One day long from now a person from earth will meet a life form from another planet. It will happen. Not in my lifetime. and when it does – we can only hope to goodness that curiosity matters more than hostility.
A new phase in the search for life elsewhere is about to begin
The Economist 24 August 2016
“We’ve been wondering what planet we’re first going to look for life on. Now we know.” Rory Barnes, of the University of Washington, puts it nicely. Proxima Centauri, the star closest to the sun, has a planet. That planet weighs more or less the same as Earth and is therefore presumably rocky. And it orbits within its parent star’s habitable zone—meaning that its surface temperature is likely to be between 0°C and 100°C, the freezing and boiling points of water at sea level on Earth.
A prize discovery, then, for astrobiologists such as Dr Barnes. And the discoverers are a transnational team of astronomers who have been using telescopes at the European Southern Observatory (ESO) in the Atacama desert, in Chile, for planet-hunting. Though they have not seen the new planet directly (they have inferred its existence from its effect on its parent star’s light), their paper in Nature describes what they have been able to deduce about it.
Proxima Centauri b, as it is known, probably weighs between 1.3 and three times as much as Earth and orbits its parent star once every 11 days. This puts its distance from Proxima Centauri itself at 7m kilometres, which is less than a twentieth of the distance between Earth and the sun. But because Proxima is a red dwarf, and thus much cooler than the sun, the newly discovered planet will experience a similar temperature to Earth’s. It is not the only Earth-sized extrasolar planet known to orbit in a star’s habitable zone. There are about a dozen others. But it is the closest to Earth—so close, at four light-years, that it is merely outrageous, not utterly absurd, to believe a spaceship (admittedly a tiny one) might actually be sent to visit it. Before this happens, though, it will be subjected to intense scrutiny from Earth itself.
That scrutiny will probably be led by ESO. The data which led to Proxima Centauri b’s discovery came from the observatory’s 3.6 metre telescope at La Silla, in Chile. But ESO is also building a much bigger device, the 39-metre European Extremely Large Telescope (E-ELT), at another site in Chile. Since the late 2000s Markus Kasper has led a team at ESO which is designing a specialised planet-spotting instrument, the Exoplanet Imaging Camera and Spectrograph (EPICS), to fit on this telescope. Dr Kasper’s camera has a price tag of €50m, and there have always been questions in the past about whether it is worth the money. But EPICS stands a better chance of producing actual pictures of Proxima Centauri b than any other camera in the world (or off it). Its future can now scarcely be in doubt.
The problem for astronomers trying to catch a glimpse of Proxima Centauri b is that, though close to the Earth by interstellar standards, it is even closer to its parent star by more or less every other standard short of that of walking down the road to the chemist. Seen from Earth, star and planet are 35 thousandths of an arc second apart (an arc second is a 3,600th of a degree). Producing a picture that separates the two objects thus requires a telescope with a resolution good enough to distinguish between the left and right headlights of an oncoming car in Denver from the distance of Berlin.
Things get worse. Dim as it is, Proxima Centauri (pictured above, as seen by the Hubble space telescope) is still more than 10m times brighter than its planet is expected to be. It is as though one of those headlights in Denver was actually the open door to a furnace, while the other was a tea light. This is what makes the E-ELT and EPICS crucial. EPICS contains a coronagraph—a tiny shield that blocks out a star’s light so that adjacent planets can be seen. Unfortunately, a coronagraph reduces a telescope’s resolution, meaning you need an even bigger one to see the target in the first place. To observe Proxima Centauri b using a coronagraph, and looking in the infrared wavelengths that are likely to provide the best information about its atmosphere, you need a telescope at least 20 metres across; 30 metres would be better.
Two other large telescopes besides E-ELT, of 27 metres and 30 metres diameter, are under construction and planned. But some suggest the first of these, the Giant Magellan Telescope, also in Chile, is not well suited to the use of a coronagraph, and the second, the Thirty Metre Telescope, is planned at the moment for Hawaii, which is in the northern hemisphere. Proxima Centauri is in the southern skies, and therefore not so easy to study from north of the equator.
There may, just possibly, be a short cut. Though Proxima Centauri b was discovered by the radial-velocity technique, there is about a 1.5% chance that it might also be detectable by the transit technique. This would require Earth to sit in the same plane as the planet and its star, so that, seen from Earth, it crosses the face of its parent and thus slightly dims it. Various southern-hemisphere telescopes are already looking for the 0.5% dimmings of Proxima Centauri’s brightness which would be observed if Proxima Centauri b were transiting. If it is, then it will be possible to study its spectrum by subtracting the star’s spectrum when the planet is hidden behind it from the spectrum measured during a transit. That would reveal the chemical composition of its atmosphere, if it has one—which might, in turn, give a clue as to whether it harbours life.
