A planet with at least 1.3 times the mass of Earth has been discovered orbiting within the habitable zone of the nearest star to our Sun, Proxima Centauri, which is just 4.2 light years away.
The milestone discovery, made by an international team of scientists using the 3.6-metre telescope at the European Southern Observatory (ESO) in Chile, is the successful result of the Pale Red Dot project that had the goal of finding a planet around Proxima Centauri, which is a dim red dwarf star.
“The importance of this discovery is not that we’re reporting the first ever planet or the last planet, but instead a special planet that is very close,” says the leader of the Pale Red Dot team, Guillem Anglada–Escudé of Queen Mary University of London. Proxima’s proximity means that the planet will come under our intense scrutiny and is even in striking distance of ambitious plans to launch interstellar probes by the middle of this century. However, while the planet seems to be roughly Earth-sized, there is no evidence yet that it is ‘Earth-like’ in terms of having an atmosphere, liquid water or life.
The type of star that Proxima Centauri is makes the discovery even more fascinating, because worlds orbiting red dwarfs have quite different characteristics to those orbiting Sun-like stars. Red dwarfs are cool stars – Proxima Centauri has a surface temperature of only 2,775 degrees Celsius, half of the Sun’s surface temperature of 5,500 degrees Celsius. This means that the region where temperatures are similar to those experienced on Earth, which are suitable for liquid water, is very close to red dwarf stars. It’s fortunate then that, because red dwarfs are so small – Proxima is just 14 percent the diameter of the Sun and has just 12 percent of the mass – their planetary systems are scaled down, with their planets orbiting extremely close to them.
The planet, named Proxima b, is orbiting its star every 11.2 days, at a distance of seven million kilometres – close enough and warm enough to potentially be suitable for life as we know it. The distance from the star is in stark contrast to Mercury, the innermost planet of the Solar System, which orbits the Sun every 88 days at a distance of around 58 million kilometres, or Earth at our distance of about 150 million kilometres from the Sun.
Standing on the planet
Since red dwarfs are cool, they radiate most of their light in invisible infrared, while visible light tends to be dominated by longer, redder wavelengths.
“Standing on the planet, the light would be like at dusk on Earth,” Anglada–Escudé tells Astronomy Now. “Also, because the light is redder, there wouldn’t be as much blue light scattering, which causes the blue sky on Earth. The sky on Proxima b would look reddish around the star, while the rest of the sky will be darker.”
Furthermore, Proxima Centauri would not appear to move in the sky or, at most, only very slowly. This is because when planets are so close to their stars they become trapped in gravitational lockstep so they always show the same face to the star, just like the Moon does to Earth. This is known as tidal locking – the planet is still rotating, but doing so at a rate that matches its 11.2-day orbit, meaning that as the planet is following the curve of its orbit it is rotating to keep the same face pointed to the star.
Alternatively, the planet could be rotating with a 3:2 resonance like Mercury, whereby it spins three times for every two orbits that it makes. Either way, this makes for some interesting scenarios given that Proxima b is in the habitable zone where temperatures should be appropriate for liquid water.
If Proxima b has water on its surface and if it is tidally locked, then that water would be most likely found around the equator on the daylight hemisphere. The permanent night side could be too cold for liquid water, although a thick atmosphere could feasibly transport heat to the night side. If the Proxima b is in a 3:2 resonance, then the water may exist in a tropical belt ringing the planet.
Of course, Proxima b might have no liquid water at all – it could be airless, or covered in a thick atmosphere creating a runaway greenhouse effect like on Venus. Furthermore, Proxima Centauri regularly unleashes flares of X-ray and ultraviolet radiation that could irradiate the planet if it doesn’t possess a powerful magnetic with which to protect itself. It’s not even 100 percent certain yet that the planet is rocky – an uncertainty that stems from how the planet was discovered.
How did they find it?
On every clear night between 15 January and 1 April earlier this year, the team searched for the planet using the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument on the 3.6-metre telescope. They were looking for a slight wobble in the rotation of the star caused by the presence of the planet.
Strictly speaking, planets don’t orbit stars; rather, they orbit the centre of mass between a planet and a star. Because stars contain over 99 percent of the mass in a planetary system, the centre of mass is always located inside the star, but its offset from its centre, meaning the star seems to wobble around it. This wobble can be tiny, just a few metres per second, but enough for HARPS to see it as a Doppler shift as the star wobbles towards us and away from us. The wobble incurred by Proxima b is just five metres per second.
