Space

Scientists Believe a Potentially Habitable World Orbits Sun’s Closest Star

The planet’s existence hasn’t been confirmed yet – but if it exists, the Pale Blue Dot could be joined by a Pale Red Dot around the star Proxima Centauri as another home for life in the universe.

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Caption and credit: ESO/M. Kornmesser

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Caption and credit: ESO/M. Kornmesser

Astronomers say there could be a potentially habitable exoplanet close enough for us to explore with probes in the near future.

They reported in this week’s issue of the journal Nature that our closest stellar neighbour, Proxima Centauri, harbours a rocky planet that’s squarely in the star’s habitable zone. And at 4.3 light years away from Earth, not only is this the closest planet beyond the Solar System but it is also the closest potentially habitable planet. In the coming years, a lot of attention will be directed on this planet.

Its name is Proxima b. It is estimated to weigh at least 1.3-times as Earth does, and orbits its host star Proxima Centauri at a distance of 7.5 million km. That’s barely 5% of the distance between Earth and the Sun. However, Proxima Centauri is a ‘red dwarf’ star – it weighs a tenth as much as the Sun does and emits one-sixteen-thousandth as much light. From Proxima b, Proxima Centauri would be just as bright as our Sun would seem to be from Saturn. On the brighter side, this allows Proxima b to have temperatures on its surface suitable for liquid water to exist. This is a big deal because wherever we’ve found water on Earth, we’ve also found life.

This planet and its star are a part of the Alpha Centauri star system. The core of this system consists of two stars, Alpha Centauri A and Alpha Centauri B. These two orbit each other and form a binary star. The distance between them as they move in their orbits stretches out to as much as the distance between the Sun and Pluto. They are located nearly 4.4 lightyears away from us, and appear to be one star to the naked eye. Together, they are called Alpha Centauri AB. They’re hard to miss in the night sky, being the third brightest stellar object after Sirius and Canopus.

Proxima Centauri is the third star in this system. It is a red dwarf and orbits Alpha Centauri AB at a distance of 2.2 trillion km (0.24 light years). Its orbit around the binary star lasts between 100,000 and 500,000 years. It is not visible to the naked eye, but it is the star that comes closest to the Solar System in its orbit. Hence the ‘Proxima’ in its name.

Stars are named in a fixed format: the biggest central star is given a name and other stars in the system are designated by subsequent capitalised letters. Planets around these stars (called extrasolar, or exo-, planets) are indicated by lower case letters. Alpha Centauri B was once thought to have a planet around it and it was christened Alpha Centauri Bb. This theoretical planet turned out to be an error in calculations and never existed. But Proxima Centauri b most likely does.

Proxima Centauri (often simply called Proxima) is one of the most studied stars in the universe. We have paid especial attention to it since the early 1990s. The team that published the paper about the new planet belongs to a project called Pale Red Dot, which was formed with the express intent of discovering Earth-like planets around Proxima. The project is a collaboration between eight countries and over thirty astronomers and astrophysicists.

Guillem Anglada-Escudé, an astronomy lecturer at the Queen Mary University of London, and his team at Pale Red Dot studied Doppler data collected over sixteen years by two European Southern Observatory telescopes: the High Accuracy Radial velocity Planet Searcher and the Ultraviolet and Visual Echelle Spectrograph, in Chile. This includes 54 individual observations logged earlier this year.

Just as with the famous example of a siren passing by, the Doppler effect can be observed with light as well. As a star moves closer to us, there is more blue in its spectrum and it appears blue-shifted. As it moves away, its light emission shows more red and thus appears red-shifted. A star’s spectrum can show signs of this effect in the presence of a planet because even small bodies exert a small, non-negligible gravitational effect on large masses, especially if they are close by. This method of detecting exoplanets is called the radial velocity method.

To Anglada-Escudé & co., Proxima’s spectrum showed both blue-shifting and red-shifting in short, periodic bursts. This indicated that the star was being tugged a teeny bit by what could only be a rocky planet. Working backward, the team was able to deduce that a terrestrial planet orbited Proxima every 11.2 days at a distance of 7.5 million km.

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Caption and credit: ESO/M. Kornmesser

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Caption and credit: ESO/M. Kornmesser

The distance puts Proxima b within the star’s habitable zone. Scientists guess that the surface temperature would be approximately -40 ºC. However, this number is assuming there isn’t an atmosphere on the planet. “The presence of an atmosphere could warm the planet considerably, taking the temperatures to the range of -30 deg C to 30 deg C,” said Anglada-Escudé in a press conference on August 23. This is almost the average temperature on Earth’s surface.

