The Earth next door

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is

Proxima Centauri. Photo: Wikimedia Commons

It was just over 20 years ago—a blink of a cosmic eye—that astronomers found the first

planets orbiting stars other than our sun. All these new worlds were gas-shrouded giants

like Jupiter or Saturn and utterly inhospitable to life as we know it—but for years each

discovery was dutifully reported as front-page news, while scientists and the public alike

dreamed of a day when we would find a habitable world. An Earth-like place with plentiful

surface water, neither frozen nor vaporized but in the liquid state so essential to life. Back

then the safe bet was to guess that the discovery of such a planet would only come after

many decades, and that when a promising new world’s misty shores materialized on the

other side of our telescopes, it would prove too far away and faint to study in any detail.

Evidently the safe bet was wrong. On Wednesday astronomers made the kind of

announcement that can only occur once in human history: the discovery of the nearest

potentially habitable world beyond our solar system. This world may be rocky like ours and

whirls in a temperate orbit around the sun’s closest stellar neighbor, the red dwarf star

Proxima Centauri just over four light-years away. Their findings are reported in a study in

the journal Nature.

By Lee Billings, Scientific American on 08.30.16

Word Count 3,226

Although technically still considered a “candidate” planet awaiting verification, most

astronomers consulted for this story believe the world to be there. Scarcely more than the

planet’s orbital period and approximate mass are known, but that is enough to send

shivers down spines. Proxima Centauri shines with only about a thousandth of our sun’s

luminosity, meaning any life-friendly planets must huddle close. The newfound world,

christened “Proxima b” by scientists, resides in an 11.2-day orbit where water—and thus

the kind of life we understand—could conceivably exist. And it is likely to be little more

than one-third heavier than Earth, suggesting it offers a solid surface upon which seas and

oceans could pool. In a feat of discovery that could reshape the history of science and

human dreams of interstellar futures, our species has uncovered a potentially habitable

planet right next door.

“Succeeding in the search for the nearest terrestrial planet beyond the solar system has

been an experience of a lifetime, and has drawn on the dedication and passion of a

number of international researchers,” says the study’s lead author Guillem Anglada-

Escudé, an astronomer at Queen Mary University of London who spearheaded the

observations. “We hope these findings inspire future generations to look beyond the stars.

The search for life on Proxima b comes next.”

For some, Proxima b is a fitting capstone to the astronomical revolution that began when

the first exoplanets were found. “For more than 20 years the history of exoplanets has been

defined by studying stars tens to hundreds of light-years away, when the Holy Grail—a

small, rocky, potentially habitable planet—was just waiting to be discovered around our

closest neighbor,” says astronomer Debra Fischer, a veteran planet hunter at Yale

University who has led independent surveys of the Alpha Centauri system. “When we

launch our first robotic explorers to stars beyond the solar system, we know where we

should send them!”

Caleb Scharf, director of astrobiology research at Columbia University, says the new planet

represents “a tremendously important psychological moment for the field, as well as for our

species. Discovering who lives in the house next door can change perspectives and

priorities—and that's what Proxima b will do.”

Although it is barely more than four light-years away, Proxima Centauri is too faint to be

seen with the naked eye. It drifts at the outskirts of the twin sunlike stars Alpha Centauri A

and B, forming a stellar trio that appears as a single gleaming point in the southern

constellation of Centaurus. The tiny star is fated to slowly slip farther away from us on the

Milky Way’s celestial currents but will remain the closest one bearing a planet for perhaps

the next 40,000 years.

“For the first time, we have an exoplanet within our reach that could be a host to biological

organisms,” says study co-author Mikko Tuomi, an astronomer at the University of

Hertfordshire. “And that makes Proxima b not only one of the most fascinating discoveries

astronomers have made but also one of the most important that can be made.”

Even so, it is a discovery that almost didn’t happen. “People seem to think we just found

the planet. But no, we have believed it was there for years,” Anglada-Escudé says. “We

just had to build an argument to convince others it exists.”

