Alien worlds keep turning up that break the rules we’ve learned from our own solar system. Are we the real renegades?
JOHN JOHNSON remembers the night he found his first blue lion. He was observing at the on top of in Hawaii. Earlier that evening, before he had driven up the rubble road to the volcanic summit, his collaborator Josh Winn had phoned from Massachusetts and joked, as he always did: “Tonight’s the night we find a retrograde planet.”
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A retrograde, or backwards, planet is one that orbits its parent star in the opposite sense to the star’s rotation. No planet in our solar system does this, and few astronomers seriously expected to find one around another star. “It’s like going on a safari and stumbling across a blue lion,” says , who is at the California Institute of Technology in Pasadena.
Yet ever since the first extrasolar planet was discovered in 1995, nature appears to have supplied us with green zebras, orange rhinos and purple buffalos. For a start, there are the hot Jupiters, gas-giant planets a hundred times closer to their parent star than Jupiter is to the sun. Then there are bloated planets far too big for their masses; planets more massive than Earth but less than Neptune, which are conspicuously absent in our solar system; and planets in orbits so elongated that they are more reminiscent of comets. “No one ever guessed that when we discovered other solar systems they would be so extraordinarily different from our own,” says Johnson.
Planets are the latest celestial bodies to be spotted travelling in reverse. A small number of asteroids, Neptune’s moon Triton and even certain types of galaxy disobey the rules (see “Backwards moon” and “Backwards galaxies”). In fact there is so much weirdness out there that we are now realising that planets and other bodies could have a far more complex life than we once thought. What we know about planets from our own solar system is likely to be just the tip of the iceberg, and backwards planets are giving us a glimpse of how much more there is to discover. We might even find out that rocky little Earth, with its stable existence in a habitable zone, is in fact the oddity.
The story of backwards planets began in 2005. Mindful of the surprising nature of extrasolar planetary systems, Johnson, then at the University of California, Berkeley, and Winn, at the Massachusetts Institute of Technology, homed in on a fundamental property of these systems that had not yet been measured and might conceivably yield new findings: the angle between a planet’s orbital plane and its star’s equator.
Topsy-turvy orbits
In the solar system, this angle averages only about 7 degrees – that is, all the planets orbit in the same sense as the sun’s rotation and pretty much in the same plane as the solar equator. This is generally thought to be a direct consequence of the solar system being formed from a spinning cloud of gas and dust. This contracted under its own gravity, becoming a flattened, spinning disc. “Since the sun and the planets congealed from disc material swirling around in the same direction, the planets have naturally ended up orbiting in the same plane as the sun’s equator and in the same direction it is spinning,” says Johnson.
But is that the whole story? “Given that extrasolar planets have continually confounded us, we were hopeful we might find something interesting,” says Johnson.
So much for hope. Having examined the orbits of nine such planets and found eight to be perfectly aligned with the plane of their solar equator, and one just teensily misaligned, Johnson and Winn realised that patience for their project was wearing thin. By 2008, they were refused more observing time on the Keck telescope they had been using on Mauna Kea.
Undaunted, they successfully applied for two nights on the neighbouring Subaru telescope. But Johnson and Winn were not the only ones looking for extrasolar planets with unusual orbits. Astronomers from a consortium of UK universities called the were also on the case. They had found a planet, christened WASP-14, whose orbit was misaligned by 70 degrees with its star’s equator. “It was not retrograde but it was a strong hint that there were unusual planets out there,” says Johnson.
On his first night at Subaru, Johnson measured the spin alignment of WASP-14 by observing subtle changes in light from the star. As a star rotates, one side of it recedes and light from that side is red-shifted to longer wavelengths. Conversely, light from the portion of the star approaching us is blue-shifted (see diagram). Things change subtly when a planet transits or crosses in front of the star and blots out part of its light. Comparing when the red and blue-shifted light dims tells you whether the planet orbits in the same or opposite direction as its star.
Using this technique, Johnson revised the WASP team’s estimate of 70 degrees down to 30 degrees. But it was still big and unusual.
Smash or grab
On the second night, he observed a planet called HAT-P-7, but did not examine the measurements closely until hours later. When he did, his jaw dropped. Its misalignment was so huge it had to be a retrograde planet. He had found a blue lion.
Knowing how heretical the discovery would be, Johnson kept the result quiet while he checked and rechecked his measurements. “In science, your only currency is your reliability,” says Johnson. “You really cannot afford to use it up.”
So he beavered away for three weeks, excluding all possible mundane explanations. Finally, there was no doubt about it: the planet was going around its star the wrong way. “Only then did we start popping the champagne corks,” says Johnson.
The pair quickly wrote a paper and submitted it to the journal Science. Incredibly, they were told they had been pipped to the post by the WASP team, led by David Anderson at Keele University in the UK. The WASP team also claimed the discovery of a retrograde planet, which they named WASP-17b. “It was a marginal detection, but such is life,” says Johnson. “We were second. We submitted our paper to The Astrophysical Journal Letters instead” ().
HAT-P-7 is a Jupiter-mass planet in a circular orbit around a star about 300 light years away. Now, amazingly, six backwards planets are known, five of which have been found by the WASP consortium. “Retrograde and misaligned planets are utterly shocking,” says , an astronomer at the University of California, Berkeley, who has discovered more extrasolar planets than anyone else. “I still can barely believe them. Their topsy-turvy orbits seem to defy everything I learned about solar systems, including our own.”