If there is an atmosphere on the planet, the odds of there being life on its surface are hiked significantly, with clouds to regulate the temperature. Proxima b is also likely tidally locked with synchronous rotation to its star. This means that the same side of the planet always faces the star – getting baked by starlight and excess radiation – while the other side would be cold and dark. Given the planet’s distance from the star, the radiation would include a lot of X-rays and other high-energy radiation considered harmful to the sustenance of life. An atmosphere could save the day, though. Clouds would redistribute the heat throughout the planet and preserve an average temperature between both sides. The additional presence of a magnetic field would mean a definitive protection against radiation bursts from Proxima.

At the same time, we can’t be all that hopeful about a healthy atmosphere. It’s possible that Proxima b might have a really thin one or none at all. Its closeness to its host star would have had several effects on the planet. The most obvious of these concerns how hot the planet got when it formed. If Proxima b had actually formed closer to Proxima and then moved further away, it would’ve been subjected to intense heat. This in turn would’ve caused gas molecules to expand, thin out and escape the planet’s gravity. Another problem is the stellar wind – a stream of energetic particles emitted by a star, known to strip away atmospheres and do to planets what our Sun is thought to have done to Mars. It is possible that even today, the star’s winds, flares and radiation could be attacking Proxima b to leave only a frayed blanket of gases in their wake.

Astronomers hope for a thick, protective atmosphere because it would enable an active water cycle like the one on Earth. However, the fact that water could exist on the planet doesn’t necessarily imply that it does. The only way to deduce the presence (or absence) of water at the moment is to study the planet’s history and formation. “Where and when the planet formed offers large clues to whether it holds water, and if it does, how much,” said Pedro Armado from the Institute of Astrophysics, Andalusia, at the press conference. “If the planet formed at a considerable distance from Proxima and then migrated inwards, it would be full of water still. If it formed very close to the star, near where it is now, there probably wouldn’t be any water.”

It’s heartening that when our own planet formed, there was no water on it. It is commonly believed that water was brought to Earth by the asteroids peppering its surface during its early years; Proxima b could have availed the same treatment. Moreover, we can’t entirely rule out Alpha Centauri AB’s disrupting the stellar system, either, using its gravity to fling water-laden bodies toward Proxima b. “There is a non-zero probability of atmosphere – and therefore water – existing on the planet,” added Anglada-Escudé. So there’s a high chance the planet does indeed hold liquid water.

The Pale Red Dot team agrees that more research is needed before drawing any solid conclusions on the habitability of this new planet. It also thinks a detailed investigation of Proxima b is easier than of a system similar to our own Solar System because the planet is so much closer to its star. In the future, telescopes like the upcoming James Webb Space Telescope, the ASH2 at San Pedro, Chile (used by Pale Red Dot), the SALT, South Africa, and Mission Centaur’s projected 2019 space telescope could make it easier for the planet to be observed for months at a time.

One important goal is to find the planet transiting its star – i.e., passing against the face of the star as viewed from Earth. This would offer compelling evidence of the planet’s existence. Finding Proxima b would also offer hope for affirming the existence of hundreds of thousands of habitable planets, since red dwarfs like Proxima Centauri make up over 80% of stars in the universe today. The Pale Red Dot team’s paper also mentions a possible, unconfirmed reading of a second planet around Proxima. If this were to be confirmed as well, the odds of finding habitable planets all around us would improve exponentially.

The Proxima b announcement could not have come at a more opportune time for the Breakthrough Starshot initiative – the $100 million research grant Russian billionaire Yuri Miller announced earlier this year under his Breakthrough portfolio. Starshot would have hundreds of tiny probes called nanocrafts fly to the Alpha Centauri system at a third of the speed of light in 20 years. Once there, they’d rapidly study the stars and any planets around them. It is the largest grant ever announced for research in space exploration and requires intensive research in multiple domains, as The Wire reported at the time of the announcement. Proxima b’s presence provides a great incentive for not just this project but also for competitive research and development in astrophysics, specifically in the fields of telescopes and interstellar travel.

In the coming decades, such and other missions to Proxima will start taking shape. And in the next couple centuries, robotic exploration of Proxima b may even become common and easy. “Proxima Centauri will exist for several hundreds or thousands of times longer than the Sun,” wore Artie Hatzes, director of the Karl Schwarzschild Observatory, Germany, in an article accompanying the announcement. “Any life on the planet could still be evolving long after our Sun has died.”

Sandhya Ramesh is a science writer focusing on astronomy and earth science.