Dead stars could be a great place to look for life according to a theoretical study carried out by a pair of researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) and Tel Aviv University, Israel, which suggests that a survey of the 500 closest white dwarfs have a one in three chance of harbouring a rocky world similar to Earth.
White dwarfs, which are much smaller and fainter than the Sun, are very dense, low mass stars; remnants of the expansive end of swollen red giants that are glowing white hot after dispelling their gaseous layers. These pint-sized stars might be dead, but theorist Avi Loeb at the CfA and his Tel Aviv University colleague Dan Maoz theorise that any rocky planets that might be orbiting them could be very much alive.
Sitting in their closer than average habitable zone – where temperatures support the existence of liquid water – which sees them whipping around their star in a year equal to 10 hours, the duo have a couple of thoughts as to what these alien worlds might be like on their surfaces. “On the one hand, they could be quite similar to Earth because they are rocky and they are in [their] habitable zone so they could have oceans, rivers and rain,” Maoz suggests to All About Space.
“On the other hand, such planets will most likely be tidally locked to their white dwarfs, with the planet always showing the same side to their [star],” Maoz continued. It is here, he goes onto say, that there will be a permanent day side to the planet, which will be quite warm and a permanently cold side which looks out into space. “Life, if present, would probably thrive mostly in the temperate ‘twilight zone’ around the day-night terminator of the planet,” he adds.
However, while the researchers are busy piecing together the unknown, we still have yet to find these elusive worlds – which, if tidally locked to their star, have already been estimated to have summers that, almost in the blink of an eye, rapidly turn into winters in a short timeframe of just 5 hours.
Searching these dead stars for planets is something that exoplanet hunter Matt Burleigh at the University of Leicester attempted back in 2011 alongside lead author Francesca Faedi of the University of Warwick with the help of SuperWASP, two robotic observatories situated at Roque de los Muchachos Observatory on the island of La Palma in the Canaries and at the South African Astronomical Observatory in South Africa.
Of the 200 white dwarfs he and his team pinpointed, sadly nothing was found. “Loeb and Maoz’s work is an interesting addition to a group of recent papers speculating on the possibility of Earth-like planets around white dwarf stars,” says Burleigh who believes that the proposed ratio of an Earth-like world existing around one in three white dwarfs is very optimistic. “[However] the biggest problem is that we don’t know if terrestrial planets exist in orbits near white dwarfs where water would be in liquid form. I suspect we may have to survey thousands of suitable white dwarfs before we find an Earth in a stable orbit in a white dwarf’s habitable zone.”
Finding a terrestrial planet hugging a white dwarf as it passes across the disk of its stellar companion might seem like the hard part for now, but if such a world is found, NASA’s yet to be launched James Webb Space Telescope will be put on the case of sniffing out the gases that hint towards the existence of an organism friendly world – such as oxygen and water vapour.
Loeb and Maoz’s work suggest that the abundance of heavy elements found embedded in the limbs of dead stars could be the smoking gun for the possibility of rocky worlds around them. “Their presence and their abundance patterns reveals that white dwarfs frequently swallow up rocky bodies around them,” said Maoz. “Such rocks could be the remains of larger rocky planets, or they could be potential building material for a new generation of rocky planets.”
“We should keep in mind that a couple of decades ago it was believed that [Jupiter-sized planets] should not reside close to their parent stars,” says Loeb who led the study which has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. “As a result observers were denied time on telescopes when they proposed to look for close-in Jupiters (even though they could find them technologically) and the discovery of the first exoplanets was delayed for over a decade. By now observers have discovered a large population of hot Jupiters that could have been discovered a decade earlier. The lesson from this experience is that in searching for exoplanets we should not limit our investigations by a prejudice.”
Burleigh was able to do his 2011 survey using free archive data collected by SuperWASP. However, he agrees that getting telescope time to strike out into the darkness can be hard. “Sadly, it’s difficult to get time to do fishing expeditions, but just one detection would change that”
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Images courtesy of ESA and NASA