Inside the Search for Life on ‘Nearby’ Proxima b

Canadian space telescope helping to unlock planet’s mystery.

By Eric Nadal 23 Nov 2016 |

Eric Nadal is a Vancouver-based writer and UBC graduate with degrees spanning physics, planetary science, ethics and the philosophy of science.

Some of the brightest, best understood objects in the universe, from supergiant stars to superluminous galaxies, are also the most distant. By contrast, some of the closest things to us are so dim we don’t even know they are there.

Like a hatchling just breaking out of its shell, our species is still getting to know our solar system and its immediate neighbourhood, which were bound to have a few surprises in store for us.

A major one came in August with the announcement of a potentially Earth-like planet, not across the galaxy or even a ways down the road but right next door, orbiting our sun’s closest neighbouring star, Proxima Centauri.

It’s a stunning development even for astronomers now swimming in thousands of planet discoveries that suggest potential Earths — planets similar in size to ours, orbiting in their stars’ Habitable Zones — are fairly common. By conservative estimates there was only a six per cent chance that our closest neighbouring system would actually have one.

Now, a team using Canada’s MOST space telescope is weighing in on the planet, “Proxima b,” and its small but feisty “red dwarf” sun. Around red dwarfs, the Habitable Zone hugs dangerously close to what are typically active “flare stars,” subjecting a planet like Proxima b to fierce bursts of charged particles, X-rays and UV light. With an unprecedented sampling of this activity, the MOST study is the first detailed space weather report for another solar system.

If Proxima b doesn’t give us next door neighbours, it will still provide something remarkable — an “amazing laboratory” to understand potential Earths under extreme stress from their star, says University of British Columbia astronomer Jaymie Matthews, MOST mission scientist and principal investigator. He likens the opportunity to that of a metallurgist testing a material “to the breaking point.”

It’s not clear if any magnetic field and atmosphere on Proxima b can be maintained for long, let alone provide adequate shielding to preserve any water and life at the surface. Worlds like Proxima b may be near the limit of what’s “habitable” and “Earth-like,” and we don’t know which side of the line they’re on.

“It’s like that song, ‘New York, New York’... if I can make it there, I’ll make it anywhere,” said Matthews. “The flares and all this ultraviolet can be a bad thing for life but it may also be an essential spark to trigger life in the first place.”

It might be suitcase-sized but MOST, Canada’s first space telescope, packs a mean punch, boasting a history of overturning reported discoveries of “exoplanets,” or planets orbiting other stars.

“We have been planet killers in the past and we could have been a planet killer in this case,” said Matthews, conveying his confidence that our newfound neighbour is real.

MOST lets scientists precisely measure brightness variations in a star and distinguish between the effect of a planet and the effect of sunspots and other changes that have fooled planet hunters in the past. Happily, the team found no modulation of the star’s light that could have been masquerading as the reported planet.

Even 10 years ago most astronomers didn’t think red dwarfs, small and relatively cool stars, were a good place for planets to form, much less support a biosphere. Now, the flare activity and other complications of being so close to the star are no longer thought to be complete show stoppers for life.

“It could mean we actually have neighbours,” Matthews said of Proxima b.

The strangest of Earths

Proxima b orbits extremely close to its sun, a blazing white orb tinged red that would appear nine times larger in its sky than the Sun appears in ours. Close enough to generate powerful tides on the planet, this sun would have slowed Proxima b’s rotation in a process called “tidal locking.” This would likely yield a day 22.4 times longer than ours, maybe even one that never ends.

To see the landscape in its full glory would require eyes that could see in the infrared band, where the star puts out most of its energy.

To human eyes, Matthews estimates that “at high noon on Proxima b, the sky would be about as bright as a clear Earth sky just before sunset, or heavy overcast at noon.”

For many latitudes and long stretches of time you would be in twilight.

You would be treated to some rather exotic weather. Wave after wave of the red sun’s flares and resulting space weather could help light up the sky in a much brighter version of our northern lights, produced on a global scale.

Assuming the planet is not airless and lacking a magnetic field, these “Proximan lights” would be 100 to 10,000 times brighter than our aurorae, according to Rodrigo Luger and a team out of the University of Washington. This could help light up a relatively dim landscape and “decloak” the planet for astronomers on Earth, rendering it directly visible with new advanced telescopes.

As strange as they may sound, planets like Proxima b appear to be the most common kind of potential Earths out there, since 75 per cent of all stars are red dwarfs. Thus, whether these systems are prone to life, or just prone to flares, may determine how prevalent life will be in the galaxy.

