The sun’s closest neighbour, Proxima Centauri is not a lone star. A century after its discovery, our team has just demonstrated that this red dwarf is actually bound by gravity to the pair of stars Alpha Centauri, around which it orbits every 550,000 years.
Among the multitude of stars of the Milky Way, the photograph shows the orbit of Proxima around Alpha Centauri and, on the right, the constellation of the Southern Cross as a landmark. Photo © ESO / Serge Brunier / Frédéric Tapissier / Pierre Kervella.
Johannesburg, South Africa, October 1915. Comparing two photographic plates taken five years apart of a region close to Alpha Centauri, then the closest known star to the sun, the Scottish astronomer Robert Innes discovered another, very faint star, which seemed to be moving in the same direction. Additional observations enabled the distance to this newcomer to be measured, and it appeared to be slightly closer than Alpha Centauri. This star, baptised Proxima Centauri by Robert Innes himself, took over as the closest star in the solar system.
Soon after this discovery, astronomers suspected that Alpha Centauri, comprising two very close stars (A and B) and similar to the Sun, and Proxima Centauri, a much smaller red dwarf, were actually bound by gravity and constituted a trinary star system. This was the best way of explaining how these three stars in the constellation of Centaurus, located respectively 4.37 and 4.24 light-years from the Earth (a little over 40,000 billion kilometres) seem to move the same way in the celestial sphere and are located in a tiny volume of space.
This argument is not exactly a proof, however. Since then, several unsuccessful attempts have been made to formally demonstrate this link. For the first time, using observations made from the European La Silla Observatory in Chile, we have proven that the three stars indeed form a single stellar system, in which the star Proxima orbits the pair Alpha Centauri.
To reach this conclusion, the key was measuring the relative speed of Proxima to that of Alpha Centauri with a high degree of precision. If the speed is too high, it means that Proxima is escaping the gravitational attraction of Alpha Centauri A and B. If it is sufficiently low, it means that it is orbiting these two stars. The threshold speed between these two scenarios is known as the escape velocity. The higher the gravitational pull exerted by the object, the lower this speed. On Earth for example, the escape velocity of an object on the surface of the planet is about 40,000 kph - the speed that space rockets need to be given to send them to explore our solar system.
For Alpha Centauri and Proxima, we needed to know their masses and the distance separating them to calculate the escape velocity. The positions of the three stars in the sky were measured very precisely by the European astrometric satellite Hipparcos more than twenty years ago. Their distance to the Earth is known with a relative accuracy better than 0.1%. We therefore deduced that the current separation between Alpha Centauri and Proxima is 13,000 astronomical units (an astronomical unit being the average distance between the Earth and the Sun), or 2,000 billion kilometres.
As shown by this comparison of the sizes of the stars studied, while Alpha Centauri A and B (ᾳ Cen A and ᾳ Cen B) are similar to the Sun (within 10%), Proxima is a red dwarf. Smaller than the rings of Saturn, it is less than two thousandths of the brightness of the Sun. Photo © Pierre Kervella / NASA-ESA-HST /(STSC Aura)-NSA / SDO.
The masses of the three stars are also well known, as are their other physical properties. Near twins of the Sun, Alpha Centauri A and B have masses within 10% of our own star. Proxima however, is very different and “weighs” eight times less than our star, has a diameter that is six times smaller and only two thousandths of the luminosity of the Sun. With these values, the escape velocity of Proxima (at its current position) relative to Alpha Centauri is 545 m/s (plus or minus 11 m/s), or about 2,000 kph.
It only remained to measure Proxima’s speed relative to the pair of stars, and to an accuracy better than a few tens of metres per second. The level of accuracy must be equivalent to only a tiny fraction of Proxima’s escape velocity, otherwise it would be impossible to know whether the star’s speed is greater or less than this limit. To determine the speed of our nearest neighbour, we have to add up its movements along all three directions in space, that is, in the plane of the sky (two directions) and along to the line of sight (the direction Earth-Proxima). Its speed in the plane of the sky is known with precision thanks to long-term astrometric observations which, for decades, traced the changing position of the star in the sky.
However, its speed with along the line of sight - its radial velocity (the speed at which it moves towards or away from the Earth), had never been measured until now with enough accuracy. Using spectrography, this is a more delicate measurement to make. The aim is to analyse certain specific spectral lines in the light emitted by the star. Through the Doppler Effect (*), the wavelength of these lines (or their colour) is shifted towards the blue if the star is moving closer to us, or towards the red if the star is moving away from us. By precisely measuring the apparent wavelength of a spectral line, and knowing its intrinsic rest value through laboratory experiments, we can determine its shift due to the Doppler Effect, and from there, measure the star’s radial velocity.
Until now, spectrographs had not reached the necessary level of precision and stability to measure the radial velocity of Proxima. Things have changed with the recent arrival of a new, generation of ultra-precise and extremely stable instruments, used to search for exoplanets, that are also using the Doppler Effect. For this work, we employed the French-Swiss spectrograph Harps, installed on the 3.6-metre diameter European telescope La Silla. It was this same combination of telescope and spectrograph that discovered many exoplanets, including the temperate terrestrial planet Proxima b. Around 1.3 times larger than the Earth, Proxima b was detected in 2016 around Proxima. Even better - it wasn’t even necessary to make an additional observation in the telescope to measure Proxima’s velocity, as we analysed spectra already collected over the last few years to search for planets around this star.
Measuring Proxima’s speed however has an added difficulty compared to detecting exoplanets. During their orbital movement, planets cause their star to jig ever so slightly, and it is this tiny variation that signals the presence of one or more planets. The measurement is based on comparing the successive light spectra of the star with an average, reference spectrum. Using several thousands of spectral lines, the relative shift of one spectrum to the reference is measured with a very high precision. To discover Proxima b, the 1.4 m/s velocity variation induced by the planet on its parent star was measured with a remarkable accuracy of 20cm/s.
