Astronomers are being forced to rewrite their theories of planet formation
Washington DC-based Nasa and others are struggling to explain them
Previously it was thought our solar system was a model of other systems
But the discovery of bizarre planets in odd orbits has challenged theories
Some huge planets are in tight orbits that defy our current laws of planets
Lots of other systems also have super-Earths, but our system has none
The discovery of these new planets is leading astronomers to change tack
A few decades ago astronomers were pretty confident they knew how planets formed.
Based on our own solar system they thought small, rocky planets formed near their host star and larger, gaseous or icy planets formed further out.
So when they started finding planets that didn’t conform to any of these theories they were confused: were their prevailing theories of planet formation wrong?
Theories of planet formation are being tested as astronomers find bizarre new worlds in the universe. This artist's illustration shows the planet designated HD 209458b orbiting close to its host star, despite being the size of Jupiter. Known as a 'hot Jupiter', this is a type of planet that was once thought not possible to exist
For example, some planets have been found so close to their star that they orbit in just days - but density studies indicate these planets are somehow made of ice.
Other rocky planets have been found so big that they have led astronomers to question exactly how planets can form.
The current model of planet formation is that they are born out of the dust and gas that also creates the star at the core of a planetary system, known as the core-accretion process.
As the central star rotates it spins the surrounding material and heats it.
Other time this material clumps together, with materials with high melting points like iron and rock forming nearer the sun.
Further out in the system it is colder, allowing ice to form, while planets also accumulate some of the gas in their vicinity, becoming ‘gas giants’ like Jupiter and Saturn in our own.
Why, then, have we found systems where there are gas giants in orbit a tenth the distance of Mercury in our own solar system (known as 'hot Jupiters')?
Why do some planetary systems have giant ‘super-Earths’, huge rocky planets devoid of a gaseous exterior, orbiting in their extremes?
And why, too, do some planets orbit in wildly elliptical orbits rather than in a flat ‘plane’ like those in our solar system?
The answer: we just don’t know.
Although the number of planets we know of has increased since then, it shows how many different types of planets we found, many of which are not even planets that exist in our own solar system
It's possible that, in some systems, planets are knocked into wild orbits by others, or they are captured by a star as it passes.
The process of planet formation itself might, too, be more chaotic than we once thought.
‘The first detections of exoplanets revealed bodies which are utterly unlike any solar system planet,’ says Nasa, ‘and subsequent discoveries have shown that many exoplanet systems are very dissimilar from ours.
‘In some exosystems, planets as massive as Jupiter orbit so close to their star that they are heated to high temperature and their upper atmospheres are swept into space.
‘In other systems, planets follow elongated orbits (in contrast to the nearly circular orbits of the solar system).’
However, this is not necessarily a bad thing.
Finding planets that don’t conform to our theories merely means that we haven’t quite nailed down how planet formation works yet.
It may even be that our solar system is fairly unique when compared to other planetary systems.
Another mystery is the abundance of super-Earths (illustration of Kepler-62f shown) elsewhere in the universe. Why do other planetary systems host at least one of these giant rocky planets, whereas our own solar system has none? Astronomers will be hoping to answer this question in future with new theories
‘Out studies of exoplanets are just beginning, and it is not possible to be sure what will prove to be “typical” planets among our neighbouring stars,’ says Nasa.
‘Will most planet systems prove to be much like our own, or are we exceptional in more ways than we can imagine? Only years of further study will tell.’
That is not to say there are not exoplanet systems like our own, though; the star 55 Cancri, 41 light years away, has a system of five planets with a similar distribution to our own.
But it may be that our theories for how these planets formed in the first place, and what sort of systems they inhabit, may need to be revised.
‘Perhaps the most interesting question, and one of the most difficult to answer, concerns the uniqueness of the Earth,’ concludes Nasa.
‘Are there planets similar to the Earth around other stars and does life exist on any other planet beyond our own Earth?’
By 2000, astronomers had found 30 exoplanets; by the end of 2008, 330. Then NASA launched Kepler, which spent the next four years searching for exoplanets in a single patch of sky containing some 150,000 Sun-like stars. Kepler identifies planets by detecting the slight dimming in a star's light that occurs when an object passes in front of it. This 'transit' method can find planets much smaller than the radial-velocity technique can, giving astronomers a chance to detect other Earths. Kepler has now found 974 exoplanets, with 4,254 further candidates waiting for confirmation by ground-based measurements. If all of Kepler's candidates are confirmed — and they do tend to be — then the techniques taken together will have found well over 5,000 exoplanets.
Kepler's planets run in odd systems. The Kepler-56 system, for example, has two planets, of 22 and 181 Earth masses, both orbiting at 45° to the star's plane. In the Kepler-47 system, two planets both orbit a binary star. Kepler-36's planets are closer together than any others yet seen: they orbit the star every 14 days and 16 days, respectively. One is rocky and is eight times as dense as the other, which is ice. “How did they get so close together?” wonders Richardson. “And how are they so different?” Kepler-11 is orbited by six planets, five of which are among the smallest and least massive ever found. Their densities, says David Charbonneau of the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, “are shockingly low, they must be mostly ice or have significant gas envelopes” — yet all five are tucked in together within 0.25 AU of their star.
