A web exclusive story from Astronomy magazine

The exoplanet next door

There’s no star closer to us than Proxima Centauri — and now we know it has an Earth-mass planet in its habitable zone.

The hunt for exoplanets has, in some ways, been about the hunt for an Earth-like planet – something warm where water could exist. Headlines tout each discovery as “the most Earth-like planet yet.” Many of those planets are far away.

But a new discovery published August 24 in Nature hits closer to home, with an Earth-mass planet in the habitable zone of its star. What’s more, that star is Proxima Centauri, only 4.24 light-years away. That means that there is no solar system that will be closer to Earth in our lifetimes.

And so far, the exoplanet, named Proxima Centauri b, is shaping up to be quite Earth-like, roughly the mass of our planet and in just the right place where, if it has an atmosphere, liquid water could exist on the surface.

This is as in our backyard as it gets.

“I think it actually marks a transition,” Jeffrey Coughlin, a SETI Institute scientist not involved in the study who assembles the Kepler catalog, says. “Twenty years ago, we were finding the first exoplanets and it was totally exciting,” he says. Then there was the Kepler telescope, which found thousands of planets, including some in the habitable zone, and some within a few dozen light-years of us.

And now there’s a planet of 1.3 Earth masses right next door, zipping around its star in 11.2 days. Its distance of 4,349,598 miles (7 million kilometers) from its star may seem tiny, at just one-fifth the distance between Mercury and the Sun, but Proxima Centauri is the runt of the litter in the Alpha Centauri system. At a diameter of 124,274 miles (200,000km), it’s only 1.43 times the diameter of Jupiter,.

So how was there a planet hiding around the closest star to us, just waiting to be discovered? The simple answer: Finding a planet is really hard. Kepler found thousands of planets by staring at 145,000 stars in a minute region of the sky at the tail end of Cygnus, waiting for the 1 percent chance a planet would directly pass in front of a star and cause a dip in its light, in a method known as transiting.

But the problem with the Proxima Centauri planet is that it doesn’t transit — at least not from our vantage point. In order to witness a transit, the orbital plane of the planets must be at or near our line of vision, but not all solar systems have the same orientation. A star might have all of its planets aligned at a 90-degree angle from us, with the planets orbiting in such a way that they never pass in front of their star for our telescopes to see. While some planets have been found by direct imaging (that is, appearing in a photo along with its star) it’s not possible of yet with Proxima, a 5 billion year old planet. Unless the planets are very young and very large, no instruments are currently capable of directly imaging these planets.


How to find a planet (that doesn’t want to be found)

That’s why the Pale Red Dot project, tasked with finding a planet around our nearest neighbor, had to turn to indirect — but reliable — methods of detection. The researchers chose radial velocity, a process that looks for shifts in a star’s light due to the tug of a planet, sometimes called the Doppler shift method. Subtle movements of gravity cause the light of a star to move toward the blue end of the light spectrum, which means it’s moving toward us, or the red end of the spectrum, which means it’s moving away. Based on those changes, researchers can give a mass estimate, and the frequency gives an idea of the orbit.

The planet itself was found over a series of nights from January 19 to March 31, 2016, during which Proxima was monitored closely for subtle variations on the European Southern Observatory’s HARPS instrument.

“Instead of applying for a few nights over a semester and repeating the same thing over the years, what we did here is try to convince ESO it was worth doing intensive campaigns to monitor the star 60 days in a row, only 20 minutes per night,” principal investigator Guillem Anglada-Escudé said in a press conference.

The end result is a planet as close to us as any could be, outside of a similar discovery in the same system.

The project team, however, has been cautious every step of the way. They ignored weak evidence of the planet’s existence stretching back to 2013 in favor of stronger observations over the subsequent three years, factoring in other studies of the star stretching back 16 years. Though fairly convinced it’s a planet (above 99 percent certain, in fact) based on their data, the researchers still refer to the planet as a candidate. A possible second planet in the system with a 60-day or longer orbit only gets a passing mention in the paper, ignored in favor of the solid, concrete evidence.

The researchers also kept a tight lip on their findings, awaiting the end of the peer review process. But as their intended press conference approached, an anonymous ESO astronomer leaked the story to the German press, sending Pale Red Dot into damage control, with the team keeping a tighter lid on its findings as rumors swirled in the astronomy community. Nature was forced to address the rumors in its materials to the press before the official announcement.  

Technically, the planet was spotted as early as 2013. That signal was weak, however. Subsequent observations bolstered the planet’s case, but it wasn’t until the most recent observation campaign that the evidence became a solid match for a planet and not some other stellar phenomenon. Given this history, the team has been methodical every step of the way.