In our case however, the goal is not to measure Proxima’s variation in velocity, but the absolute value of the velocity itself. In these conditions, it is impossible to analyse the same lines usually used for exoplanets. Why? Because in the spectrum of a cold star like Proxima, these so-called absorption lines, created when light from the star is absorbed by various molecules present in its lower atmosphere, are extremely numerous. As a result, they overlap, making it impossible to analyse them individually. Furthermore, due to the complexity of the molecules at play, the intrinsic wavelength of these spectral lines at rest in the laboratory (before they are Doppler shifted) cannot be measured with enough precision. We therefore looked at another type of spectral lines, never before used in this type of study, but already present in data collected by Harps; emission lines, produced directly by the gas present in the star’s upper atmosphere, its chromosphere. We focused our work on four lines in particular, caused by the presence of calcium and sodium atoms. As Proxima is an active star with a very hot chromosphere, these intense emission lines can easily be identified in its spectrum. And, unlike absorption lines, their wavelengths can be measured with high precision in the laboratory (these are single element lines, easily identifiable in spectra).
The trajectory of Proxima around Alpha Centauri could be traced in projection on the bottom of the sky. Its position is indicated for millennia to come: the period of its orbit is 550 000 years. Photo © Pierre Kervella / ESO / Digitized Sky / Survey 2 - David de Martin/Mahdi Zamani/Observatoire de Paris.
By analysing 260 spectra containing these lines, collected using Harps between 2004 and 2016, we determined Proxima’s radial velocity relative to the Sun: it is moving towards us at the speed of 22,204 m/s, plus or minus 32 m/s. To reach such a high precision, in addition to the Doppler shift due to the star moving through space, we also took into account two other subtle effects that affect the light spectrum and “skew” the radial velocity we are trying to measure.
On the one hand is the star’s gravitational field, which “retains” the photons on the surface. They of course escape but lose a tiny fraction of their energy in the process and appear slightly reddened (red light carries less energy than blue light). On the other hand, the boiling inside the star (convection movements) brings hot matter up to the surface. As the hot and brighter matter is moving towards us, its colour is shifted towards the blue by the Doppler effect. Both effects can be accurately estimated as we know the mass, the radius and the temperature of Proxima.
With all these data in hand, we were finally able to determine Proxima’s velocity in relation to Alpha Centauri: 273 m/s plus or minus 49 m/s, or 980 kph. As this value is far lower than the escape velocity (545 m/s; about 2,000 kph), this means that the three stars are bound by gravitation.
Knowing Proxima’s velocity and its position in the sky, we were also able to calculate its orbit; it makes one revolution around Alpha Centauri every 550,000 years. This is longest known orbital period for a multiple-star system. Projected onto the sky, Proxima’s trajectory appears to have a large angular size of more than three degrees, which roughly equates to the width of two fingers at arm’s length. In April 2018, the European satellite Gaia provided a very precise measurement of Proxima’s distance and movement, that is in agreement with the previously available measurements.
A CHOICE TARGET
Our discovery has considerable implications for knowledge of the Alpha Centauri system, and the planets it contains. As the capture probability of a star like Proxima Centauri is low, its gravitational link to Alpha Centauri suggests that the three stars formed together from the same cloud of gas and have therefore of the same age. Until now, we only knew the age of the A and B components, estimated between 5 and 7 billion years. Proxima’s age was unknown however, as such a small star evolves very slowly and hardly changes its appearance throughout its long existence - several hundreds of billions of years, in theory. As a result, we can estimate that the planet Proxima b is also the same age, as it formed at the same time as its star. This planet is therefore 1 to 2 billion years older than the Earth. This is significant, as we know that Proxima b, probably a rocky planet like the Earth, is in its star’s habitable zone, where water can exist in liquid form. If the conditions were right from the start, life could have had enough time to appear on the surface.
The interest of astronomers in Proxima is thus even higher now. Its closeness already made it a prime target for observations, as it is easily accessible for telescopes. The discovery of a potentially habitable planet has added to the fascination. So much so that, now, all eyes are turned towards the star and Proxima b, to better characterise them and try to discover other planets. Some are even envisaging sending a space mission to study this star system in situ (see inset).
Furthermore, the fact that Proxima belongs to a very particular trinary system, comprising two stars very similar to the Sun, close to one another, and a red dwarf far away from the main pair, makes the star even more original. Astronomers’ excitement for our closest neighbour is not going to wane anytime soon.
(*) The Doppler Effect is the apparent change in frequency of a wave signal (sound, light) emitted by a moving source and received by a fixed observer. The variation in frequency is proportional to the relative speed between the observer and the source.
A mini ship, pushed by laser, could go to study Proxima B (view of artist). Photo © Breakthrough Initiatives.
What if we sent a mission to Proxima to observe its planet more closely? This is the ambitious idea of project Breakthrough Starshot, funded by the Russian billionaire Yuri Milner. Objective: to launch, in the coming decades, a fleet of miniature vessels weighing a few grams and accelerated at 20% of the speed of light (60,000 km/s) by a powerful laser shot from the Earth. At this speed, they would reach Proxima and its neighbours Alpha Centauri A and B in “only” twenty years. With no braking system, the probes would briefly gather information about the star system. One option currently being studied, plans to slow the craft down using the light from the three stars, to place them in orbit around Proxima or even its planet. The possibilities for scientific study would be considerably richer. The only downside to braking is that the cruising speed would be reduced, lengthening the voyage time to more than a century!
Astronomer at Paris Observatory (Lesia).
CNRS director of research at Côte d’Azur Observatory in Nice.
Teacher and researcher at Geneva Observatory.