By the time you read this, the number of planets found outside our solar system may have hit the 1,000 mark: a historic landmark in astronomy. Or it might not. It depends on who you ask: the Extrasolar Planets Encyclopedia claims, at the time of writing, 999; Nasa admits to just above 900.
However you count them, not one – as yet – has been “Earth-like”: small, rocky and located in that slender area around its star that allows for liquid water, which is known as the “Goldilocks zone”. It’s finding such worlds that most excites those whose job it is to look for places where alien life might exist.
“I’m not particularly surprised that we haven’t yet found anything truly Earth-like, because most of our telescopes aren’t sensitive enough to detect Earth-like planets,” says Dr Lewis Dartnell, an astrobiologist (or alien-life-hunter) at the University of Leicester. “They’re fiendishly difficult to spot. Fat planets near to their stars are easiest to find; small, rocky ones with an orbit of one year are much harder.”
The search for worlds other than those orbiting our sun is one of the most exciting fields in science. Before 1988, it was not known whether there were any extrasolar planets (“exoplanets”) at all. In the past few years, we have been discovering them at a rate of nearly one a day – or we were.
The astonishing rate of discovery was due to a remarkable machine, the Kepler space telescope, which studied 145,000 stars at a time, looking for planets. “Kepler was so important because it had the capability to spot small planets in the habitable zone,” says Dr Dartnell. If we wanted to find a really plausible candidate for an alien homeworld, Kepler was our best tool.
But sadly, it’s broken. Earlier this year, Nasa announced that it was giving up trying to repair it, after one, then a second, of its four “reaction wheels” – gyroscopes that can point it, with astonishing accuracy, to a given point in space – failed. It had enough redundancy built into the system to survive one such failure, but with the second gone, it was left with only its far cruder rocket thrusters. Unlike the Hubble space telescope, the Kepler is too far away to be fixed.
Like most planet-spotting telescopes, Kepler didn’t actually see the planets directly – it detected tiny, brief dimmings of alien suns’ light that would indicate a planet passing in front of them, a technique known as the transit method. Other telescopes use the “radial velocity” technique, looking for wobbles in the star’s movement caused by a planet’s gravity. These indirect methods aren’t certain, which explains the dispute over the number of planets discovered: different authorities require different levels of confirmation.
“Kepler has thousands of candidates, but they need to be checked off, one by one, by other telescopes, to make sure they’re not just a fluctuation in starlight, or a sunspot, or something,” says Dr Dartnell. Discrepancies arise when there’s a disagreement over whether a planet has been confirmed or not. “This happens all the time in science. Results aren’t absolutely certain at first when you’re right on the cutting edge.”
The next stage is not looking for planets, but determining what they’re like. The next generation of telescopes will be able to look directly at the light coming from these worlds’ surfaces. It’s already been done with some super-large, super-hot planets, but funding has already begun for missions to look at smaller, rocky examples – similar to Earth.
And that is where it gets exciting. Scientists will be able to analyse the chemical make-up of the atmospheres, and look for the tell-tale signs of life as we know it: oxygen and methane.
“You need to look at a planet for a really long time for that to work,” says Dr Dartnell, “so these telescopes won’t be looking for planets themselves. The planet-hunting telescopes will pass on a ‘shopping list’ of potentially habitable candidates, and they’ll study the light that’s come through the planet’s atmosphere, stripping away the much brighter light of the star.”
We’re also looking for life on Mars, and other places in our own solar system. But, says Dartnell, the smart money is on exoplanets. “If there’s life in this system, it’s not conspicuous. And there are maybe three or four places where it might exist. But there are potentially millions upon millions elsewhere in the galaxy. My hunch is that, if there is life out there, we’re likely to discover it far away first, on an Earth-like planet orbiting a different sun.”