The Pale Red Dot

Tuomi was the first to see hints of the world in 2013 while planet-hunting with Anglada-

Escudé. As relatively junior researchers, the two astronomers had struggled to secure

prized telescope time to search for planets and instead were seeking overlooked worlds

by carefully reanalyzing public data released by other teams. Proxima b’s faint signal first

appeared in several years’ worth of combined observations from two planet-hunting

spectrographs, HARPS and UVES, which are operated on telescopes in Chile by the

European Southern Observatory (ESO). Both HARPS and UVES had monitored Proxima

Centauri for years, watching for the star being tugged to-and-fro by any unseen pirouetting

planets, but the teams running them had claimed no detections. Orbiting worlds impose a

distinctive periodic wobble upon their stars, sometimes so gently that they sway their suns

slower than the pace of a crawling baby. The 11-day wobble Tuomi thought he saw in the

combined HARPS and UVES Proxima Centauri data was slightly stronger—1.4 meters per

second, an adult’s average walking speed. Along with several colleagues, Tuomi and

Anglada-Escudé quickly wrote and submitted a paper reporting the potential planetary

signals.

But many things can move a star. For example, heaving waves and vortices of magnetized

plasma flowing upon its surface can mimic or mask the wobble caused by a small planet.

And even the best planet-finding spectrographs are subject to calibration errors that can

cause further confusion. In 2012, the HARPS team had announced a wobble possibly

caused by a small rocky world around Alpha Centauri B—but that planet ultimately proved

illusory, a phantom produced by starspots, stellar rotation and questionable statistical

analysis. It was a cautionary tale: If the HARPS team had been fooled by Alpha Centauri B,

one of the most quiescent stars in the sky, hopes seemed slim for finding planets around

neighboring Proxima, which constantly erupts with “superflares” that can easily scuttle

careful observations.

Proxima’s reputation as a cantankerous flare star had kept it from the top of the HARPS

team’s priorities—and had also cast doubt on Tuomi’s and Anglada-Escudé’s claims. Their

paper was rejected; reviewers found their evidence unconvincing. Anglada-Escudé

responded by spending the next two years developing the “Pale Red Dot” observing

campaign, named for the famed “Pale Blue Dot” Voyager 1 image of Earth from deep

space popularized by the late astronomer Carl Sagan. It would be an audacious departure

from most previous planet hunts, which tended to favor skimming large numbers of stars

for easier, more obvious worlds rather than hammering away at any single target.

Anglada-Escudé and the rest of the Pale Red Dot team persuaded ESO to give them a 20-

minute chunk of Proxima-focused time on HARPS for 60 consecutive nights in the spring of

this year. The team bolstered their HARPS work with concurrent observations from two

other telescopes to monitor for flares and starspots that could masquerade as planets. As

the observing campaign began, Anglada-Escudé also worked with study co-author and

Carnegie Institution astronomer Paul Butler—a grizzled planet hunter who helped found

the field 20 years ago—to successfully extract the 11-day wobble from the UVES data

alone. Pairing the old data with the new, the signal soared above the sea of stellar noise,

cresting into unquestionable statistical significance. Within the first 10 days of the months-

long observing run, the Pale Red Dot team knew they had found Proxima b.

“I’m totally convinced,” Butler says. “Nature is malicious and tries to hide things, but you

don’t accidentally find two perfectly matching signals from two separate instruments.”

Members of the HARPS team, whose data proved so crucial to the discovery, are

convinced as well. “The signal is significant and is due to a planet,” says Christophe Lovis,

an astronomer at Geneva Observatory who developed the HARPS team’s data-analysis

software. “It is the recent, high-cadence datasets that make the difference… [The Pale Red

Dot team] simply tried their luck and it worked.”

Goldilocks Dreams And Nightmares

Perhaps the most surprising thing about Proxima b is that its existence is not really

surprising at all.

In recent years, thanks in large part to the work of the HARPS team as well as results from

NASA’s planet-hunting Kepler space telescope, astronomers have converged on a

statistical estimate for the number of potentially life-friendly worlds in our galaxy.

Somewhere between 15 to 30 percent of the Milky Way’s stars, it seems, should harbor

“Goldilocks” worlds—planets neither too big to be smothered by thick atmospheres nor too

small to lose their precious air to space, in a not-too-hot, not-too-cold orbit where liquid

water could exist on their rocky surfaces.

Similar to the way granules of rock outnumber massive boulders on a sandy beach, wee

stars like Proxima Centauri are far more abundant than larger ones like our sun. Small stars

are more efficient with their nuclear fuel as well, shining hundreds of billions—even trillions

—of years longer than sunlike stars. So we should expect most Goldilocks worlds to exist

around red dwarfs like Proxima Centauri. But should we expect them to actually be

habitable? Of this, not even the experts are sure.

