There might be a planet better than Earth – right next door

There might be a planet better than Earth – right next door
There should be worlds out there so balmy they make Earth look stale, and there are signs of one just four light years away. That's close enough to visit...

By MacGregor Campbell

“WHEN I was a kid, I was always looking at Alpha Centauri,” says Eduardo Bendek. One of the things he discovered about it while growing up in Chile was that our closest neighbouring light had a secret: it is not one star, but two.

More than 30 years later, Bendek, now an astronomer at NASA’S Ames Research Center, suspects that his favourite celestial beacon might just be hiding another, more marvellous secret. There could be a planet orbiting one of the stars. And not just any old space rock. This could be a place so bursting with life that it makes Earth look post-apocalyptic.

And at a mere 4.4 light years away, we might feasibly develop a probe that could visit within decades. That’s precisely what a project backed by Stephen Hawking and billions of dollars now plans to do. We could catch our first glimpse of this bucolic world within a generation.

We are used to thinking small when it comes to alien life. Our list of living worlds has a sole data point, Earth, and even our convivial planet seems to have been a tricky place for life to get started. How could we expect more than a self-replicating bag of biomolecules anywhere else?

That might be too lofty a view of Earth. After all, huge areas of our planet, including the poles and deserts, are rather barren. And whole epochs of time were inhospitable to life.

Time was perhaps the most important attribute identified by astrophysicist René Heller of the Max Planck Institute for Solar System Research in Gottingen, Germany, when in 2015 he was considering the factors that make a planet more habitable than Earth. Habitability isn’t just about having the correct balance of temperature and chemicals. Life takes aeons to get started, so it’s also about how long those persist.

One reason Earth might not top the charts in terms of its habitable lifetime is the size of the sun. The smaller a star, the more efficiently it uses its fuel and the longer it lasts before blowing up, taking any nearby planets with it. The sun is a G-type star, the third smallest variety. So planets orbiting diminutive K and M-type stars can expect longer lives.

But water is a factor here too. The presence of liquid water is thought to be a prerequisite for life. And to host this life-giving stuff, planets should ideally reside in a habitable zone: the band of space around a star with the appropriate temperature. The habitable zones of M-type stars must be far closer in than in those of warmer stars like our own. So snug, in fact, that the star’s gravity is likely to wreak havoc on any planets. The difference between the gravitational tug on the front and back of the planets can deform them into an egg-shape, which eventually stops them spinning. Hardly ideal for life. A K-type star, on the other hand, would have just the right conditions. And that’s exactly what Alpha Centauri B, the smaller star of the pair, is (see Graphic below).

Alpha Centauri
Alpha Centauri is a star we could conceivably visit
ESO/Digitized Sky Survey 2/Davide De Martin
A second thing that makes a planet more likely to be benign for longer is its size. Rocky planets like Earth can have liquid metal cores, a boon for life because it drives plate tectonics, which in turn refreshes atmospheric gases. The liquid metal also spins, creating a magnetic shield around the planet that deflects biomolecule-destroying radiation. Once the core cools and solidifies, those effects vanish. A planet bigger than Earth would take longer to cool because there is more bulk for the heat to dissipate through.

Could a large rocky planet be orbiting Alpha Centauri B? In 2012 Xavier Dumusque at the Harvard Smithsonian Center for Astrophysics and colleagues turned their telescope on the star, looking for the characteristic wobble caused by a planet’s gravitational tug. They found evidence for a planet about 10 per cent larger than Earth with an orbit of just over three days.
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Cue serious excitement. These initial observations made the planet look a little hot, but here nonetheless was evidence for a planet with some traits that could make it superhabitable – and right in our cosmic back garden.

The excitement was short-lived. Last year, Suzanne Aigrain at the University of Oxford showed that the wobble Dumusque spotted was almost certainly a measurement error. Still, that is no reason to give up hope. NASA’s Kepler telescope, which hunts for exoplanets in the Milky Way, has found so many that a star without one now seems unusual. Many are “super-Earths” – precisely the large rocky worlds that fit the bill for superhabitability.

Also encouraging are the measurements that rule out rocky planets more than three times the size of Earth. That’s good because when planets reach such a size they become less likely to be hospitable to life. The increased pressures halt internal heat convection and the crushing gravity shuts down plate tectonics.

Plus, Christophe Lovis at the University of Geneva says measures of the star’s wobbles rule out any gas giant planets orbiting close to it. Traditionally, such a planet would have been thought likely to destabilise the orbit of a rocky world or swallow it up, although that’s now being questioned (see “Pinball planets“). “I think it’s more likely than not that there is a potentially habitable planet around at least one of Alpha Centauri’s stars,” says Bendek’s collaborator at Ames, Ruslan Belikov.

To know for sure we need pictures, so Bendek and Belikov have proposed a mission to get them. To snap an exoplanet, you need a telescope equipped with a coronagraph, a shade that blocks the blinding glare from the parent star. Alpha Centauri’s two stars make things even more difficult.

Existing space telescopes such as Hubble were not designed to block the light from stars as close to one another as the Alpha Centauri pair. So Bendek and Belikov came up with an instrument they call ACESat. This would suppress both stars’ light using a coronagraph in conjunction with a deformable mirror. Subtle ripples in the mirror deflect the light of each star separately, and simulations show it would be able to reveal Earth-sized planets in the habitable zone of either one. “I realised you could directly image an Earth around Alpha Centauri with a telescope as small as 25 centimetres,” says Belikov. That is small enough to fly into orbit at what is, in space freight terms, rock bottom price.

Sweet spot
Once it’s there, Belikov knows exactly where to point it. Computer simulations show there’s nothing further out from the stars than roughly 2.5 times the Earth-sun distance, since such an orbit will not be stable, he says.

At the moment, however, NASA is not about to take up the Ames duo’s plan. A mission focused on just two stars is simply too chancy for it, says Bendek. In contrast, the Kepler mission has surveyed more than 100,000 stars. In terms of funding, “when you start to look at single targets, exoplanet discovery gets a lot more difficult,” he says.

Undeterred, Bendek and Belikov are exploring private financing. That may not be a bad move. After all, Russian billionaire and physicist Yuri Millner has already hatched a plan to visit Alpha Centauri. Announced last month, the Breakthrough Starshot initiative plans to send a tiny spacecraft there.

It doesn’t seem beyond the pale, even if not all of the required technology exists yet. The plan involves accelerating the craft to 20 per cent the speed of light by equipping it with a solar sail and pushing it with a giant Earth-based laser.

It has been 37 years since Bendek first looked at Alpha Centauri through a telescope. It will be at least as many before we get a close-up look at the stars. But never before has the idea of visiting another star been within the realms of possibility. And we have a fantastic reason to make the trip. There could be a paradise next door.

This article appeared in print under the headline “Paradise next door”
MacGregor Campbell is a consultant for New Scientist based in Portland, OrgeonMagazine issue 3074, published 21 May 2016
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