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Elizabeth Tasker
Astrophysicist. Writer. Cat lover. Tea drinker. (Order may inaccurately reflect applitude)
Astrophysicist. Writer. Cat lover. Tea drinker. (Order may inaccurately reflect applitude)

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Considering buying a +BMW? You might want to rethink that.
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Reading about yourself as a character within a book is one of the most bizarrely fantastic experiences I've had to date! Meet Grace Carter; she wants to become an astrophysicist.
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We're about the launch to Mercury! Like... right now! Check out the live feed here:
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ESA & JAXA are teaming up to launch to Mercury with the BepiColombo Mission! Launch is in about 24 hours (!) but why are we heading to our inner most planet?
Elizabeth Tasker
Elizabeth Tasker
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We need to talk about Proxima Centauri b. Again.

Last week, the media exploded like overripe roadkill with this headline:

The Closest Exoplanet to Earth Could Be “Highly Habitable”

An article that rapidly spread through Scientific American, & Live Science. Which was a pity because it was complete garbage.

In truth, the actual article is fine but the headline is more misleading than raisins in a cookie labelled “chocolate chip”. Why? Let’s just review what we know about our “closest exoplanet”.

Our neighbour is Proxima Centauri b. Orbiting our nearest star (Proxima Centauri: don’t we name exoplanets well?), this is as close as close gets. Short of the passing of a rough world that has been kicked out of its star system, we will never find an exoplanet nearer than this one.

Basically, I’m OK with the first five words of the headline.

Now, let’s get to the details which are about as meaty as a chicken foot kebab. Proxima b was discovered in 2016 by an exoplanet hunting method known as the “radial velocity technique”. This measures the small shift in the star’s position due to the pull from the gravity of an orbiting planet. It gives you the minimum mass of the planet.

And 👏 That 👏 Is 👏 All 👏.

Proxima b’s minimum mass is 30% larger than the Earth. But its true mass could be much larger. It all depends on whether we’re viewing Proxima b’s orbit exactly edge-on, (in which case the minimum mass is the true mass) or if it’s tilted at an angle. There is no reason at all to suspect one inclination angle is more likely than another.

Increasing the planet’s mass increases the gravitational pull, which umps the amount of gas a planet can hold onto and thickens the atmosphere. The planet also becomes able to hold onto lighter gases that escape from the Earth’s atmosphere into space, such as hydrogen and helium. Even the minimum Proxima b has a stronger gravitational pull than the Earth. If the inclination is more than about 25 degrees away from edge on, we’re probably detecting a gas planet like Neptune and not a rocky world at all.

The only other measurement that’s currently possible to make that is relevant to planet conditions is the distance of the planet away from the star. This gives us the amount of starlight that the world should receive during its orbit (assuming that orbit is circular. Which we don’t know). Proxima b’s orbit takes just 11.2 days, but the star is a dim lightbulb of celestial elemental fusion known as a “red dwarf”. This means rather than blasting Proxima b more strongly than Mercury, the planet receives around 65% of the radiation level the Earth receives from the Sun. This nearly-comparable level puts Proxima b in the so-called “Habitable Zone”.


Let’s just briefly note that the edges Habitable Zone are defined for the Earth: that is, a planet of our mass, radius, atmospheric composition, magnetic field etc etc being radiated by a star pretty similar to our Sun. We only have one lower limit on any property of Proxima b and it doesn’t match that of the Earth, plus it’s orbiting a completely different type of star. Stating the planet is within the Habitable Zone is therefore about as meaningful for habitability as saying you must be King Arthur because you were born somewhere in Europe.

We cannot measure anything about conditions on the surface of Proxima b, what its atmosphere might be like or whether water exists. In short, anything that you might vaguely associate with habitability we do not know.

At this point you might reasonably ask how any article about Proxima b exists that is longer than:

“There’s a planet that’s at least 30% bigger than us around a squiffy star. The end”.

And that my friends, would be the most fact-filled article ever penned on the subject. However, this story is actually based on a paper published in the journal of ‘Astrobiology’ by an author list led by Anthony Del Genio from the NASA Goddard Institute for Space Studies. Del Genio and his collaborators are experienced climate modellers and this paper is really interesting: it just doesn’t say that Proxima b is actually likely to be “highly habitable”.

