Any day now, the thousandth exoplanet discovery will be logged, but Earth's twin is not among them. Where are the habitable planets and why can't astronomers find them?
Imagine Earth's twin planet: shining blue with oceans and laced with white clouds. It orbits a star that is virtually indistinguishable from the sun, and is – of course – teeming with alien life. The problem is that try as they might, astronomers have not been able to find such a world. Even after two decades of searching, an Earth-sized world, in an Earth-like orbit, around a sun-like star eludes us still.
Jean Schneider at the Observatoire de Paris curates The Extrasolar Planet Encyclopaedia, which currently lists 998 exoplanets. He thinks that the focus on Earth's twin distracts from the real goal. "What we are interested in are habitable planets, even if they are not exactly Earth-like," he says. Yet there are still problems, even after widening the goalposts. The majority of known exoplanets are completely unlike Earth. They are either too big, or too small, or just too bizarre. Take the case of CoRoT-7b. It is so hot that astronomers theorise it could rain pebbles, which would condense out of the atmosphere in the way water droplets do on Earth.
This is pure speculation, though, because currently there is no way to analyse the atmosphere of most exoplanets. To do so would require aspace telescope dedicated to the task, but this is exactly what astronomers could get if the EChO space mission is approved by the European Space Agency (Esa). EChO is the Exoplanet Characterisation Observatory. At a meeting of the Royal Astronomical Society in London last week, astronomers and engineers discussed the way this mission would work.
It would target stars known to have planets that pass in front of them. The starlight would pass through the atmosphere of those worlds, and the gases there would naturally absorb certain wavelengths of light.
EChO would capture the remaining light, allowing astronomers on Earth to analyse which wavelengths were missing, and hence which gases are present in the planet's atmosphere. If they saw silicon compounds on CoRoT-7b, for example, then the pebble rain might begin to sound a little more plausible.
The EChO technique has been pioneered using ground-based telescopes to look at the biggest and brightest exoplanets. Now, principal investigator Giovanna Tinetti at University College London thinks it is ripe for taking into space. "Now would be the perfect moment for this mission. The next step for exoplanets must be to analyse their atmospheres and we will have several hundred planets to target," she says.
Until now, missions such as Nasa's Kepler Space Telescope have been designed simply to find planets. Together with follow-up observations from ground-based telescopes, they could provide bulk properties such as the planet's mass and diameter, and its orbit.
From such crude measurements, a rough surface temperature for the planet can be calculated and this has led to a number of headlines about "the most habitable exoplanet so far". In reality, what that means is that their temperatures may be in the range that allows liquid water to exist. And water in liquid form is thought to be a prerequisite for life.
For starters, there are a number of planets orbiting the red dwarf starGliese 667C that may be clement enough for liquid water. There is also Gliese 581g, which is by far the most similar world to our according to the University of Puerto Rico at Arecibo's Earth Similarity Index. The trouble is not all astronomers are convinced that Gliese 581g actually exists. It shows up in some analyses of the data but not in others.
EChO could open the way to a new era in exoplanet research. It could lead to a time when much of the current speculation about conditions is replaced by plausible hypotheses. And it could put us on track to finding worlds with habitable atmospheres; maybe even Earth's twin.
However, EChO is not a done deal. It is a candidate for Esa's M3 launch opportunity and faces stiff competition from four other excellent mission possibilities. Each of the teams is racing to complete their studies to persuade the power-brokers at Esa that their project should be the one to receive the half-a-billion Euros on offer.
The decision will be made in February 2014, and the mission must be ready to launch between 2022 and 2024. In the meantime, if you need an excuse for a party, keep an eye on the exoplanet counter. Any day now, it will clock up the 1000th discovery.