Seven (TRAPPIST-1 planets) for the Dwarf-lords in halls of stone

“Hey Elizabeth, do you know what the NASA press conference is about tomorrow?”

I hadn’t a clue. Having stepped off a plane from Japan the night before, I was twirling around in a swivel chair in one of the student offices at McMaster University while I tried to bully my brain into action. Until that moment, I wasn’t aware NASA had announced a press conference.

 The NASA vintage travel poster for one of the Trappist-1 planets.

The NASA vintage travel poster for one of the Trappist-1 planets.

The NASA site did not reveal much. Tomorrow’s event was to “present new findings on planets that orbit stars other than our sun.” It was exoplanet news, but the lack of details left us speculating. 

“It’s an atmosphere detection for Proxima Centauri-b!” 

“Can’t be. The planet doesn’t transit.”

This fact made our nearest exoplanet something of a disappointment. Proxima Centauri-b had been found by detecting the slight wobble in the position of the star due to the planet’s gravity. However, without an orbit that took the planet between star and Earth, there was no opportunity to examine starlight passing through the planet’s atmospheric gases. Such a technique is known as ’atmospheric spectroscopy’ and can uncover which molecules are in the air to reveal processes that must be occurring on the planet’s surface — the location relevant to habitability. The next generation of telescopes including NASA’s JWST and ESA’s Ariel are focussed on using this method to finally probe planet surface conditions. The uselessly orientated orbit of Proxima Centauri-b however, removes it from the target selection lists. 

This took us back to the problem of what NASA were about to announce. 

“It can’t just be another planet.”

“It could be a possible biosignature?”

“… do we have anything that could measure that yet?”

This was the crux of the mystery. It is amazing that in the scant 25 years since the first exoplanet discoveries, finding a new world beyond our solar system has become insufficient to warrant a press conference. We now know of nearly 3,500 exoplanets, roughly a third of which are less than twice the size of the Earth. The news had to be bigger than a simple additional statistic. 

However, a discovery of alien life seemed to be too premature. It is true that the presence of biological organisms may be detected by their influence on a planet’s atmosphere. It is also true that the Hubble Space Telescope (HST) can do atmospheric spectroscopy, although not nearly at the resolution of the future instruments. As far as I am aware, HST has examined the atmosphere of three super Earth-sized planets and only seen features in 55 Cancri-e, which orbits so close to its star that a year is done in hours. So … a biosignature was not impossible. It just would have meant we had got very very very very very lucky.

Nobody’s that lucky. Especially not in 2017.

We were evidently not alone in our speculation, since the news was leaked later that day. Seven Earth-sized planets had been discovered orbiting the ultracool dwarf star, TRAPPIST-1. It was a miniature solar system and NASA were about to infuriate me by gabbling non-stop about the prospect of life.

Let’s make something clear:

Apart from roughly the same number of planets (by astronomer standards, 7 basically equals 8. Or 9.) the TRAPPIST-1 system is very unlike our own. 

That is what makes it cool.

Also, the system takes its name from a Belgian beer. 

 Trappist beer. Oh yes.

Trappist beer. Oh yes.

Last year, three planets were discovered around TRAPPIST-1. The star was named for the telescope that was used in the discovery, the robotic Belgian 60cm ‘TRAnsiting Planets and Planetesimal Small Telescope’. It sounds like a perfectly reasonable acronym until you learn that Trappist is a Belgian brewing company. Astronomers have no shame. It’s all kinds of wonderful.

The news was that further inspection of the system had added another four planets. The fresh observations had used a number of telescopes around the world and finished with an intensive stint on NASA’s infrared Spitzer Space Telescope. 

(Interesting fact: Launched in 2003, Spitzer was never designed to be able to see planets. Some swanky engineering tricks from the ground allowed a 1000 times improvement in measuring star brightness that led to the tiny dip from a transiting planet being detectable. Cool stars like TRAPPIST-1 are a 1000 times brighter in the infrared than at optical wavelengths, making Spitzer a kick-ass planet grabbing machine.)

What was still more exiting is that all the planets transit, leaving the door wide-open for some rocky planet atmosphere spectroscopy rock n’ roll. 

Were alien climates ammonia cloudy with a chance of methane meatballs? The next five years might reveal the answer to that question. 

The planets were all on short orbits, with years lasting between 1.5 and 13 days. This close packed system meant that neighbouring planets would appear larger than the Moon in the night sky. The in-your-face sibling-ness also allowed for the planet masses to be measured.

While transit observations normally yield only the planet radius, the gravitational tugs from planets in the same system can vary the time between successive transits. These ‘transit timing variations’ can be used to estimate the size of the tug, and thereby measure the mass of the planets. With the exception of the outermost planet —whose single transit measurement is only enough for a radius estimate— the TRAPPIST-1 planets got both radius and mass measurements. 

And you know what that means.

(Density. It means density.)