The paper’s introduction starts with a clear nod to the lack of facts known about this exoplanet. In fact, they go further and note that red dwarf stars are rambunctious bastards that are much brighter during their early years and now throw out bouts of intense radiation. The paper notes,

“This would probably have driven [Proxima b] into a runaway greenhouse state during the star’s luminous premain sequence phase, and even today would subject it to X-ray and extreme ultraviolet fluxes and stellar winds that might lead to catastrophic atmospheric and surface water loss.”

Do you know what a runaway greenhouse is, people? It’s Venus. And do you know what Venus is not? It’s not “highly habitable”.

The paper then points out:

“Nonetheless, Proxima b has a poorly constrained and potentially complex evolutionary history.”

Basically, the planet likely isn’t habitable BUT YOU CAN’T PROVE IT!

And since we know jack, we’re going to pretend Proxima b has somehow avoided the most likely fate of absolutely all scenarios and miraculously managed to obtain and retain a somewhat Earth-like atmosphere and water. Then from this fairytale princess start, we’re going to model the climate.

The only part of this paper that is actually particular to Proxima b is the radiation received from the star. Even the surface gravity of the planet is estimated assuming (based on zero evidence) the measured minimum mass is the true mass and the planet composition gives a density similar to the Earth.

None of that’s to say this paper isn’t good. It presents a super interesting set of comparative climate models that show how how relatively small changes can make a big difference to the conditions on a planet’s surface. There’s just no particular reason to believe that planet is Proxima b.

Buuuut maybe this is terribly unfair. The paper does say it's relevant to Proxima b in the title and there’s no proof this definitely isn’t an accurate model of our nearest neighbour. So surely the media article headline was reasonable?

That might be true if the conclusion of the paper was that this particular “Proxima b” was “highly habitable”. It isn’t.

Having given their planet the best possible start in life, Del Genio and colleagues develop a computer model of its climate and eye up the resulting surface conditions. A major feature here is the inclusion of an ocean capable of moving heat around the planet.

A “dynamic ocean" is a computationally intensive addition to the climate model but the reason for including it is important. It is easiest to detect planets that orbit close to their star. This means that most of the smaller, Earth-sized, planets we have spotted are in very close-in orbits, including Proxima b and the TRAPPIST-1 system. These will be the target of observations by our next generation of telescopes —including the successor to the Hubble, JWST— that aim to detect a planet’s atmosphere. The proximity of the star means these worlds are likely to be in “tidal lock”. Like the moon, one side of the planet will always face the star, creating hemispheres of eternal day and eternal night. Without a way to redistribute heat around the planet, the surface will boil and freeze simultaneously. In order to understand the future observations of these worlds, we need to understand how climate develops in a tidally locked system.

Since the authors want to study the impact of including a dynamic ocean, most of their models are of so-called “aqua planets”; water worlds that are entirely covered with a relatively shallow sea. Whether planets without land can actually support a stable climate is also debatable, but with shallow oceans it’s considered possible. Once again: we’ve no solid evidence either way, so the paper hopes for the best and goes onto compare the climate in 18 different scenarios, changing up the quantity of greenhouse gases, the rotation of the planet and (in a couple of cases) the amount of exposed land.

So here’s the thing… only one of these 18 cases has an average surface temperature above zero. And that one has a pure carbon dioxide atmosphere.

Do you start to see my gripe with “highly habitable”?

The authors do make an eye-rolling nod to the fact that this planet sits within the Habitable Zone but in 17/18 actually extremely Earth-like scenarios still doesn’t manage an average temperature above freezing. Not only that, but the mean temperature between the models varies by 47 degrees celsius, despite all (except one) receiving the same level of radiation from the star. The paper notes that is because surface temperature is much less to do with how much radiation is received (a value that gives you the usually quoted “equilibrium temperature” of a planet) than the amount of that radiation that is absorbed. This depends on the atmosphere composition, the type of star (different stars radiate strongly at different wavelengths: you’ll be astounded to hear ‘red dwarfs’ are strong in redder wavelengths) and the amount that is reflected by the surface.