Not like the others
Kepler's biggest surprise has come from statistical summaries of its findings. The planets seen so far can be said to fall into three categories: hot Jupiters; giant planets with idiosyncratic orbits; and super-Earths. Worlds in this third category are generally found in compact systems of two to four planets each, orbiting their stars at distances from 0.006 to 1 AU in periods ranging from more than 100 days down to hours. Although there are no super-Earths in our Solar System, they orbit at least 40% of all nearby Sun-like stars, which makes them the most common type of planet found. “The hot Jupiters are freaks, less than 1%,” says Joshua Winn, a physicist who studies exoplanets at MIT. “The long-period eccentric giants are maybe 10%. The 40% — that makes you wonder.”
The question is how to account for all this planetary-system diversity. In general, astronomers begin with the standard core-accretion theory then add in processes that probably did not play out in our own Solar System.
To explain hot Jupiters, for example, they suggest2 that the planets did not stick around at their birth place in the cold outer reaches of stellar disks. Instead, the infant giants spiralled inwards as viscous gas in the disk slowed their orbits. At some point, for reasons unknown, they stopped their death spirals and settled into stable orbits close to their stars. Despite the extreme temperatures, the giant planets had strong-enough gravity to keep hold of their gas.
Eccentric giants could be the result of gravitational interaction3. If several giant planets started to migrate, they might have passed one another closely enough for their gravity to sling them in crazy new directions. They could have scattered out of alignment with the rest of the system, got knocked into orbits opposite to the star's rotation or even been flung from the system entirely.
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Super-Earths are harder to account for. For one thing, the term has no agreed definition, says Winn: some of the smallest, closest-in planets might actually be the stripped cores of migrating giants that came too close to their stars and got their gas blown off. “Super-Earths are probably not nice, stereotypical birds,” says Eric Ford, an astrophysicist at the Pennsylvania State University in University Park. “Maybe some are more like penguins.”
The sheer size of the super-Earth flock requires explanation. The standard theory cannot do that because in existing models, the central regions of stellar disks contain much too little material to create several close-in super-Earths. But theorists have found ways around that problem. Laughlin and Eugene Chiang, an astronomer at the University of California, Berkeley, have shown4 that compact systems of super-Earths can grow from disks with much greater masses, distributed closer to their stars. Murray and Brad Hansen, an astrophysicist at the University of California, Los Angeles, have also proposed5 a more massive disk, but one in which super-Earths are born from planetesimals that formed farther out in the disk, then migrated in before they collected into planets.
Astronomer Douglas Lin of the University of California, Santa Cruz, and his colleagues have tried to merge all the categories of planet into what Winn calls “an all-singing, all-dancing model” that can account for all the systems seen6. It starts by assuming that the distribution of mass in the disk will vary from system to system. After that, says Lin, it's “migration, migration, migration”: all types of planet grow to full size in the middle to outer part of the disk, and then move inwards in order.
Such models are appealing, but the concept of migration, especially of the smaller planets, gives some researchers pause — if only because no one has ever seen it happening. The necessary observations may not be possible: stars young enough to have planets migrating through protoplanetary disks are still surrounded by dust, and their light flickers, making it extremely unlikely that current methods will be able to pick out the dimming caused by a transiting planet. The theory is not settled, either. Modellers have found it hard to explain why migrating planets, big or small, would stop in the orbits that astronomers have observed. In simulations, says Winn, they don't: “the planets plop right down on the star”.
Perhaps the biggest question is why our Solar System is so different. Why doesn't it contain the one kind of planet most common around other Sun-like stars? Why are there no planets inside Mercury's orbit when that's where most of the exoplanets are in other systems? Why do we have a balance of large and small planets when most other systems seem to choose one or the other but not both?
Astronomers still don't know how different we are. Observations of exoplanets are seriously biased: neither of the two main techniques would find our widely spread-out Solar System, nor are they sensitive to systems with both large and small planets. It might be that we are not unusual at all.
Future observations may give some answers. Kepler has been hobbled by a failure of the mechanisms that keep it pointing at its original target patch of sky, but last month it was approved to keep taking data. The longer it does so, the larger the exoplanet orbits it will be able to see. Ground-based programmes are starting to operate with improved instruments, some also capable of seeing planets 5 AU or more from their stars. And from 2017, NASA's planned Transiting Exoplanet Survey Satellite (TESS) will look for planetary transits across all the bright stars in the sky. The wider range of possible exoplanet candidates makes it more likely that astronomers will spot a Solar System like ours — if one exists.
Meanwhile, researchers continue to nurture their mess of models, which have grown almost as exotic and plentiful as the planets they seek to explain. And if the current theories are disjointed, ad hoc and no longer beautiful, that is often how science proceeds, notes Murray. “Life,” he says, “is like that.”