boundaries of what's possible using current technology.
"It is exciting if this planet is really there. It will be one of the closest planets to the solar system ever found, full stop," Dr Horner, who was not involved in the research, said.
But, he cautioned, the data is not definitive.
"I wouldn't say it's the most convincing discovery, but it's an interesting suggestion there could be something there," Dr Horner said.
Barnard's star is less than six light years away from our sun.
Dr Horner said the finding was very timely with the advent of new telescopes such as Gaia and James Webb.
"If the planet that we have found really is there then we should be able to detect it in the Gaia data in the next couple of years," Dr Horner said.
"[The authors] aren't confident that the statistics are 100 per cent guaranteed yet, but if they announce it now people can look with all these other tools and try and confirm it.
"And that's typically how science works, especially when you are pushing the boundaries back."
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