If this paper had done all their calculations in one dimension, we would only know the average temperature of the planet and write them all off an inhibitable iceballs of Hoth. While that might have improved the media headline, it would have been less interesting than what the paper actually found.

The presence of a global heat-shifting ocean in the models was very effective at shifting the heat around the planet. This meant that rather than producing a so-called “eyeball” situation on the planet surface, whereby there is a small hot patch directly facing the star, a warmer region was spread outwards in what the authors describe as a “lobster shape”. The result was that while average temperatures were below zero, there were patches of the planet that were not frozen.

The paper describes the results as a “slushball”, which for some reason didn’t capture the imagination of the media headline maker quite as much. These planets have cold but above-freezing tropical oceans that actually may have happened on the Earth in the past. How much ocean is unfrozen depends on the model. Aside from the pure carbon dioxide atmosphere world, the model with the largest amount of open ocean was the one with a high water salinity.

The “High Salinity” model was actually one of the chiller worlds, with a maximum of zero degrees celsius, minimum of - 43 celsius and average of - 10. However, by increasing the ocean salinity to 23% (seawater on Earth is on average 3.5%, equivalent to 35g of salt in every litre, although the Dead Sea has a salinity of 33.7%), the freezing point of water could be lowered to - 19 celsius, allowing 87% of the planet global ocean to remain liquid.

This is the planet model that the paper describes as the most habitable, due to that large fraction of liquid ocean. They do note that this hardly fits the traditional notion of habitability, but that halophilic (salt-loving) life does exist on Earth in similar levels of salinity although at warmer temperatures.

The authors conclude that a cold, highly salty, water-covered Proxima Centauri b is possible (assuming an extremely fortuitous start and the ability to maintain a stable climate) and that it could host a salt-lovin’, cold-chillin’ form of life.

I liked this paper for two reasons:

1. The 18 comparative climates showed what huge variations you could achieve in planetary surface conditions by pretty small variations in the properties of the atmosphere and oceans, the conditions of which are currently completely unknown.

2. The paper challenged our view of what is “habitable”, pointing out that a pretty Earth-like system in the habitable zone around a red dwarf may be below freezing on average, but then showing that some areas might still support the extreme-end of life that we know.

In my opinion, these points were utterly undermined by a headline declaring “highly habitable”.

Such a front-liner strongly implies to a general audience that the planet could support a sizeable chunk of average Earth life. The reality is that all the models here are unlikely scenario for Proxima b, and that it would take the most extreme form of Earth’s life to survive even these conditions. We have no idea whether such life could even develop to begin with, or whether such extremophiles require more temperate conditions to evolve.

If one manages to get past the dreadful headline (which was most likely not written by the article author), the details in the article are really pretty good. But the headline destroys take-home point of the research. Frankly, I think that’s insulting to the article writer, the paper authors and most of all, the readers who have given up their time to learn something outside their field.


Image credit: Y. Beletsky (LCO)/ESO

Astrobiology paper (behind firewall):

#Proximab #Exoplanets #journaljuice
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Weather not looking good? Before you think it's time to move planets, best check out the storms on our neighbours, warn the panel of planetary and solar experts (moderated by me!) in "Storms of the Solar System and beyond"!
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We're all star dust, but where did that come from? A joint proposed space telescope between +European Space Agency, ESA and +JAXA | 宇宙航空研究開発機構, SPICA is an instrument to trace our origins back to the first atoms in the Universe.
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Let's talk spacecraft! I discuss JAXA's missions to the asteroids and Martian moons with "Seldom Sirius" podcast crew!
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Where the Earth's magnetic field lines dip down to our planet's poles is the polar cusp. The solar wind smacking our atmosphere here gives the aurora and dictates our habitability, but we still don't understand these complex interaction (magnetic fields are hard y'all). JAXA is part of an international team to use sounding rockets to explore this region.
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The new planet hunter, +NASA TESS, launched today to seek out the worlds closest to our own Sun. Why is this important and how is it going to bring us closer to understanding how planets like our own were born?

I take a look at the plans in a post for the NASA NExSS Many Worlds blog.
Marc Kaufman
Marc Kaufman
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