Despite resembling Earth in mass and exposure to starlight, Proxima b “is not an Earth

twin,” says Franck Selsis, an exoplanet atmospheres expert at the University of Bordeaux.

The same 11.2-day orbit that places Proxima b in its star’s habitable zone also subjects it

to a rogue’s gallery of deleterious effects that could eradicate a biosphere—or prevent one

from forming in the first place.

For Rory Barnes, an astrobiologist at the University of Washington whose gloomy outlook

on habitability has gained him a reputation as a “destroyer of worlds,” all those potential

obstacles suggest Proxima b may not be the life-friendly planet we’re looking for. “In

general, any planet should be considered unlikely to support life,” Barnes argues. “This

one has different requirements than our own, and probably has more hurdles to overcome

than Earth did.”

Chief among them, Barnes says, is the fact that red dwarfs tend to have violent, unstable

youths—rather like a human being who lives for millennia but consequently suffers

centuries of turbulent adolescence. Because they are so small, such stars are thought to

form very slowly, spending many millions of years accreting mass and shining far brighter

than they do for the rest of their lives. If Proxima b formed where we see it today, Barnes

says, to be habitable “it would have to somehow avoid being baked to a Venus-like

runaway greenhouse state for hundreds of millions of years.” However, Barnes also offers

one possible remedy using another bit of creative Goldilocks reasoning: a not-too-thick,

not-too-thin blanket of hydrogen that could act as sunscreen for the young planet,

gradually evaporating under the harsh starlight and only dissipating after Proxima Centauri

settled into adulthood.

Extreme tides produced by Proxima Centauri’s pull upon its diminutive companion could

be another killer. Those tidal effects could cause Proxima b to rotate just once per orbit,

effectively “locking” one side of the world in darkness while the other faces the star—

although many researchers now believe most conceivable atmospheres would circulate

heat between the two sides to keep hope for a biosphere alive. More troubling is a

phenomenon called “tidal heating,” friction produced inside a planet by flexure from its

star’s tidal tug. If Proxima b’s orbit is (or ever was) significantly elongated, swooping close

to the star at one end and far out on the other, the resulting tidal heating could boil off any

ocean all on its own without any help from starlight.

Jim Davenport, a postdoctoral fellow at Western Washington University, believes Proxima

b’s biggest obstacle to Earth-like conditions is likely to be Proxima Centauri’s continual

flares, which can be ten times more energetic than any ever observed on our sun. Harsh x-

rays and ultraviolet radiation from the flares could strip Proxima b of its atmosphere,

leaving it barren and airless. But hope remains in this case, too—a protective magnetic

field much like Earth’s or a thick and steamy atmosphere could conceivably fend off the

worst of the flares. “To play on Hamlet, there are more things in heaven and exoplanets

than are dreamt of in our textbooks,” Davenport says. “But right now we just don’t know.”

Sooner or later, that will change. Already the discovery is fueling new interest in searches

for radio- or laser-based messages beamed toward Earth from any technologically

talkative aliens on the planet, as well as futuristic proposals to send robotic probes

voyaging to our nearest star system—even if these efforts are unlikely to deliver results in

the near future. In the shorter term, as news of Proxima b reverberates through the

scientific community, astronomers are preparing a full-court press to observe and study it.

Meet The Neighbors

Due to Proxima b’s short orbital period, Anglada-Escudé says, planet-hunting

spectrographs besides HARPS and UVES could conceivably confirm the planet’s

existence in a matter of weeks. More extensive studies of Proxima Centauri’s wobbles

could then better constrain the planet’s mass and orbit, placing tighter limits on the

possibilities for life there and potentially revealing more planets.

But the greatest hope among the planet hunters is that Proxima b transits, by chance

passing across the face of its star as seen from Earth so as to cast a minuscule but

measurable shadow toward our waiting instruments. David Kipping, an astronomer at

Columbia University, is now leading a team searching for signs of Proxima b’s possible

transit in recent observations of Proxima Centauri taken by the Canadian Space Agency’s

MOST space telescope.

“We are optimistic that it transits,” Kipping says. “If it does, it ticks that last box and

becomes almost as optimal as possible, and that seems so perfect it gives me pause for

concern.” Accounting for the star’s fluctuating brightness due to its flares, Kipping says,

will delay a conclusive result until sometime in September. If convincing signs of a transit

appear in the MOST data, astronomers will likely seek airtight confirmation using bigger

hardware—NASA’s infrared Spitzer space telescope.