“The authors do a great job in their analysis. They follow all protocol and all standard techniques,” Sara Seager, an MIT exoplanet researcher and astrobiologist with knowledge of the paper, says. “And they do say that they looked at all the different types of stellar activity and other things that could generate a spurious Doppler signal at 11 days, but after looking at all that, they concluded the variability in the data is best explained by the presence of a planet.”

Part of the caginess may arise from a 2012 detection of a planet around another star in the system, Alpha Centauri B. That planet, aptly named Alpha Centauri Bb, was too hot to sustain life, but instantly became the closest planet to us by default.

Or it would have, if it actually existed.

That detection was riddled with problems, drawn out from spurious data, and ignored a low signal-to-noise ratio in search of a sensational new planet, the kind science fiction has long dreamed of. Instead of becoming an Earth-shattering revelation, serious doubts were cast on the detection, which also used radial velocity.

“[Pale Red Dot] actually just said the most likely explanation is the presence of a planet,” Seager says. “If you remember Alpha Centauri Bb … I just think there's a concern in the community that every retraction looks bad, even though that one wasn't officially retracted.”

Thus, the team was rigorous — and transparent — every step of the way. After all, they couldn’t repeat the mistakes of Bb. The end result? A solid detection of an Earth-size planet in a place called the “Goldilocks zone” because it’s neither too hot nor too cold for liquid water to exist – even if the researchers do use the word candidate to describe a detection with Kepler-catalog-like certainty.

“Because there have been previous claims of other planets (in the system), we had to verify as much as possible that [something else] was not causing this candidate signal,” Anglada-Escude said.


But is it habitable?

Potential habitability and a lush world of liquid bodies of water and a thick atmosphere are two very different things. In our own solar system, three planets are technically in the habitable zone. Venus is on the inner edge, while Mars is in the outer. (Hint: the third is Earth.)

Both Mars and Venus likely had bodies of water at some point in their history. But solar winds and other stellar events ripped away layers of lighter elements and evaporated lakes, oceans, and streams away. As the water boiled away, the hydrogen escaped into interstellar space while the oxygen came back down and bonded with carbon atoms.

For volcanic Venus, this meant a series of heavy elements and molecules created a permanent smog that ensured the planet remained a dry hellscape free of all but the slightest traces of water vapor. For Mars, this meant a thin carbon dioxide atmosphere with what little water remained trapped in frozen lakebed glaciers buried under oxidized iron soil, or in seasonal floes of brine mixed with trace amounts of water.

In either case, these planets didn’t last long as habitable worlds, at least for any life form beyond a microbe.

One of the big culprits is the lack of a present-day magnetic field on either planet, which, like the energy shields in Star Trek, deflects the worst the Sun and the universe can throw at Earth.

Even if Proxima Centauri b is in the habitable zone, it could have had an early atmosphere ripped away by the first billion years of violent stellar activity common with red dwarfs. This means that, even if the planet is in the right place for liquid water, a lack of atmosphere could have evaporated that water long ago, leaving a cold, barren planet of -40° F (-40° C).

“The planets are a lot closer to the star [than ours to the sun], so they're closer to these big energetic events — you're just potentially getting bombarded with more of that for potentially habitable planets around M-dwarfs,” Elisabeth Newton, a Kavli post-doctoral fellow at MIT who studies red dwarf systems (also called M-dwarfs), says.

This could be especially compounded by the planet’s tidal locking to its parent star. Because of the small separation in the system — the distance between Centauri b and its star is just 5 percent the distance of between Earth and the Sun — the same side of the planet faces Proxima Centauri at all times, much like the same side of the Moon faces Earth at all times.

However, if Centauri b still has an atmosphere, it could reach temperatures up to 86° F (30° C) on its sunlit side, and -22° F(-30° C) on its darker side, bringing it into quite Earth-like temperature ranges.

The key to preserving an atmosphere would be a magnetic field. Researchers have gone back and forth about if tidally locked planets could generate magnetic fields, which are a consequence of materials in the planets’ core stirring with its rotation. Since red dwarf planets are in lock-step with their star, some believe the cores would be inert.

Mercedes Lopez-Morales, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has modeled the possibilities of magnetic fields around red dwarf planets, and a picture is gradually emerging: The planets likely form in the outer parts of their solar systems and migrate in. This means they start out life rotating, and possibly generating a magnetic field if they have the right materials in their core.

“On Earth, we're only here because we have a magnetic field that shields us from any solar wind,” she says. “Any solar storm could wipe us out otherwise.”