“Retrograde planets are utterly shocking. They defy everything we know about solar systems”
The big question is: how does a planet get to be that way? One theory is that as planets form in their protoplanetary disc, small bodies and debris get sent flying everywhere. In the chaos this creates, an almighty collision could occur between three bodies, with one being hit so hard that it reverses its orbital direction.
Such three-body scattering, albeit on a smaller scale, is believed to have knocked Neptune’s giant moon Triton into its retrograde orbit. “It is certainly possible that some of the retrograde planets were captured in three-body gravitational encounters,” says Craig Agnor of Queen Mary, University of London, who came up with the explanation of Triton’s weird orbit ().
Another possibility is that backwards planets were captured from the planetary entourage of another star that came too close. Or perhaps it was the star itself that flipped – so the star is backwards not the planet. According to a team led Dong Lai of Cornell University in Ithaca, New York, such a flip could be caused by the interaction between the star’s magnetic field and the magnetic field of the swirling protoplanetary disc ().
Johnson favours another mechanism, proposed by Japanese astronomer Yoshihide Kozai in 1962 to explain why some asteroids have orbits that are highly inclined relative to the plane of the planets. According to Kozai’s prescription, gravitational interactions with planets and other bodies can tilt the orbital plane of a planet so much that it ends up going backwards; that is, the angle of its orbital plane changes by more than 90 degrees.
Recently, Johnson’s student Tim Morton has studied the spin-orbit angles for a sample of 32 misaligned planets to see whether the Kozai mechanism can explain the distribution (). He finds that it cannot on its own – though it does seem to work when you combine it with collisions between planet.
Whatever the true story, it is an extraordinary one, as is the very existence of backwards planets. They show that there is far more going on than our simple model of the formation of the solar system would suggest.
So far we have found over 500 planets orbiting stars outside our solar system. Yet we may not even have a representative sample. “Our detection techniques tend to find very massive planet in close-in orbits, so we’re not sure yet how this is skewing our overall picture,” says Johnson. Who knows what strange planets might turn up over the next few years? We may yet find out that our solar system is the oddball.
Marcy, however, is already convinced. “Our solar system is unusual,” he says. “Its rocky planets survived in the habitable zone and their orbits have remained nearly circular even though large planets may have crossed their path. We may find that worlds like our own – Earth-sized, rocky planets in circular orbits – are actually quite rare.”
Backwards moon
Neptune is the farthest planet from the sun and home to the only backwards moon of substantial size in the solar system. While Neptune spins one way on its axis, its largest moon, Triton, circles the planet the other way.
Large moons form out of the debris swirling around a newborn planet, so they should circle in the same direction as the planet’s rotation. The fact that Triton, whose diameter is one-fifth that of the Earth, does not circle in this way suggests that its origins lie elsewhere.
The obvious place is the Kuiper belt, a distant band of icy rubble left over from the birth of the planets. Growing up on the edge of the Kuiper belt, the infant Neptune could easily have encountered Kuiper belt objects and snared one for its moon. The trouble is that a body as massive as Triton could be captured only if it was moving improbably slowly.
There is a way around this, say Craig Agnor of Queen Mary, University of London, and Douglas Hamilton of the University of California, Santa Cruz. Triton had a companion. Agnor and Hamilton’s simulations show that, in a three-body encounter with Neptune, Triton could have lost speed at the expense of its companion, which was ejected ().
Such an encounter is not unimaginable, and many Kuiper belt objects are known to be binaries. The missing culprit could be Pluto. It is about the same size as Triton and its orbit crosses Neptune’s. Conceivably, Pluto and Triton are brothers.
Backwards galaxies
Backwards motion may be behind the most energetic objects in the universe. Every galaxy is thought to harbour a spinning supermassive black hole which sucks in gas to form a disc of super-hot matter whirling around it. But in some galaxies, known as active galaxies, gas in this accretion disc is so hot that the core can be hundreds of times brighter than all its stars put together. Some can even project their power across millions of light years along thin jets stabbing out into the intergalactic medium.
No one knows for sure what causes the jets, but they are believed to be driven by twisted magnetic fields in the accretion disc. Herein lies the connection with backwards motion.
In February, a team led by at the Jet Propulsion Laboratory in Pasadena, California, published the results of a study that modelled active galaxies. Their calculations showed that active galaxies with jets had accretion discs turning in the opposite sense to their black holes, whereas in jet-less galaxies they were spinning in the same direction ().
The researchers speculate that the large change in angular momentum necessary for in-swirling matter to get close to an opposite-spinning black hole makes such an encounter almost impossible. This leaves a gap between the inner edge of the accretion disc and the black hole for magnetic fields to “pile up”, leading to a powerful magnetic catapult to launch the jets.
But how do accretion discs get to be spinning backwards in the first place? According to a team led by Chris Nixon at the University of Leicester, UK, the gas flowing from a galaxy into its central black hole gets random kicks from supernovae and the stellar winds associated with star formation. This means it is as likely to be rotating against the black hole as with it. In fact, the team believes that a retrograde accretion disc may be essential for galaxies to merge ().