This illustrates that being so close to us didn’t necessarily make it easier to find the planet. Regardless of whether Proxima is 4, 40 or 400 light years away, HARPs would still be able to detect the signal. Instead, the key factor is the size of the signal relative to the star.
Clearly the planet’s mass has an impact on the size of the signal – the more massive the planet relative to the star, the bigger the wobble – but activity on the surface of the star can also act to hide the planet’s signal. Pulsations, flares, starspots, coronal mass ejections – all of these can create a noisy background in Doppler shift measurements. Red dwarfs like Proxima are particularly notorious for having lots of stellar activity. To account for this, both the Las Cumbres Observatory Global Telescope (LCOGT) network and the 0.4-metre ASH2 telescope at San Pedro de Atacama in Chile provided observations that measured the amount of stellar activity on Proxima, allowing the Pale Red Dot team to subtract it from their observations.
Uncertain mass
The measurement of Proxima b’s mass as 1.3 times greater than Earth’s mass is described as a ‘minimum’ mass. The Doppler shift is strongest along the orbital plane of the planet, but we don’t yet know the angle at which we’re seeing Proxima b’s orbit. The larger the angle, the less of the true wobble we see.
“If the orbit is edge-on then we will see the maximum signal,” says Anglada–Escudé. “If the planet’s orbit was face on to Earth, we would see nothing at all.”
Although its possible that we are seeing the planet’s orbit edge on, and therefore the full amount of its wobble, it’s more likely that we see something in between, with the planet’s orbit at an angle. Therefore, since we would not be seeing all the wobble, the mass of the planet could be even larger than 1.3 Earth masses. How much larger is unknown, but Anglada–Escudé is optimistic that the true mass of Proxima b will keep it in rocky planet territory.
“We’re being very careful with our words and calling it a candidate rocky planet for now,” he tells Astronomy Now. “The thing is, a lot of stars like Proxima have small planets around them, so the chances of this being a rocky planet are extremely high.”
There’s also a chance that Proxima b isn’t alone. Anglada–Escudé says that some of the Doppler shifts seen in Proxima haven’t been fully explained yet and that these features could be one or more additional planets on orbits between 50 and 500 days around Proxima. Alternatively, there could be planets around Proxima that are too small for HARPS to detect. It seems that there could still be much to discover about Proxima Centauri’s planetary system.
Getting to know Proxima b
The next step is to gather more data. David Kipping of Harvard University is currently leading a project using the Canadian MOST satellite to search for transits of the planet across the face of Proxima and hopes to have results before the end of the year.
“We are also doing ground-based searches, because now that we have the Doppler signal we know when the transits are most likely to happen so we can optimise a bit for that,” says Anglada–Escudé.
If transits can be detected, they will provide the diameter of the planet and, combined with the mass, will help indicate its density and determine whether it truly is rocky or not. It will also be possible to study the planet’s atmosphere by performing spectroscopy of Proxima’s light as it passes through the atmosphere and is partially absorbed by different molecules that might be present.
“What we’d really like to do is take a picture of it,” says Anglada–Escudé. However, this may require scientists to build dedicated instruments designed for the task of imaging Proxima b. Not even the James Webb Space Telescope, launching in 2018, will be able to resolve the planet in an image.
A picture of the planet as a dot of light taken from Earth would be revelatory, but getting to see the world up close would be even better. This could be achievable around the year 2060 if the Breakthrough Starshot initiative is successful. Funded by Yuri Milner’s Breakthrough Foundation, Starshot intends to send hundreds of tiny nanoprobes, riding beams of laser light, to the Alpha Centauri and Proxima Centauri planetary systems.
“We could launch them in a generation’s time and they’d take 20 to 25 years to get there,” says Pete Worden, Chairman of the Breakthrough Foundation and former director of NASA’s Ames Research Center. “We don’t have the technology to send anything bigger yet – that might not happen for centuries – but with the technology we have today we can do a fly-by of Proxima Centauri and get images of the planet and even answer the question of whether there is life there.”
The details of the planet’s discovery are published in the 25 August issue of the journal Nature.