Once these planets migrate in, their star strips off the early atmosphere of lighter elements. But heavier elements — like oxygen — could be left behind. The magnetic field shields the planet from the worst excesses of its star, which then settles into a state of relative dormancy it can stay in for trillions of years.

Volcanism and other mechanisms could replenish the atmosphere. With the star less active, that atmosphere could stick around. Lopez-Morales also says that the magnetic field could stay active for billions of years, even after a planet becomes tidally locked.

In other words, the hope for life can stay alive, even after the brutal first eon of the planet’s life.

“There's no reason why a planet like this could not keep a magnetic field long enough for life to develop itself,” Lopez-Morales says.

That means it could hold on to liquid water. But does it?

That’s where things get a lot trickier.



In Transit

The Pale Red Dot team found a planet. It seems to be the right mass and the right distance from its star to put something somewhat similar to Earth in our cosmic backyard. But reality is way more complicated than that, as seen by the afore-mentioned histories of Venus and Mars. 

Astronomers need to observe the planet in greater detail in order to further characterize it. The problem is that Centauri b was detected indirectly, and there’s very little to draw out of the data besides its size.

“The planets are so small, the signals are so weak, it takes a huge amount of resources to make a detection at all,” Seager says. “If you want to do better, it almost needs its own dedicated telescope just to hammer away at it and do better and better.”

The easiest way to study a planet’s atmosphere — and easy is a very, very relative term here — is to watch the planet pass in front of its star and to watch the spectra of any gasses that distort the star’s light. The problem with this method is that thus far, no transits have been detected around Proxima Centauri, though it hasn’t been ruled out as a possibility.

But to Newton, “it's basically the best target for future efforts to look for biosignatures in the atmospheres of other planets.” You just need to know how and where to look.

Currently, no instrument in space or on the ground is sensitive enough to pick up reflected light from older and smaller planets. But the James Webb Space Telescope might be, as will other mega-telescope projects currently under construction on the ground.

Catching these glimmers of light, however faint, could indicate whether or not there’s an atmosphere, and even what it’s made of.

“You find the spectrum of the planet, and from that you can detect molecules in the atmosphere of the planet,” Newton says.

Seager mentions the possibility of using stellar suppression techniques in the future, a process in which blocking the light of the star from the vantage point of a telescope allows the instrument to gather more light from the planet or planets around that star.

One of the other possibilities is viewing in infrared. Cullen Blake, a University of Pennsylvania researcher who studies low mass-stars and their planets, says, in visible light, “you definitely have a pretty severe limit to the distance to which you can see these measurements.”

Infrared eliminates some of those hurdles. This could show the planet’s own glow, without the need for starlight. Because of atmospheric distortion, virtually all infrared astronomy has to be performed by space-based telescopes or high-altitude flying observatories like NASA’s Stratospheric Observatory for Infrared Astronomy, a telescope mounted to a Boeing 747SP.

Future space observatories like Webb or Hubble-like telescopes built for infrared with apertures of around three meters could also aid in the hunt.

“We can look at the light from the star that gets reflected on the atmosphere of the planet or we can look at the light in the infrared coming directly from the planet,” Lopez-Morales says.

Looking to the future

Now we know — or know with only a sliver of a percent of doubt — that there’s a planet slightly more massive than Earth the next star over. It’s a banner accomplishment, one that has the scientific community salivating at the possibilities.

“To find one around the nearest, best-studied star … maybe we're just really lucky, or maybe there really are just billions of M-dwarf planets out there waiting for us to find them,” Newton says.

Low-mass stars are some of the most plentiful in the galaxy. Of the 10 closest star systems to Earth, only one does not contain a low-mass star (the Sirius system consists of a blue giant and an ultra-compact white dwarf, the remnant of a Sun-like star). Beyond those two, only Proxima Centauri’s bigger brothers Alpha Centauri A and B are larger stars.

Nearly every star is suspected to have a planet. Some of those could be habitable. If it ends up that Proxima Centauri b is barren, then perhaps we’ll have better luck looking at the next star over, Barnard’s Star, where planetary detection has remained elusive. It could be that, like Proxima Centauri, we haven’t been looking for the right kind of planet with the right kind of dedication.

Or maybe the real solution is at Wolf 359, 7.7 light-years away. Or Lalande 21185, 8.2 light-years away.

“If you just make a list of the closest stars to the Sun, there's a handful there that would make good targets for these kinds of observations,” Blake says.

We may not need to go clear out to Wolf 1061 at 13.8 light-years away to find the next closest potentially habitable planet. All we need to do is stretch our instruments to the limits and take a dedicated look for the next pale red dot.

John Wenz is an associate editor at Astronomy magazine. Follow him on Twitter: www.Twitter.com/johnwenz.