Out of the 100 billion stars in the Milky Way and the many multiples beyond it, one, the red dwarf Proxima Centauri, is our closest stellar neighbour.

On Wednesday, a team of astronomers announced they had discovered a planet orbiting Proxima Centauri — and that the planet is likely rocky, has a mass similar to Earth’s and sits in the “Goldilocks” zone where liquid water could exist on its surface.

Twenty five years ago, astronomers had no direct evidence of planets beyond our solar system. Today, an Earth analogue — a primary target in the search for life elsewhere in the universe — is sitting a cosmic stone’s throw away.

Much remains unknown about the new planet, including whether X-ray flares far more intense than we receive from our sun may have eroded any atmosphere that once formed. Just because water and life could exist doesn’t mean they do.

But one fact is certain: the discovery will set off a massive push to analyze the planet and piece together its history, using current ground-based observatories, soon-to-be-launched spacecraft and maybe — a big maybe — ambitious interstellar missions that entrepreneurs and scientists want to see launched within their lifetimes.

“If and when we do send interstellar robotic probes, I would say this is it — this is the prime target for any such effort,” said Ray Jayawardhana, dean of science and an astronomy professor at York University, who studies the origin and diversity of planetary systems.

“At least as alien worlds go, our nearest neighbour is not that far away. It’s as near as it could possibly be,” said Jayawardhana, who was not involved in the new observations.

Proxima Centauri is one of three stars in the neighbouring Alpha Centauri system, which has long been a favourite of science fiction: the movie Avatar is set on a fictional moon in Alpha Centauri, and the Transformers’ home world originally orbited there.

The two main stars in the system are a binary pair. Proxima Centauri is a dimmer, low-mass red dwarf that can’t be seen at night with the naked eye: it is less than 1 per cent as bright as our sun. But because of its proximity to us (hence the name), a relatively close 4.2 light years away, Proxima Centauri is one of the best-studied stars of its type.

Beginning in 2000, astronomers at the European Southern Observatory, which sits in Chile’s Atacama Desert, collected data that hinted at the presence of a planet orbiting Proxima Centauri. But red dwarfs can be very active, with star spots that can be mistaken for a planet, and the researchers couldn’t be sure.

So in January of this year, an international team co-ordinated by Queen Mary University of London’s Guillem Anglada-Escude embarked on a hunt: the Pale Red Dot campaign.

Nearly every night for ten weeks, the team used the ESO’s High Accuracy Radial velocity Planet Searcher (HARPS) to study the starlight emanating from Proxima Centauri. As a smaller object like a planet orbits a star, it produces a gravitational wobble, and the resulting shifts in the light that reaches Earth can be measured by instruments like HARPS.

In 2012, astronomers announced the discovery of a planet orbiting one of the stars in the brighter binary near Proxima. Upon closer analysis, it turned out the planet did not exist. The Pale Red Dot team wanted to be sure, so they combined careful observations of Proxima from other instruments to rule out stellar activity and older wobble data collected in the previous decade, making it very unlikely their observations are a fluke.

The evidence was clear: A planet with a minimum mass of about 1.3 times that of Earth — which suggests that it is likely rocky — was orbiting Proxima Centauri every 11.2 days. Their findings are published Wednesday in the journal Nature.

Proxima b, as the planet is called, sits about seven million kilometres from the star, much closer than infernally-hot Mercury orbits our sun. But because the red dwarf is so much dimmer, that puts the planet in the middle of the habitable zone where its estimated surface temperature would allow water to exist in liquid form.

“Succeeding in the search for the nearest terrestrial planet beyond the solar system has been an experience of a lifetime,” Anglada-Escude said. “The search for life on Proxima b comes next.”

Some features of the planet may lower the likelihood of it actually supporting life. Because it sits so close to the star, it is very likely tidally locked, with one side in permanent darkness and the other in permanent daylight. It experiences X-ray and ultraviolet radiation at least a hundred times more intense than Earth does from our sun. How these features affect habitability is a matter of fierce debate in the academic community.

“For sure the difference between Earth and this planet is that it is receiving much more high-energy radiation, but we don’t think it’s a showstopper,” said Anglada-Escude.

In the coming months and years, astronomers will try to analyze the planet with our current ground-based observatories. If the planet transits the disc of its star from the perspective of astronomers here on Earth, they can study the light that passes through any atmosphere it may have, looking for telltale absorption signals that provides information about the atmosphere’s chemical components.

The James Webb Space Telescope, set to launch in 2018, will be able to beam back even clearer information about any atmosphere the planet may have without the complicating effects of our own blurry atmosphere.

Earlier this year, the Russian entrepreneur Yuri Milner, backed by a cast of scientists including Stephen Hawking, announced a plan called Breakthrough Starshot. Its aim is to send a flurry of interstellar probes to Alpha Centauri within the next generation.

Loading... Loading... Loading... Loading... Loading... Loading...

Our current interstellar spacecraft would take tens of thousands of years to get there, so any mission to reach Alpha Centauri in this century would likely require billions of dollars and huge leaps in technological capability, if it is even possible at all. Other scientists speak about interstellar probes of this range as a possibility in the distant future.

Since the discovery of the first planets outside our solar system in the 1990s, scientists have realized that such exoplanets are quite common. The Kepler space mission resulted in staggeringly high estimates of Earthlike planets orbiting sunlike stars.

The thrill of Proxima b is how close it is to us, and the science that enables.

“This is ... a dream for astronomers, if we think about follow up observations,” said Ansgar Reiners, another researcher with the Pale Red Dot campaign.

Burning questions for Proxima b

Can I go?

Sorry. Our next planetary target for human exploration is Mars, which will be hard enough to achieve by 2040 as NASA hopes. To name just one challenge, we haven’t quite figured out how to shield astronauts from the intense radiation they would absorb on the minimum yearlong journey to Mars. Proxima b is more than 500,000 times further away.

Can robots go?

The initiative will spend $100 million (U.S.) over the coming years to demonstrate proof-of-concept for a truly ambitious plan: to build a massive array of lasers that would beam thousands of lightsail-equipped miniature “nanocrafts” towards Alpha Centauri at 20 per cent the speed of light, a journey that would take twenty years. One hurdle — whether any interesting exoplanets exist there — was resolved on Wednesday. But staggering engineering challenges remain, and other scientists believe it will take centuries for such a journey to become feasible.

Fine, what can we do now?

Our current class of ground-based telescopes is already sensitive enough to start studying Proxima b’s atmosphere, if it has one — especially if it transits the face of Proxima Centauri, though the Pale Red Dot team did not observe that occurring and believe there’s only a small chance it will. A new generation of mega-telescopes are under construction, and as they come online in the next decade, astronomers will have even more powerful instruments to detect light from the planet directly. The closeness of Proxima b boosts their chances of success.

What about next year?

NASA’s Transiting Exoplanet Survey Satellite, set to launch in 2017, will spend two years in space monitoring 500,000 nearby stars, searching for drops in brightness that indicate the transit of exoplanets. The scientists building TESS expect to catalogue about 500 Earth-sized and Super Earth planets transiting very bright nearby stars. There’s only a small chance TESS will be able to provide data on Proxima b, but an excellent chance that TESS will find other prime planetary candidates for followup observation.

And the year after that?

One of the key goals of the James Webb Space Telescope, an international collaboration including Canada that should be space-bound in 2018, is to study the atmospheres of exoplanets like Proxima b and those discovered by TESS. Different molecules in the atmospheres of these planets — water, sodium, methane — absorb light at specific wavelengths. By exploiting this phenomenon, JWST will help astronomers track down planets with similar atmospheric compositions to Earth’s, if they exist.