The third brightest star in the night sky is Alpha Centauri. It is our closest stellar neighbour, the fictional birth system of the Transformers, Small Furry Creatures and Pan Galactic Gargle Blasters… and now our closest exoplanet.
Alpha Centauri is actually a triple star system. It consists of a binary pair, Alpha Centauri A and B, and a more distant dwarf star called Proxima Centauri. It is around this dim third wheel that a planet has been detected.
The discovery has exploded my social media feeds. The new world has a minimum mass 30% larger than the Earth and receives a comparable amount of light and heat. Anyone familiar with exoplanet news knows this is sufficient to start packing a suitcase and buying shares in the inevitable Really-Star-Bucks coffee franchise.
Anyone familiar with my feed knows I am about to ice bucket challenge this baby.
To be fair, this is exciting. It's really exciting. In fact, I'm so excited I've ditched my morning chore of cleaning my newly empty apartment to sit on a cardboard box and write this post [*].
Let's start by disembowelling the description of those planet properties. Proxima Centauri b has been found by the Radial Velocity Technique: this is the slight wobble in the star's motion due to the pull from the planet's gravity. The bigger the wobble, the stronger the gravity of the planet and thus, the more massive the planet. All true, but it runs us into our first caveat:
CAVEAT #1: We only know the minimum mass
We can only measure the star's motion directly towards the Earth. This means we only see part of the planet's effect on the star.
This is like trying to judge how far a hot air balloon has moved by looking at its shadow. You'll probably underestimate the distance travelled, because the balloon has moved upwards (causing no change in its shadow position) as well as horizontally. You might think the balloon didn't need much gas to move such a short distance, but actually it needed a lot of fuel to climb vertically. Likewise, the Proxima Centauri's planet might be much more massive but most of its force is pulling the star 'upwards' compared to our line of sight.
How close our measurement of 1.3 Earth masses is to Proxima Centauri b's true mass depends on the orientation of its orbit around the star. If we're looking at the orbit exactly edge on, then 1.3 Earth masses is the true value. If it's nearer to face-on, then the mass could be 70 times the mass of the Earth; the regime of the gas giants. If we assume the orientation is completely random, then the planet is most likely to be about 2.6 Earth masses.
So… what does this mean?
To gain even a rough inkling about what Proxima Centauri b is really like, we need its density. A high density would indicate a world with a solid surface, while a low density would suggest a Neptune-like gas giant. For density, we need size.
For all those headlines out there that have been proclaiming "Earth-sized planet discovered!" — be ashamed. We don't know jack about Proxima Centauri b's dimensions. This means the planet could be a rocky super Earth, or a gaseous Neptune.
However… if we were to take a guess… a rocky planet is likely. There is empirical evidence that planets smaller than about 1.5 x Earth size are more typically rocky than gassy. This boundary corresponds to a planet mass of roughly 4.5 Earth masses, assuming an Earth-like silicate rock composition. This is bigger than the most probable mass for Proxima Centauri b. So let's be optimistic and say we have a planet with a solid surface, but remember this is an educated guess based on only one measurement.
Let's move on to talk about the light and heat issue. Proxima Centauri b is much much closer to its star than the Earth is to the sun. In fact, it orbits at just 5% of our distance. That's way nearer than Mercury, which sits at 40% of the Earth-sun distance. A year on Proxima Centauri b is over in just 11.2 days. However, Proxima Centauri is a weakling among stars. It's a red dwarf with just over 10% of our sun's mass. It therefore only delivers 2/3rds of the radiation to Proxima Centauri b that we get on Earth.
This means that if you were to coat Proxima Centauri b in Earth's atmosphere, the surface temperature would be chilly… but it could support liquid water. That is, Proxima Centauri b is squarely in the 'Habitable Zone'.
CAVEAT #2: The Habitable Zone does not say ANYTHING about habitability.
I hate the term 'Habitable Zone' because any rational individual would believe it marks a location suitable for life. You know, with the term HABITABLE being right there in the name.
All the 'Habitable Zone' means is that a planet with an Earth-like atmosphere and surface pressure could host liquid water. Do we have ANY INDICATION WHATSOEVER THAT PROXIMA CENTAURI b HAS AN EARTH-LIKE ATMOSPHERE?
One guess. Two choices. And the answer isn't 'yes'.
The Habitable Zone tells us nothing at all about the planet, only about its location. If Jupiter sat at the Earth's position, it would be in the Habitable Zone, but certainly not any more habitable. The Habitable Zone is still interesting, since it can be used as part of a selection tool for follow-up studies: with a zoo of over 3,000 known planets, we need to pick out the best candidates for further observations. But this doesn't mean we're selecting Earths.
The Habitable Zone also doesn't tell us that much about the star. Which brings us to the third caveat:
CAVEAT #3: Red dwarfs have behavioural problems.
Red dwarfs make up for their poxy size by spewing strands of stellar material called 'flares'. The sun has flares too, but Proxima Centauri has way bigger ones and the planet is much much closer. The net result is an X-ray bath at Proxima Centauri b that is 400 times that on Earth. This value is the present-day one: the early years of the star would have been far more dangerous. Such radiation levels (past or present) could strip the atmosphere, evaporate any water and nuke all life on the planet.
