This week’s news that the first exo-moon might have been found, about 4000 light years away, is startling for several reasons.
The first astonishing bit is that it was found at all. The majority of planets orbiting other stars have been found by light-curve analysis as the planet passes directly between the parent star and our telescopes – for the most part, via the Kepler space telescope. The new exomoon is one of them, orbiting the planet Kepler 1625b. This is a gas giant about 10 times as massive as Jupiter, although it’s actually about the same size as Jupiter because, you know, physics. It’s also orbiting in the ‘habitable zone’ of its primary star, Kepler 1625, a star similar to the Sun which is estimated to be about 3,956 light years from Earth.
Kepler 1625b was detected in Kepler data from the way the light dips, it’s possible to not just find the planet, but to also discover a good deal about it. In theory, any moon orbiting the planet would create a secondary dip of its own – but on a vastly smaller scale. And that’s the issue; even planetary detection starts to involve variations in only a few handfuls of photons. Now imagine detecting a moon.
But there were hints of that in the Kepler data, and that was exciting enough to get discoverers David Kipping and Alex teachey of Columbia University, some 40 hours of time on the Hubble Space Telescope, back in October 2017. From that data they were able to deduce the characteristics of the proposed exomoon.
Another reason why it’s amazing is that, although the detection hasn’t been conclusively proven yet, odds are on that it’s real. And while it’s obvious that solar systems other than our own will be as complex – with all kinds of planets, moons, comets and the rest whirling around – from a strict scientific perspective that demands proof.
The final awesome bit, at least for me, is that exomoon is, itself, about the same dimensions as Neptune or, in point of fact, Uranus. What sort of place is it? The size suggests a gas giant. It is, however, just 1.5 percent the mass of its primary world. This isn’t such an unusual differential; it’s estimated that relative to its planet, this exomoon is smaller – proportionately – than our Moon relative to Earth.
But there might be variants on the theme. A paper written by Rene Heller in August 2017, before the Hubble data was available, looked into various scenarios of the nature of both planet and moon. Data available at the time pointed to a variety of possibilities. One question was whether the moon had formed in situ with its parent planet, or been captured later; the maths showed that an object the mass of Neptune (or Uranus) could be captured by Kepler 1625b. The temperature of the moon has been calculated to be 80 degrees C. One possibility is that the moon has similar mass to Earth but has been ‘puffed up’ by heating. Nobody knows – yet. What this says is that there’s going to be some exciting science coming up.
What it doesn’t say, though, is that there will be exomoons out there like Earth, orbiting gas giants and maybe populated by CGI-style humanoids who live in a Morally Better Way. Nor are there likely to be any single-climate forest moons filled with cute teddy-bear cuddly creatures who, nonetheless, can defeat Imperial Stormtroopers in battle.
In point of fact, I don’t think it’s plausible to have an Earth-like world orbiting a gas giant, even if the gas giant is in the right part of the star’s ‘habitable zone’.
Here’s why. To get the kind of setting beloved of a particular sci-fi movie that I fell asleep while watching, twice – you know, where the crescent gas-giant looms large in the sky – you have to be in a fairly close orbit. That implies that such a world would be tidally locked, forever showing the same face to the gas giant. That’s going to make the day at least several Earth days long, or possibly a week. It also implies that the moon is orbiting splat in the middle of the gas giant’s radiation belts, just as Io is around Jupiter. Any life on the world? It’s reduced to glow-in-the-dark toast, even supposing it could emerge in the first place.
You could always assume that this putative Earth-like satellite had a large iron core, enabling a magnetic field that might deflect most of the radiation, but this swiftly leads you down a path of assumed characteristics and events that start becoming highly unlikely. We think, for instance, that Earth’s large iron core – which is virtually unique in our own solar system – was the result of an ancient collision between proto-Earth and a Mars-sized body named Thiea. The resulting wreckage produced both modern Earth and the Moon – and the heavy stuff sank to the centre of the gravity well, meaning Earth got most of the iron.
For that to happen around a gas giant – where the gas giant itself would likely soak up the lion’s share of the wreckage if two moons collided – starts looking less likely. We might find that exomoons around gas giants consist of lighter material. Maybe such moons would be primarily made of hydrogen and helium. Or water. That last doesn’t preclude life, if the temperature’s right – but it does mean that the moon won’t be much like Earth. And in this calculation we also have to remember the problems of tidal heating – again, looking at Io as an example. That place is absolutely boisterously volcanic.
There’s a possibility, then, that the newly discovered exomoon might be essentially a large blob of boiling water with a small rocky core. Who knows? That doesn’t mean it couldn’t host life – think about Earth’s extremophiles. But it does mean that the place isn’t going to be like Earth.
My take is that exomoons – which must be out there – will have characteristics of their own, and we can’t know what they are for sure until we’ve detected a fair few. One thing’s for certain; these conditions are unlikely to be much like Earth, even for an exomoon in the ‘habitable zone’. No, they’re going to be strange – and wonderful to discover.
Copyright © Matthew Wright 2018