A transit would be a treasure trove for astronomers. The planetary silhouette would allow

them to directly measure Proxima b’s size, precisely pin down its mass, and even calculate

its density and estimated composition. Moreover, starlight limning the edges of a transiting

Proxima b could allow astronomers to determine the presence and bulk composition of the

world’s atmosphere, if it has one. Such observations would likely require the observational

heft of NASA’s 6.5-meter James Webb Space Telescope, launching in October of 2018.

Even if Proxima b proves not to transit, it still offers a unique opportunity for a coming

generation of extremely large ground-based telescopes presently under construction

around the world. Set to debut in the 2020s and armed with light-gathering mirrors

stretching 30 meters or more across, such telescopes could conceivably obtain direct

images—actual pictures—of Proxima b, unveiling otherwise-unavailable information about

its composition and history. The biggest of these next-generation behemoths will be ESO’s

European Extremely Large Telescope (E-ELT), which could begin operations in Chile as

early as 2024.

“Proxima b might be the only (or at least one of the very rare) habitable zone planet that

could be imaged with the E-ELT,” Selsis says. “Even without talking about life, this could

represent a revolution in planetary science.” Observations of the planet in transit—or direct

images from a gargantuan ground-based telescope—could reveal whether the planet has

a thick, watery atmosphere. If it does, Selsis says, “we would then know that red dwarf

planets can keep their water despite stellar activity and be habitable. That would be

fantastic.”

Looking further ahead to the 2030s and past James Webb, NASA’s next large space

observatory will be WFIRST, a souped-up version of the agency’s wildly successful Hubble

telescope with a wider, infrared-optimized field of view. Present plans call for WFIRST to fly

with a high-performance coronagraph, an instrument capable of blocking a star’s light so

that faint accompanying planets can be directly imaged. Alas, WFIRST’s coronagraph is

optimized for stars like our sun, not red dwarfs like Proxima Centauri. According to Jeremy

Kasdin, the Princeton University astronomer who leads development of the project’s

coronagraph, WFIRST “will not be able to see Proxima b due to its closeness to its host

star and the planet’s low intensity at the telescope’s wavelengths.”

For now, this means the goal of thoroughly probing Proxima b and other nearby worlds for

convincing signs of life—so-called “biosignatures”—may remain out of reach for decades.

“The longer-term goal of directly imaging these planets is to see if their atmospheres are

conducive to or even influenced by a biosphere, to look for gases like oxygen that are very

far from thermodynamic equilibrium, gases that on Earth are produced by living things,”

says Butler, the planet-hunting veteran who has been pursuing this dream for most of his

life. “People ask me, ‘how will you ever prove a planet has life?’ If you take a spectrum of a

potentially habitable planet and see water and some gas out of equilibrium, you flip that

question from ‘prove there is life’ to ‘prove there isn’t.’ My great hope is that this will happen

in a generation.”

As generations go, many younger astronomers are less patient. Instead of waiting for

another large space telescope even further in the future after WFIRST, some now say

Proxima b has changed the rules. Just as its discovery required a dedicated, intensely

focused observation campaign, seeking signs of life there might best be served by

pushing for single-purpose space telescopes that are smaller, cheaper and faster than

NASA’s lumbering multipurpose flagship missions. Already, some maverick NASA

researchers have suggested such an approach for Proxima Centauri’s neighboring, more

sunlike stars, Alpha Centauri A and B.

“We may be in a new race now,” Anglada-Escudé says. “Building a massive observatory to

take pictures of planets around a hundred stars is very expensive. But people now know

exactly what to look for, so you can design your telescope and instruments to look only at

this planet, and optimize them for that single task.”

Sara Seager, an astrophysicist at MIT who has helped plan WFIRST and other next-

generation missions, sees the result as a profound new opportunity for exoplanetary

science. “This gives permission for those of us in the field to put all our eggs in one basket,

rather than throwing darts randomly at the sky,” Seager says. “Before Proxima b, you’d

scarcely imagine sending up a space telescope for one star, but now it’s imaginable. There

are downsides. If we all pick the wrong thing, what happens then? Might there be less

work for people to do, because we will be focusing on fewer objects? We could end up like

the particle physicists, with thousands of authors for one paper. But this is the path to

finding the most promising planets around the very nearest stars.”