Space

An Interview With the Discoverer of Proxima b, the Exoplanet Next Door

Guillem Anglada-Escudé, an astronomer at Queen Mary University of London and leader of the team that made the discovery, says Proxima b is “an ideal target” to look for life on.

Guillem Anglada-Escudé. Credit: Ita Mehrotra

Guillem Anglada-Escudé. Credit: Ita Mehrotra

The recent discovery of an Earth-sized planet orbiting Proxima Centauri, the star closest to the Sun, within within the star’s habitable zone has caused a lot of excitement among the international astronomy community.

Guillem Anglada-Escudé, an astronomer at Queen Mary University of London and leader of the team that made the discovery, says Proxima b, as the planet is called, is “an ideal target” to look for life on.

Located within a system that is just 4.3 lightyears, or 40 trillion kilometres, from Earth – a distance regarded as next door in cosmic terms – Proxima b is now among the most coveted exoplanets to determine habitability.

Proxima b is 1.3 times the mass of Earth and orbits its red-dwarf star every 11.2 days. Although the spectrographic evidence of such a planet has been observed since 2000, it was only in January 2016 that Anglada-Escudé and his team decided to make a definite determination using the European Southern Observatory (ESO) facilities. Between January 19 and March 31, they studied it for 20 minutes each night. The finding of their work was reported in the journal Nature to international headlines.

Adding to the excitement over the discovery are the expectations of a breakthrough in laser-propelled interstellar probes that can travel at 20% the speed of light, covering the distance to Proxima b in 20 years.

The question of whether life can actually exist on Proxima b is very much in the realm of speculation. Its distance, although relatively close to its star compared to Earth’s from the Sun, is mitigated by the fact that it orbits a red-dwarf star that is smaller and dimmer than the Sun. Its distance may make it friendly to liquid water on its surface but given that it is tidally-locked with its star makes thing much more difficult.

Being tidally-locked means it is always the same side of the planet that faces its star – the way the same side of the Moon does Earth. This causes a dramatic difference in surface temperatures between either halves of the planet.

For Anglada-Escudé, there is rather personal joy to discovering the planet because of his lifelong passion for such exploration, and his particular interest in a science fiction novel named Proxima by Stephen Baxter. The book is centred on a planet orbiting Proxima Centauri.

He answered questions from The Wire by email. Excerpts:

Is it fair to say that when it comes to exploring life on exoplanets there could not be a better location than Proxima b in terms of its proximity to Earth and given likely advances in laser-powered interstellar probes?

Yes, It is an ideal target because of proximity to Earth and also Star-planet contrast. The only drawback is that the orbit is relatively close to the star. Some direct imaging instruments might be able to resolve it, but it is really at the limit of current plans. However, designs might be slightly tweaked given that the planet is now known to be there.

In the very recent past, there have been a few possible candidates for Earth-like exoplanets but Proxima b seems most promising. Can you describe the fundamental features that you looked for to establish its credentials?

I would not say is the best Earth-like planet found so far. I would say this is one of the possibly Earth-like planets where we have better chances of getting more information. All boils down to its observational advantages. From a philosophical point of view, it would be way better detecting an Earth-like planet around more massive Sun-like stars (like the two Alpha Centauri stars, A and B), but observationally speaking at least we have realistic chances of being able to deduce some basic information from its atmosphere using near future observatories.

Would you describe the detection of starlight shift that helped you determine the existence of this planet?

We used the Doppler method. It consists on measuring the radial velocity of the star caused by its motion around the centre of mass of the system. That is, the planet and the star orbit the common centre of mass. What we measure is the back and forth motion of the star that follow the orbit of the planet.

Would it be accurate to say – if and when Proxima b’s friendliness to water and life is confirmed – that the circumstances under which life can evolve around the universe are pretty diverse and eclectic?

Yes. If by any chance we detect evidence of life on Proxima, is would very likely mean that the universe is full of inhabited planets. The contrary (absence of life) would not be very informative though. At least, if we get information on the putative atmosphere we can calibrate our models to narrow down the best spots for life.

I ask because in terms of their sizes and other parameters our sun and the red-dwarf around which Proxima b are so different. Also, one is struck by the orbital differences of our 365 days compared to Proxima b’s 11.2 days. And yet, it could also harbor life simply because the habitable distances are different.

The number itself is not that important. The compact orbit has consequences in terms of the tidal/rotation state of the planet. For example, we strongly suspect that it is synchronously rotating with the orbit in the same way the Moon is locked to Earth (we always see the same side). In this sense, we would have a side of the planet in permanent light and the other side on permanent darkness. The star would always be hanging from the same point of the sky. This has consequences on the possible climates but it has been found that it is not a major concern in maintaining a wet atmosphere.

You have been quoted as saying that there is a reasonable expectation that this planet may be able to host life. Do we have the ability to detect signatures of life from this distance in a broad sense or do we have to be on or close to Proxima b to be able to ascertain?

With the nextgen instruments we might be able to figure out the presence of a few molecules on the atmosphere of these planets. That is the next step. However, the presence of water and O2 only would not be a definitive evidence for life. What you need are chemical species that would be destroyed in the presence of the other (For instance, O2 + methane would react relatively quickly unless one of them is replenished). Out of equilibrium chemistry would be a really strong evidence for life on a given planet. Of course, we would still want to see it, but one step at a time.

How do you get around the datedness of the data given the time it takes light to travel from there to here, 4.3 years.?

We always see a delayed picture of the system. There is no problem on this because the delay is always more or less the same.

How do you compare the tug of Proxima b on its star to Earth’s on our sun? I am particularly interested in the numbers of the movement both cause on their stars?

Earth moves the Sun at 10 cm/s with a cycle of one year. Proxima b moves the star 1.5 m/s (at least) over an orbital cycle of 11.2 days. The combination of this larger signal with the shorter orbital period makes Proxima b much easier to detect that exact Earth analogs around Sun-like stars.

You have a lifelong passion for exploration of precisely the kind of planets that Proxima b appears to be. Also, you were drawn to Stephen Baxter’s book ‘Proxima’ which strangely foreshadows some of the very things that you might be on to. Describe to me how fiction and fact can converge in the most unexpected ways.

We were already hunting for the ‘signal’ at the time I found and read Stephen Baxter’s world. I found it rather amusing that the basic properties of the planet he describes were pretty close to the ones we were chasing. He makes up a lot of things but he based his world (called Per Ardua) on plausible climate models on scientific literature, so the world he describes is a plausible scenario (best case possibly…).

Can you describe the actual process of observation that you were engaged in? I believe you and team observed it 20 minutes every night between January 19 and March 31 this year using the ESO’s planet-hunting instrument. Do you actually peer into the sky using it or study the spectrographic information produced by this planet-hunting  instrument?

We used HARPS installed at the 3.6m ESO telescope at La Silla [in Chile]. The observations were taken by an astronomer on site each night (We didn’t travel to the observatory this time). The only thing we see there are the spectra measured by HARPS.

We followed the star simultaneously with two other observatories (ASH2-SpaceObs and LCOGT). In these cases we did take images of Proxima (many hundreds of them!), but we used those to basically monitor its brightness and activity to be sure the activity was not related to the signal we were trying to confirm.

Finally, what more evidence would you be looking for now and if found, is it your expectation that we may mount an exploration in a reasonable future?

The most obvious thing we are trying to do is to see if the planet transits in front of the star. There is a small (but non-negligible) chance that this happens but it we need to be lucky. If that is the case, we could start characterising its atmosphere before the end of the year! In the likely event that there are no transits, we will have to wait for E-ELTs and space-based instruments.