My weekend slid downhill when I began an article that started:
"The discovery of alien life could be a step closer after scientists found a newly discovered planet is ‘likely' to harbour life forms."
My friends, that be pretty big talk for a planet for which we only know the minimum mass.
The planet in question is Proxima-b, whose discovery around our closest star was announced in August. You may remember its name from my previous editions of OMG-PLANET-NEWS-GET-UR-SHIT-TOGETHER.
This article (in the UK newspaper, the Independent) covered recent research published in the scientific journal, MNRAS. Unfortunately, it represents the work more poorly than the Hollywood adaption of your favourite novel. One you really, really liked.
Now, I too find reading research papers a drag: they’re dry, gloss over the exciting scrumptious bits in favour of a parameter space study and sometimes the graphs aren’t even in colour. But this particular paper was less than five pages. FIVE. And that includes the plots. And those five pages do not discuss the chances of Proxima-b being inhabited by anything.
Let’s take a look at “In innards of Proxima-b: how the movie differs from the book".
The research paper asks a simple question: If we assume Proxima-b is a rocky planet, what might it be like?
Did you notice that summary started with an “if”?
Because Proxima-b has not been observed passing in front of its star, we don’t know the planet’s radius. Instead, astronomers have measured the slight wobble in the star’s position due to the tug from the planet’s gravity. This tells us how much the planet is pulling the star towards the Earth and that gives us a handle on its mass. However, since we don’t know the angle of the planet’s orbit, we can’t tell whether the planet is pulling the star directly towards the Earth, or if only part of its tug is in our direction. The upshot is we know only the lowest possible mass for the planet, with its true value being potentially much higher.
Proxima-b’s minimum mass is ~1.3 Earth masses; a value that suggests (but still doesn’t guarantee) a rocky surface. The maximum value would make the planet a gas giant such as Neptune.
For this paper, the researchers are only interested in the outcomes for a rocky composition. This leads them to consider only the follow situation:
(1) The MINIMUM MASS of the planet is the TRUE mass. Since every measurement has errors, they actually consider the planet has a mass between 1.1 Earth masses - 1.46 Earth masses.
(2) The planet has a thin Earth-like atmosphere, not a thick envelope like Neptune.
(3) The rock composition is similar to that found in the solar system, with the planet having an iron core, silicate mantle and ice or water top layer.
There is no observational evidence at all for any of these points. A familiarly solid base for the planet is assumed, and then the research asks what permutations are possible. To say this suggests Proxima-b is like Earth is akin to filling a pen with red ink and then claiming this proves all pens write in red. It’s nonsensical and it’s not the point of the paper.
The paper considers three possible masses for Proxima-b, within the error bars that surround the minimum possible value: (1) 1.1 Earth mass planet, (2) 1.27 Earth mass planet and (3) 1.46 Earth mass planet. The authors then tweak the relative amounts of core, mantle and water to see what worlds result.
To put limits on the possibilities, the research assumes a mix of silicate, iron and water typical of planets, asteroids and comets found in the solar system. Planet models that have more water than most solar system objects, or huge cores are dismissed as implausible.
The authors placed down these boundaries as the solar system is only place where we have data on what ranges are reasonable. However, Proxima-b’s star is not like our sun. Instead, it’s a dim red dwarf with a different mix of elements. It could therefore be that the rocks available to build planets have a very different blend than those around our own sun. Such differences can lead to drastic changes in planet conditions, such as producing carbon worlds with diamond mantles and seas of tar.
However —again— we have to work with the data we have. Which is very little for the Proxima system.
The result of the paper is not a single favoured model, but a range of possibilities for a rocky Proxima-b. A Proxima-b with a 1.1 Earth mass but radii between 1.2 - 1.3 Earth size could contain 60 - 70% water, compared to our own Earth’s minute 0.05%. On the other hand, an Earth-sized planet of that mass could contain no water but a fat iron core. The total composition range (for conditions 1 - 3 above) is a planet made from 65% iron / 35% rocky silicates (matching a radius of 0.94 x Earth) to a 50% silicate / 50% water world (radius 1.4 x Earth), with 200 km deep liquid ocean.