With Proxima b right next door astronomers can gain insight into these systems at a level of detail never previously imagined. Upping the ante, the Proximan system is as old or older than our own solar system.

“It's the best-case scenario for astronomers interested in exoplanet habitability,” said Luger.

A laboratory for life

Aurorae aside, Proxima b’s severe space weather doesn’t create the prettiest of pictures, at least to human eyes. “We’ve seen 10 huge flares over the span of a few weeks time,” said Matthews. “We estimate the star has flares which we would consider major flares on the sun two or three times a day. Even at its quietest it’s still pretty loud.”

That’s just what occurred while MOST happened to be looking. The team’s statistical analysis, led by James Davenport of Western Washington University, suggests that superflares 10 times stronger than anything ever observed on our sun would be produced eight times a year. To make matters worse, Proxima b has a front row seat, 20 times closer to its sun than we are to ours.

An atmosphere with an ozone layer could keep deadly UV exposure brief, and not every flare would be pointed at the planet. But a single one of these superflares would obliterate an ozone layer like ours. The team warns that any atmosphere on Proxima b may not have the time it needs (at least a few years) to fully recover between bombardments.

“It’s a challenge but I don’t think it rules out biology,” said NASA and University of Washington astrobiologist Rory Barnes, when asked about the MOST team’s findings.

Barnes’ team argues in a recent study that this scenario may simply curtail bioproductivity rather than eliminating it. “Regardless, life can happily survive in an ocean while the star flares away,” he added.

Then again, maybe Proximan life need not be so delicate. In another recently submitted paper, Jack O’Malley-James and Lisa Kaltenegger of Cornell University examine the potential of biofluorescence, where UV light is absorbed and re-emitted at safer, visible wavelengths.

Organisms on Earth, from fish to spiders, have evolved biofluorescence for all sorts of reasons. But in corals it’s a possible method of UV protection.

The study’s authors imagine evolution scaling up biofluorescence on Proxima b. At complete efficiency, which has been demonstrated with proteins in the lab, the modelled biosphere makes the planet change colour in response to a superflare.

Ironically, such hardy, luminous life, as difficult as it might be to evolve, could be easier for us to detect one day from Earth.

Testing our luck

Proxima b was already a gift to astronomy and astrobiology, and a pretty lucky one. But there was one final stroke of luck still on the table.

Perhaps the most anticipated result from the MOST team was a chance to see if the Proxima system has the right orientation in space (an orbit tilted along our line of sight) to let us view the planet transiting its star, a kind of eclipse in miniature. This would dramatically open up our ability to analyze the planet’s composition and search for signs of life.

A transit detection (a measured dip in brightness as the planet passes in front of its star) would also immediately pin down Proxima b’s mass, size and density. Its mass is probably between 1.3 and three times Earth’s, meaning it has a very good chance of being a solid, rocky planet, but a transit is the best way to confirm this.

In a recently submitted paper still under peer review, the MOST team does report a “candidate transit signal” that matches Proxima b well. In normal circumstances it might “pass the smell test,” said Matthews. However, the noisiness from the flares, and a lack of corroboration from ground-based facilities, lead them to suspect it’s not a real transit.

Trying to spot a transit amid the flares is a bit like trying to spot a whale just from the splash it makes: normally doable but try it in choppy seas during a storm. You simply “grit your teeth” and hope for the best, joked Matthews.

In the end, the flare activity was worse than expected. “We knew the star would flare a lot but I was certainly surprised by the sheer volume of flares observed,” said David Kipping, from Columbia University, who led the transit search.

Aviv Ofir, a member of the team that discovered Proxima b, thought the MOST candidate signal, while inconclusive, certainly justifies expanding the transit search. Barnes and Luger agreed.

Matthews says the team wants to continue to pursue the effort, possibly with more MOST data. “Whether it’s the MOST data that can demonstrate whether the planet is transiting, or other data... to be honest, that doesn’t matter to me. It’s just, boy, I really hope it’s a transiting system.”

Without a transit, it’s still possible that behemoths like NASA’s James Webb Space Telescope, due for launch in 2018 (and even larger ground-based telescopes planned for the mid-2020s) could detect whether it has an atmosphere and ozone layer.

“If we launch a big enough telescope into space, we will be able to image Proxima b directly,” said Barnes, looking long term.

In the meantime, the flare activity may continue to scuttle many efforts to learn about our newfound neighbour. Regardless, Proxima b has changed the exoplanet field forever and is bound to loom large in the human imagination for generations.  [Tyee]

Read more: Science + Tech

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