There’s growing evidence that not just our Earth but also the solar system we know and love is actually a rarity, as such things go. Many stars, we now learn, have planetary systems. But very few are like ours. That’s a change from even the mid-twentieth century, when we considered our solar system to be probably typical. There were even theories about a kind of mechanistic distribution of planetary distances, observed in our own system, to which all systems were expected to adhere.
Now we know better. Johann Elert Bode (1747-1826) and Johann Daniel Titius (1729-1796), who proposed this ‘law’, were simply doing the usual human thing of finding a pattern where there was no real pattern at all. But we couldn’t be sure about that until we had a data-set about other planetary systems. It turns out that this ‘law’ was an illusion. What’s more, most of the other planetary systems detected so far are smaller than our own, some crowding their worlds into orbits so close they are gravitationally related.
In part this is a result of the fact that our observing technologies are biased towards a specific type of star system. We can’t yet observe exoplanets directly: a few have, in fact, been imaged, but most of those we pick up are beyond the resolution of current telescopes. The majority are picked up variously by the radial-velocity method, or by transit-detection: spotting the light-dip as a planet passes between our instruments and the parent star.
These methods are highly effective and mean that planets can be detected at many thousands of light years distance. However, they are also limited. The radial velocity method works best for large planets that are close to the primary and relies on the fact that orbiting bodies actually revolve around the shared centre-of-gravity, the barycentre, of the system. A Jupiter-mass planet might have only a few percent of the mass of its primary, but it’ll still ‘wobble’ the primary as it orbits. The closer the world is, the more effect it has in that sense, which causes the light coming from the star to be alternately blue- or red-shifted. Instruments on Earth can detect that shift in the spectrum. The method was available in the 1990s, which is why some of the earliest planets detected are what we now call ‘hot Jupiters’; gas giants skimming around their star at less than Mercury’s distance from the Sun. These days, radial velocity variations as low as 3 metres per second are detectable.
The transit-detection method is a different approach that has largely (but not wholly) relied on space observatories such as Kepler. This method works by detecting the change in brightness of the target star as one of its planets passes in front of it, cutting off part of the light. There are two problems. One is that it works only if the angle on which the planets are orbiting is precisely in line with the detector. Exoplanetary systems at different angles can’t be detected by this method. Kepler detected a lot of systems anyway – implying that planetary systems are probably standard accessories for any star.
The other issue is that, while it’s possible that a planet at (say) the distance of Uranus from our Sun might happen to pass between our detector and its parent star, just as we’re observing, probably it won’t. That’s why many of the planets Kepler detected (and which remain to be detected, in its vast database) have been around Mercury’s distance from the sun, or less.
All this, in large part, is a function of the detection method. To properly survey an exo-system picked up by the transit method, observations have to be made continuously for many decades, and it will still miss planets on orbits that are inclined to the main plane of the system. But, even for a favourable alignment, we haven’t had the technology for the necessary length of time. Worse, the most effective tool we had for it – Kepler – has already been retired after its gyros wore out, and then the fuel driving the thrusters it used to orient itself with less fidelity was depleted. Its successor, TESS, has a slightly different mission and may last longer, but I’d be surprised if that amounted to the eight or nine decades or more needed to confirm outer planets in a solar system similar in scale to our own even if all else was favourable. What this means is that, as with the interferometry method, the transit-detection method basically works best for planets relatively close to the primary star.
Ground-based telescopes, such as the Belgian remote-controlled TRAPPIST pair, or the European Southern Observatory’s VLT (Very Large Telescope) in Chile’s Atacama Desert, can do much the same thing – and may well have a century-plus lifespan: but again, they’re relatively recent. The VLT, incidentally, was able to provide spectral data about the exoplanet Gliese 1214 b of sufficient fidelity that it was possible to detect water vapour in its atmosphere. Even larger ground-based telescopes will be completed in the near future, possibly joined by the James Webb Space Telescope, if it is ever launched.
For all that, what we have found so far is intriguing. Our own solar system’s distribution of planets – four rocky ‘terrestrial’ worlds, two large ‘gas giants’ and two smaller ‘ice giants’ – with many other bodies, some discovered at astonishing distances by earlier standards – appears atypical. Exoplanet detections have shown that Neptune-sized worlds are more common than Earth-sized. Some of these Neptunes are ‘hot’, meaning they are probably covered in water. And there is some evidence that water-worlds are more common than those with land area.
When I say ‘Neptune’, of course, I also mean ‘Uranus’. Yup, the evidence is that planets the size of Uranus are pretty common. There, I said it.
The search for an Earth clone continues, but so far none has shown up. There have been occasional Earth mass planets (plus or minus a bit) found in the right zones for Earth-type life, but that’s not the same thing. After all, Venus is an Earth-mass world (minus a bit) in the life-as-we-know-it-friendly zone of our own solar system. Thanks to a runaway greenhouse effect, it’s also got surface pressures of 90 Earth atmospheres at 470+ degrees C, all sitting under clouds of sulphuric acid. We can’t yet tell whether the ‘Earth 2 candidates’ we discover are actually Earthlike, or another Venus, or something between – or if they are different again. However, the data we have so far indicates that anything in that category is less common than Neptune-mass planets.
All this suggests that our planet, if not unique, is certainly going to be rare. As is our own solar system. And that is all the more reason for looking after it. But I wonder. If we ever get to Mars and colonise it, or if we reach some of the other planets on any scale – will we just carry on behaving as we always do and ruin them as well, perhaps through some misguided concept of ‘terraforming’?
Copyright © Matthew Wright 2019