When scanning the skies for potentially habitable exoplanets, astronomers usually look for similar conditions to Earth. That means warm Sun-like stars are great, but tiny, turbulent pulsars aren't usually given a second thought in the search for life. But a new study has calculated that these neutron stars could indeed host habitable planets – under very specific circumstances.
Lucky for us, the Earth orbits within our Sun's "habitable zone," the area around a star where planets can stay at just the right temperature for liquid water to form. Since our home planet is the only one we know of that supports life, we can narrow down the search for other potentially habitable worlds by looking for similar conditions elsewhere in the universe.
NASA's Kepler mission is doing just that. The habitable zone varies by the star: Planets orbiting big, hot stars will need to be further away, while small red dwarfs could support life on planets tucked up relatively close.
But pulsars aren't generally thought to be contenders. Usually no more than about 30 km (19 mi) wide, these burnt-out husks of stars barely emit any heat or visible light, and are constantly "pulsing" with huge bursts of X-rays and energetic particles that would sterilize most planets. Exoplanets have been discovered orbiting pulsars before, but these conditions are thought to be far too hostile for life to take hold.
To check if that assumption holds true, a new study from astronomers at the University of Cambridge and Leiden University set out to calculate the habitable zone of a pulsar. Sure enough, they found that there is such a zone, and it can stretch as wide as the distance between the Earth and the Sun.
But there's a catch. The planet has to be a so-called super-Earth, meaning it has a mass up to 10 times bigger than our little blue marble. Anything smaller than Earth would have its atmosphere blasted off in a matter of millennia – a blink of an eye, on the cosmic scale.
This super-Earth would also need an extremely thick atmosphere – a million times thicker than Earth's – in order to protect the surface from bombardment of X-rays and high-energy particles. Such an atmosphere would convert that energy into heat, helping to compensate for the pulsar's lack of warmth. The researchers say that the surface of this hypothetical planet would resemble the bottom of an Earth ocean, thanks to that soupy atmosphere.
To test their idea, the astronomers looked to a pulsar named PSR B1257+12. Lying about 2,300 light-years from Earth, this pulsar hosts three planets, which were, in fact, the first exoplanets ever discovered.
By way of the X-ray-sensing Chandra space telescope, the astronomers determined that two of those three planets could be contenders. With masses of about four or five times that of Earth, these two planets are super-Earths and are within the calculated habitable zone. It's a promising start, but we don't know enough to tick all the boxes just yet.
"The temperature of the planets might be suitable for the presence of liquid water on their surface," says Alessandro Patruno, co-author of the study. "Though, we don't know yet if the two super-Earths have the right, extremely dense atmosphere."
The research was published in the journal Astronomy & Astrophysics.
Source: University of Cambridge
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