While the inspiration of the paper was Proxima-b, there’s nothing really particular about this calculation that applies only to this planet. The results are true for any world around 1.3 Earth masses.
Should the radius of Proxima-b ever be measured, these models could help narrow down possible planet conditions or even rule out the planet being rocky at all. However, it’s worth noting that even for an exact radius and mass, different combinations of water, silicate and iron are still possible. At present, there is no way of selecting a more probably model amongst any of the options.
So do these possibilities say anything about habitability? Not a jot.
Should water be present, life would get a helpful medium for some biochemistry action. But this is only one of many many (many many) factors. The changing iron core size is liable to affect the magnetic field; a likely essential component of any planet orbiting a red dwarf. These dim stars may sound benign, but they are prone to violent outbursts of energy that could strip a planet’s atmosphere without the protection from some heavy duty magnetics. A thick mantle will have a baring on plate tectonics and is liable to determine the gases in the atmosphere. The deep water world may also have a thick layer of ice that cuts off the silicon surface from the ocean, preventing a carbon-silicate cycle of elements that helps control planet temperature on Earth.
We cannot be anymore quantitive about these properties, since we don’t know the surface conditions on any exoplanets. The next generation of instruments are just beginning to be able to sniff the atmospheres of these new worlds. This may provide us with the first clue of what the surfaces could be like.
This paper was a neat modelling experiment that drives home how varied a planet could be, even with a huge number of assumptions. So how did the news article get the message quite so wrong?
My guess is that the writer did not read the journal paper at all (despite quoting it as the source) but took the information from the very beginning of the press release by CNRS: the ‘Centre National de la Recherché Scientifique’ in France, and home institute of the research authors. The press release overall isn’t bad, but the opening paragraphs are misleadingly phrased and contain the statement; “[Proxima-b] is likely to harbour liquid water at its surface and therefore to harbour life forms."
No dude, that just ain’t true. Water is a possibility for the planet’s composition, but the research doesn’t promise that any is there.
Taking this as the full research, the news article then quotes the lead author seemingly collaborating this statement. While it’s hard to know without hearing the interview verbatim, I suspect this is an example of poor editing. The author apparently told the newspaper:
"Among the thousands of exoplanets we have already discovered, Proxima-b is one of the best candidates to sustain life."
With only the minimum mass measured, there’s no reason Proxima-b is more likely to harbour life than many other exoplanet discoveries. However, it is true the proximity of the planet makes it an excellent candidate for more detailed observations.
"It is in the habitable zone of its star, [and] even if it is really close to the star the fact that Proxima Centauri is a red dwarf allows the planet to have a lower temperature and maybe liquid water."
Note, the author said “maybe” here. Like the Earth, it is unlikely that Proxima-b formed with liquid water: its location close to the star would have been too warm for ice to be incorporated into its body. Instead, the planet would need to form further out and move inwards, or receive a delivery of icy meteorites from further from the star. Both are possible; neither are certain.
"The fact there could still be life on the planet today, not only during its formation, is huge."
I guess this is true, but its unsubstantiated based on the research paper, so I don’t understand the motivation behind the comment. I’m inclined to blame selective editing once again.
"The interesting thing about Proxima-b is it is the closest exoplanet to Earth. It is really exciting to have the possibility that there is life just at the gates of our solar system."
Yes. This is the main reason Proxima-b is exciting. We don’t yet know if the planet is rocky. We certainly don’t know if its surface conditions are similar to Earth. But while the world is too far to visit with current technology, its relative proximity gives the next generation of telescopes the best chance at finding out more.
Research paper: Brugger, Mousis, Deleuil, Lunine, 2016