In fact, the mass measurements were not particular accurate, leading to error bars as large as the measured value except in the case of planet TRAPPIST-1f. However, all measurements hinted at (and Ms Accurate TRAPPIST-1f agreed) that these planets were on the fluffy side.

With sizes less than the empirical threshold value 1.6 Earth radii, the planets were unlikely to be Neptune-like gas worlds. But their low density suggests they do have a much higher fraction of volatiles than the Earth. They could even be downright watery.

This possibility is backed up in a less obvious way by the planet orbits. The inner six worlds are in resonance, meaning that the ratio between their orbital times can be expressed as two small integer numbers. So while the innermost world orbits the star 8 times, the outer planets orbit 5, 2 and 2 times. 

Well… almost. And since we declared above that 7 basically equalled 8 or 9, I’d say we were good. 

Strings of planets in resonance are completely unsurprising and utterly predictable.

… so long as you formed somewhere entirely different.

Transits of the seven TRAPPIST-1 worlds. The orbits shown in the first few seconds show the resonance between the planets.

Resonant orbits between neighbouring planets occur when young planets migrate through the planet-forming gas disc. This gas migration can occur once the growing planet reaches the size of Mars and its gravity begins to pull on the surrounding gas, which pulls back. The net force usually sees the planet move towards the star. If multiple planets take this site-seeing tour of their system, their mutual gravity will pull on one another. These tugs only balance out when the orbital times form integer ratios, producing a resonance. The predicted result is a series of planets in resonant orbits close to the star — exactly what is seen in the TRAPPIST-1 system.

If the planets formed in cold outer reaches far from the star,  then a substantial part of their mass would be in ice. As the planets moved towards the (ultracool but still a nuclear furnace and way hotter than Colin Firth in Pride and Prejudice) star, the ice would melt into water or vapour. This would explain the low densities compared to the Earth’s predominantly silicate composition. 

Three of the TRAPPIST-1 planets stopped their mooch inwards within the star’s temperate zone (or ‘habitable zone’ if you must). This is the region around a star where an exact Earth clone could support liquid water on the surface. 

Once more for the cheap seats at the back?

EXACT EARTH CLONE.

If you’re not an exact Earth clone, then the temperate zone guarantees as much as one of Nigel Farage’s Brexit bus adverts. 

brexitbus

So how Earth-like are these temperate zone wannabes? On the plus side, they likely have plenty of water. On the down side, it’s quite likely too much.

While the majority of the Earth’s surface is covered with oceans, water makes up less than 0.1% of our planet’s mass. If we had formed further out where water freezes into ices (i.e. past the ‘ice line’), then that fraction could be nearer 50%. This would create huge oceans as the planet warmed, enveloping all land under a sea a bajillion fathoms deep (exact measurement. Prove me wrong.) 

The bottom of such a monstrous ocean would be so high pressure than a thick layer of ice would separate the water from the rocky core. This would scupper the carbon-silicate cycle, preventing the quantity of carbon dioxide in the air responding like a thermostat to global changes in climate. This would mean anything other than the absolute perfect amount of stellar heat would render the planet uninhabitable. The temperate zone would shrink to a thin slice and any slight ellipticity in the planet orbit, or variation in the star’s heat, would fry or freeze everything in site. 

It ain’t impossible for life, but it ain’t promising. It also ain’t Earth.

Even if the oceans were shallow enough to avoid this, the icy composition of the planet might burp out a crazy atmosphere. Our atmosphere was outgassed in volcanic eruptions during the Earth’s early years. But if the planet was made not of silicates, but of comet-like ices, then the gasses emerging from the volcanos would likely be mainly ammonia or methane. Not yummy. Also strong greenhouse gases, so could end up roasting planets within the temperate zone. 

Since we’ve no analogue of such planets in our own solar system, it’s hard to speculate on their surface conditions. Could such a rocky ice mix produce a magnetic field? The icy Jovian moon, Ganymede, has a weak field, so it could be possible. If it is not, then any atmosphere might be stripped by the star’s stellar wind. 

 Google doodle celebrating the Trappist-1 system

Google doodle celebrating the Trappist-1 system

The fact we’ve not the foggiest idea of what these worlds would really be like is why they’re so exciting. Here we have 7 prime candidates for atmospheric studies and we’re hoping to see not the same thing as beneath our feet, but something entirely new. This would tell us about how planets form (really migration? really ices?) to how a completely non-terrestrial geology behaves. It’s going to be so much more awesome Ewoks. 

So are we going to give these planets better names than TRAPPIST-1b, c, d, e, f and h? Speaking at the NASA press conference, lead author Michaël Gillon admitted,

“We have plenty of possibilities that are all related to Belgian beers, but we don’t think that they’ll become official!”  

DAMN YOU, IAU. 

In better news, NASA has designed a new travel poster to mark the occasion. And there’s a google doodle. Yay.