… or it could not. The Earth is protected from the sun by its magnetic field. If Proxima Centauri b has a molten iron core and some plate tectonic action, then it may have wrapped itself in a magnetic safety vest. Do we know? Not a clue — even if the planet is rocky, its ingredient mix might be entirely different to Earth. Even if it's got the same rocky recipe as Earth, there may still be no magnetic field: Venus is an incredibly close match to us in size and mass, but has next to zilch in the magnetics department.
The flaring action of the star leads to another issue...
CAVEAT #4: The planet may not exist.
Flares, star spots and general star action can produce wobbles as the star rotates that can look an awful lot like a planet. Planets are so tiny compared to stars that it is terribly terribly easy to mistake their faint whisper among stellar groans and creaks. The more rambunctious the star, the harder the detection.
The published signal for Proxima Centauri b looks reasonable, but the star's activity has led to scepticism. Independent observations are needed before we can be certain the planet is definitely there. If it does turn out to be a false positive, it will be in good company: in 2012, a Earth mass planet was announced around Alpha Centauri B, but later retracted when a fresh analysis of the data caused the signal to vanish.
If the planet does exist, its close proximity to the star may lead to another problem: it might be tidally locked. Like the moon and the Earth, one side of Proxima Centauri b may permanently face the star, while the other side is a land of perpetual night. Whether this creates a split world of deathly roast and deathly cold depends again on the planet's atmosphere. If the air can circle around and redistribute the star's heat, surface conditions might be liveable. Alternatively, it might be the worst cooked Christmas turkey ever.
There is also the issue we've no real idea what it takes to be 'habitable'. With only the Earth as a reference point for hosting life, it's impossible to tell which conditions are the most key. For example, does the planet need to be in a system of worlds to have water delivered to its surface? Is having a moon important for heating? What happens if the planet's orbit is not circular, but a bent ellipse? (And if anyone has seen the ESI number discussed, just bleach your brain.)
It's also worth remembering:
CAVEAT #5: We've seen similar planets.
Proxima Centauri b is not the nearest exoplanet to Earth in mass, nor is it the first found in the Habitable Zone. However...
CAVEAT #6: Yeah, OK, this is still big news...
Proxima Centauri b it is the nearest exoplanet that could exist [**] and that is the reason its discovery is incredibly exciting. The Kepler Space Telescope has given us fantastic statistics about the numeracy of planets and the architecture of these alien systems, but just a single radius measurement for the planet itself. To understand more about planet formation and the development of life, we desperately need details on these individual worlds. In particular, we need a rocky planet close enough to examine its atmosphere and begin to probe surface conditions. That candidate is very likely to be Proxima Centauri b.
Proxima Centauri b will never be "Earth-like", since its star is definitely not "sun-like". However, red dwarfs are the most common stars in our galactic neighbourhood and the planets around them some of the easiest to find. The science community has raged about whether such stars are the best targets (easy to find planets) or the worst (warm planets dangerously close to the star) to explore the prospect of habitability. Future observations of Proxima Centauri b will hopefully pour facts into a debate that has been speculation and models.
So what is next?
Absolutely ideally, we'd spot the planet crossing the star's surface. This is the 'Transit Technique' for planet detection: the planet blocks out a small amount of the star's light as it passes between the star and Earth on its orbit. The amount of light obscured and its duration gives a handle on the planet's diameter and confirms the orientation of its orbit. With that, we'd have an average density AND the possibility of glimpsing the contents of the planet's atmosphere as the starlight gleams around its edges.
Unfortunately, the probability of Proxima Centauri b actually transiting the star are low. Astronomers have been hopefully gazing at our nearest stars for so long, that we should have spotted the little blighter if it were acting as a periodic dimmer switch. This means that the planet either does not transit as viewed from Earth, or the transit is undetectable due to the frequent massive flares from the star.
However, another exciting prospect is direct imaging. Planets (fortunately) don't have the roaring inferno of stars, but they do emit some heat. If this can be detected, we'd actually be able to see the planet. Direct imaging is still in its infancy and normally only spots Jupiter-sized worlds far from the star. But the proximity of Proxima Centauri means we might just be able to catch a glimpse of it with our best telescopes now… or very soon with instruments such as the James Webb Space Telescope (Hubble's successor) and the ground-based European Extremely Large Telescope in the pipeline. Seeing the planet directly would also allow us to check out its atmosphere and potential surface environment. In my (OK, potentially slightly biased) view, this makes Proxima Centauri b THE most exciting target for these telescopes.
If Proxima Centauri b is so very close, could we visit?
CAVEAT #7: The closest possible exoplanet is still damn far.
Proxima Centauri is 4.24 light years away from Earth. The furthest humans have ever travelled is a loop around the moon: a teeny tiny 00000004 light years away. Voyager 1 —our furthest and currently fastest travelling space craft— would still take about 75,000 years to reach this system (and it's not pointed in the right direction).
That said… one of the craziest idea in the world recently got funding. Project 'Starshot', financed by Russian billionaire, Yuri Milner, is planning to develop a method to send a tiny probe to Alpha Centauri in 20 years. To describe this as a "long shot" is a joke on several levels. However, if it were to be workable, there is now the greatest of great destinations.
[*] I'm moving. Not to Proxima Centauri.
[**] Still orbiting a star
Good reads on Proxima Centauri b by planetary scientists:
Jonti Horner and Tanya Hill @ The Conversation
Sean Raymond @ Nautilus