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There are at least 100 billion stars in our galaxy, and 2 trillion galaxies in the universe. Results from the Kepler Space Telescope suggests that many of these stars have planets. In view of the number of worlds out there, the Fermi paradox famously asks why we haven’t been contacted by other civilisations yet. Perhaps the answer is our Solar System is unique in ways that we hadn’t previously considered.
The first planets beyond our Solar System were confirmed in 1992 by looking for stars that wobbled slightly as they were shifted off centre by the gravitational pull of a planet. This method only detects very large planets with very close orbits, so naturally it only found star systems quite different to our own. Then, in 2009 the Kepler Space Telescope began searching for planets by measuring the drop in brightness as a planet transited in front of the star. This is a much more sensitive method, and in nine years has found over 2,300 confirmed exoplanets. Now the California-Kepler Survey has refined the orbital parameters of almost 1,000 of the Kepler planets using ground-based telescopes, and the results announced recently are quite troubling. It’s not just that most planetary systems are wildly different to ours. It’s the fact that they all follow a distinct and predictable set of rules, and our Solar System is the odd one out.
Let’s start with the Sun. Our star is a G-type, main sequence star. This is already unusual because around 75 per cent of the stars in the galaxy are M-type red dwarfs, which are smaller and cooler. Even among main sequence stars ours is one of the brightest – it outshines 95 per cent of all the stars in the galaxy. It is also somewhat unusual in being a loner; more than half of all stars are part of binary systems, where two stars orbit each other.
When we look at the planets that the Kepler Survey has found so far, things get even weirder. The most common type of planet in the galaxy by far is the ‘superterran’ – a rocky planet up to tentimes Earth masses and 2.5-times Earth's radius.
The next most common is the ‘sub-Neptune’ type; a planet with a hydrogen-helium atmosphere, but is still less than the mass of Neptune. We don’t have a single example of either of these planet types in our own system, and planets that do resemble our own in size and composition are rare everywhere else.
The discrepancy becomes even more apparent when you consider the placement of these planets relative to their parent star. We have four small, rocky, inner planets and then four much larger gas giants further out. But almost all the exoplanet gas giants we have found are well into the ‘hot zone’ of their star (too close for liquid water on the planet surface), even though we used to think that
gas giants couldn’t even form that close. In fact, exoplanets in general appear to orbit their stars much more closely than ours do. Over 93 per cent of all the planets detected by Kepler are inside the hot zone of their star. In our Solar System the only planet that close is Mercury. “We really have nothing interior to Mercury's orbit,” says Dr Gregory Laughlin, professor of astronomy and astrophysics at Yale University. “There's zilch. There aren't even any asteroids down there.” Kepler-11 on the other hand is a star with five planets orbiting closer than Mercury, and this seems to be the norm.
Of course it is much easier to detect planets with tight orbits in the first place because they block more of the star’s light when they transit and they have shorter orbital periods, so it is easier to spot the cyclical pattern as they come round each time. So could this skew in the data simply be a consequence of the kind of planets that Kepler can detect? The lead scientist of one of the California-Kepler Survey studies, Dr Lauren Weiss, says not.
“Kepler was not sensitive to planets beyond about 1AU – the Earth-Sun distance. For this reason, using Kepler alone, we cannot test whether our outer
Solar System is unique. However, the statistical properties of Kepler’s multi-planet systems show us that our inner Solar System is unusual. Most Kepler planetary systems have planets that are very similar in size. In contrast, our terrestrial planets have unusually diverse sizes. Venus is more than twice the radius of Mercury, and Mars is barely half the radius of Earth.”
Most exoplanets are just 10 per cent larger or smaller than their immediate neighbours. To check whether Kepler might have missed some planets that would result in more familiar-looking systems, Dr Weiss tried building imaginary star systems with randomly sized planets and then discarded all the ones that wouldn’t be detected by Kepler. “The result… looked nothing like the regular patterns in planet size that we observe, so we rejected the null hypothesis. The similar sizes of the planets is astrophysical, not the result of a detection bias.”
