Discovery of potentially Earth-like planet Proxima b raises hopes for life

Extract from The Guardian

Thought to be at least 1.3 times mass of Earth, planet lies within ‘habitable’ zone of Proxima Centauri, raising hopes for life outside our solar system

Discovery of rocky exoplanet Proxima b close to our solar system – video report

Nicola Davis

@NicolaKSDavis

Thursday 25 August 2016 03.00 AEST 

The search for life outside our solar system has been brought to our cosmic doorstep with the discovery of an apparently rocky planet orbiting the nearest star to our sun.
Thought to be at least 1.3 times the mass of the Earth, the planet lies within the so-called “habitable zone” of the star Proxima Centauri, meaning that liquid water could potentially exist on the newly discovered world.

Named Proxima b, the new planet has sparked a flurry of excitement among astrophysicists, with the tantalising possibility that it might be similar in crucial respects to Earth.
“There is a reasonable expectation that this planet might be able to host life, yes,” said Guillem Anglada-Escudé, co-author of the research from Queen Mary, University of London.
Eamonn Kerins, an astrophysicist at Jodrell Bank Centre for Astrophysics, was among those enthusiastic about the discovery. “Finding out that the nearest star to the sun hosts not just a planet, not just an Earth-sized planet, but one which is in the right location that it could support life - and there are a lot of caveats there - really underscores that not only are planets very common in our galaxy, but potentially habitable planets are common,” he said.
Proxima b may be the closest of the thousands of exoplanets - which are planets orbiting stars outside our solar system - discovered to date, but at 4.2 light years away the prospect of quick visit to find any Proximese aliens is still remote. Based on spacecraft today, a probe launched now would take around 70,000 years to reach the new planet.

 

Writing in the journal Nature, an international team of researchers describe how they discovered the planet after scrutinising data based on the light emitted by Proxima Centauri, collected using instruments at the European Southern Observatory in Chile.

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“What we basically do is measure how the star is moving,” said Anglada-Escudé. “If you have a planet around a star, the planet is also pulling the star a bit so you see the star is moving. It is going towards you and away from you, periodically.”
This movement affects the colour of light detected from the star - as it the star moves slightly towards us we see the light as being slightly bluer - as it moves away the light appears a little redder. The frequency of this motion relates to the duration of the planet’s orbit, and hence its distance from the star, while the magnitude of the motion provides information about the planet’s mass.
While analysis of data collected before 2016 hinted at the presence of a planet, it took a further intense round of data collection earlier this year before the discovery could be confirmed.
Taking 11.2 days to travel around Proxima Centauri, the planet orbits at just 5% of the distance separating the Earth and the sun. But, researchers say, the planet is still within the habitable zone of its star because Proxima Centauri is a type of red dwarf known as an M dwarf - a smaller, cooler, dimmer type of star than our yellow dwarf sun.
Whether the planet could harbour life, however, is matter of debate.
Red dwarfs are generally very active stars, emitting powerful solar flares, with Proxima b receiving greater doses of high-energy radiation than reaches Earth from our sun. “Because they tend to have a lot of these flares and things like that, it makes it very difficult for [planets] to keep an atmosphere - these flares just blow the atmosphere away,” said Don Pollacco, professor of astrophysics at the University of Warwick, who was not involved in the research. But, he adds, Proxima Centauri is only a moderately active red dwarf, potentially making its environment less hostile than other such stars. Whether the star’s activity when it was younger could have stripped Proxima b of an atmosphere remains to be discovered, while it is also unknown if the planet has a magnetic field which could potentially protect it from such radiation.

The planet has other characteristics that could affect its potential to host life.
The research reveals that if the planet’s temperature were down to its sun alone, its surface would be, on average, a chilly -40C. “It seems cold, but then if you look at the same numbers for Earth you would get minus 20, minus 30C,” says Anglada-Escudé. “What keeps Earth warm is basically that it has an atmosphere and an ocean,” he adds, pointing out that should the newly discovered world also boast an atmosphere, its temperature would likewise be higher.
And there’s more. It is very likely that the new world is tidally locked to Proxima Centauri (just as our own moon is to Earth) with only one side of the planet ever catching the star’s rays.
“If you are standing [on the side] looking at the star, you are quite warm,” said Pollacco. “If you are on the other side it is bloody cold - continual night.”
Anglada-Escudé is sanguine. If the planet has an atmosphere or an ocean, he says, the redistribution of heat could be possible. “Even if you have tidal lock, you have a lot of circulation - it is like jet streams on Earth.”
But Pollacco believes discussion about the habitability of the new planet is somewhat premature, “These observations tell you nothing about the planet itself, it is important to realise that,” he said, pointing out that there is no evidence that the planet has an atmosphere, or any water.
Anglada-Escudé admits that there is plenty to be unpicked about the new world. While its mass is thought to be at least 1.3 times that of the Earth, its size, and hence density, is unknown, meaning that  scientists can only make an educated guess that it is likely to be rocky based on the types of exoplanets that have previously been detected around other small stars.
And there are other mysteries to solve. The planet’s location is also puzzling scientists, who say it is unlikely that the planet formed in its current location given that there would have been little planet-forming material available so close to the star. “What we suspect is that the planet forms somewhere else, or the raw materials, the dust or the rocks or the ice, condense somewhere else and then ended up there through some migration process,” said Anglada-Escudé. He points out that if the planet formed farther away from the star and migrated to its current position, it is likely to be a “water-world”, whereas if it formed in its current location from dust that migrated, it is likely to be very dry unless water was later delivered to the planet by a comet or other body.
While Pollacco describes the new discovery as symbolic, he believes the planet’s potential for hosting life could remain a mystery for many years to come, not least because the chances of seeing the planet pass in front of its star - an observation, known as a transit, that is crucial to unravelling the planet’s size and make-up - are low.
Anglada-Escudé is rather more optimistic. Even if a transit cannot be seen, he believes other techniques could shed light on the nature of the planet. “There are reasonable expectations that this planet can be detected with direct imaging within the next 10 years,” he said, adding that it might also be possible to design a space telescope to orbit around the Earth and look at the new world.
And with Proxima b just an interstellar stone’s throw away, he believes the new discovery could inspire a wave of creativity in designing approaches for probing the planet further.
“Just the discovery, the sense of exploration, of finding something so close, I think it is what makes [it] very exciting,” he said.

• This article was amended on 24 August 2016. The planet orbits at 5% of the distance separating the Earth and the sun, not a fifth as an earlier version said.