Life on Earth leaves a sign of its existence in the atmosphere, in the form of oxygen. This is produced by plants and it is such a reactive chemical that if their photosynthesis stopped it would disappear rapidly from the air. Free oxygen in Proxima Centauri b’s atmosphere would therefore get a lot of people excited—but possibly without justification, for there are ways (such as the dissociation of water by ultraviolet light) to put oxygen into atmospheres abiotically. A stronger indicator of life would be finding both oxygen and molecules associated with biology that cannot long persist in its presence, and must thus be produced continuously.
On Earth nitrous oxide and methane fit this bill, though these molecules are present in amounts much too low to be seen light-years away. But David Catling, a colleague of Dr Barnes at the University of Washington, points out that there are models of planetary atmospheres which allow methane to build up quite a lot on a planet like Proxima Centauri b. If spectra showed methane and oxygen together, the likelihood of life on Proxima Centauri b would rise dramatically.
Another way to look for life on Proxima Centauri b would be to search for radio signals. Life in general does not generate radiation at radio frequencies. But intelligent life does—at least it does on Earth. And that Earth-bound life also puts a tiny bit of effort into looking for such emissions from elsewhere, an endeavour known as the search for extraterrestrial intelligence, or SETI. There have been SETI studies of Proxima Centauri in past decades, but they have not been particularly sensitive. If the inhabitants of Proxima Centauri b were beaming powerful transmissions at Earth all day and night they would have been heard; if they were merely using radio for their own ends, in the way that broadcasters and radar systems do on Earth, they would not.
Now, though, a big new SETI project bankrolled by Yuri Milner, a Russian billionaire with a taste for things interstellar, is collaborating with the Parkes radio telescope in Australia and the Meerkat radio array in South Africa. According to Dan Werthimer of the University of California, Berkeley, who works on Mr Milner’s programme, Proxima Centauri will now be getting a lot more attention.
That there is intelligent life in the nearest planetary system to Earth’s is surely the longest of shots. And despite its nice-sounding location in the “habitable zone”, the existence of life itself on Proxima Centauri b is far from a foregone conclusion. For one thing, there are doubts about how easy it is for planets around red dwarfs to develop and retain atmospheres. Though such stars are cool for most of their lives, in their earliest stages they burn bright. A planet close enough to one to stay warm in later life might have seen its atmosphere burned off in the star’s brief blazing youth. Even if it avoids this problem, it will still be whipped persistently by the star’s magnetic field and lashed by its flares. Though they are dim, red dwarfs are given to all sorts of eruptive activity and pump out X-rays at a prodigious rate. These are both things which might make an atmosphere hard to hold on to and life itself a bit tricky.
Such doubts will not stop people looking. Indeed, the discovery of Proxima Centauri b may accelerate plans to construct space telescopes designed especially to look directly at exoplanets by employing what is known as a starshade, instead of a coronograph, to block the light from parent stars. A starshade is a second spacecraft, flown in formation with the telescope, that eclipses the parent while leaving the planet visible. Sara Seager, a planet hunter at the Massachusetts Institute of Technology, is a fan. She wants all future space telescopes to be designed with the use of starshades in mind.
The most radical form of follow-up, though, is an idea for interstellar flight that Mr Milner is working on in parallel with his SETI project. Light beams exert pressure. A powerful laser focused on a low-mass spacecraft could in principle accelerate it to a significant fraction of the speed of light. This would be awesomely difficult in practice. It would require hundreds of thousand of lasers yoked together to make a single coherent beam of 100 gigawatts or so. That is about the maximum electrical-power consumption of France. It would also require a spacecraft weighing just a few grams that could sit at the beam’s pinnacle without getting fried; a way of coping with high-speed collisions with dust particles that the craft would inevitably encounter en route; and a method of getting data back to Earth once the probe reached its destination. But it might not be impossible. Over the next six years Mr Milner’s “Starshot” programme plans to spend tens of millions of dollars to test the idea. If encouraged by the results, he may try to build a test rig in which a laser zaps a tiny object to the highest speed ever achieved outside a particle accelerator.
Meanwhile, Dr Kasper will get on with his camera for the E-ELT. Others will look for evidence of other nearby planets—including around Proxima’s bigger, more sunlike companions, Alpha Centauri A and B. The discovery of Proxima Centauri b is a huge step forward in the search for life elsewhere. But the journey is nowhere near over yet.