Another strong statistical pattern that emerged is that planet size increases as .you get further away from the star. This is quite different from our system, where the two largest planets, Saturn and Jupiter, sit in the middle. Very large planets that are more than six-times the size of Earth are already quite rare in the galaxy. Where they occur, it is generally in a system where all the other planets are also large. Another factor is that 90 per cent of
“Planetary systems have planets that are very similar in size. Our terrestrial planets have unusually diverse sizes”
Dr Lauren Weiss
all the gas giants we have found have orbits smaller than Mars’. The few that venture further out are in strongly eccentric orbits. Jupiter is particularly peculiar because it is huge, far away and has an almost perfectly circular orbit. “About one in every 2,000 stars in our galactic neighbourhood is a Sun-Jupiter system,” says planetary astronomer Dr Sean Raymond. “Those are about the odds of being picked if you apply to NASA to be an astronaut!”
However, it is the quirkiness of our gas giants that could be the key to understanding all the other strangeness, according to Dr Weiss. “The role of Jupiter and Saturn was likely very important in shaping the Solar System. A complicated dance between Jupiter and Saturn in the early Solar System is often invoked to explain the anomalously small size of Mars. Jupiter and Saturn are also likely responsible for the current orbits of Uranus, Neptune and the Kuiper Belt. Jupiter also might have helped or hindered the delivery of water to Earth by way of comets. Indeed, Jupiter and Saturn might be responsible in some not-yet-quantified way for the rise of life on Earth.”
Before the first exoplanets were discovered, theories of planet formation only had to explain our own Solar System. The earliest theories assumed that the planets formed in their current positions. "We used to look at the giant planets and think
“Jupiter and Saturn might be responsible in some not-yet-quantified way for the rise of life on Earth”
Dr Lauren Weiss
those are big, so they never moved," says Dr Kevin Walsh of the SwRI's Planetary Science Directorate in Colorado. However, computer models with these assumptions always produced a Mars that was much too big and an asteroid belt that was much too full. The only way around is to assume the gas giants are more mobile.
“That anchor point? It's gone,” says Walsh. Two main competing theories have been put forward since then. The Nice model proposes that all the large planets formed much closer in and then migrated outward, triggering a bombardment of protoplanets and comets from the outer Solar System. The Grand Tack model goes further, suggesting that Jupiter first moved inward and then migrated out again.
These models go some way in explaining Mars and the asteroid belt, but they hit a major problem when we try to apply them to other planetary systems because they rely heavily on orbital resonance. This is where one planet makes exactly two orbits for every one of its neighbour, or some other neat ratio. These orbital resonances are common in our Solar System because they are energetically stable, but as soon as we look at other stars, they are nowhere to be found. “The vast majority of the Kepler planets are not in mean motion resonances,” says Weiss. “Understanding why has been one of the major unsolved problems in planet formation theory over the past few years.”
Exoplanet orbits aren’t random according to Dr Weiss. Their orbits show regular-spacing patterns that are correlated with the planets’ sizes, but whatever rule they use to determine their position, it doesn’t involve orbital resonance. Something about the initial conditions or the movements of our giant planets has set the stage differently. “Our Solar System is a bit of a weirdo,” says Weiss.
There’s a big gap between odd and unique, though, and some astronomers are not convinced we’re all that special. "I would be very surprised if the Solar System were really strange," says Jack Lissauer, a planetary scientist at NASA Ames Research Center in California. "There are so many stars out there. Even if it's only one per cent, it's still not really rare."
When the Transiting Exoplanet Survey Satellite (TESS) begins its two-year survey of the entire sky this year, it will cover 400 times as much area as Kepler and look for planets around more than 200,000 stars, but even TESS won’t be able to see a Earth-sized planet in an Earth orbit around a star like our Sun. Predicting how rare Earth really is will still rely on scientists fully understanding how planets form and evolve.