THERE was a time when Stéphane Udry and his fellow astronomers thought they understood planets. So when they began to find alien worlds circling stars outside our solar system in the 1990s, they presumed they would see clones of the Earth and its siblings emerging from the mist: chilly Jupiters and icy Neptunes far out from a star, with hot, rocky Mercuries closer in.
The planets didn’t play ball, however. Sure enough, there were giants as large as Jupiter and bigger. But instead of orbiting sedately, hundreds of millions of kilometres from their stars, the first crop were frantically whirling round in close orbits, blistering in hellish heat. Small planets turned up too, but not orbiting sun-like stars: they were hostage to a neutron star – a tiny stellar corpse far smaller than Pluto.
Each time, it left astronomers wondering: who ordered that? “The study of the solar system was an old science and we thought that we understood everything,” says Udry, of the Geneva Observatory in Switzerland. “Now we have learned that anything is possible.”
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With around 250 exoplanets now catalogued, a new era of exploration has begun. Not just notching up new planet discoveries, astronomers are starting to explore the planets’ environments. They are finding water in their atmospheres, measuring shockingly hot temperatures on some and hints of roaring winds on others.
In June, on the Greek island of Santorini to mull over the most bizarre solar systems they’ve found to date. Here are some of the intriguing worlds that will keep them busy for decades to come.
Warm and windy nights
In May this year astronomers announced the . Although fairly crude, it hints that some pretty wild weather is blowing on a giant Jupiter-like planet.
HD 189733b in 2005. Seen from Earth, it transits its star, passing in front of it and then behind it on each 2.2-day-long orbit. Like most planets that are very close to their stars, HD 189733b has one side always facing its star in perpetual daytime while the other is permanently dark. The cooler, night side of HD 189733b faces us when the planet is in front of its star and the hotter day side rotates into view just before the planet disappears behind its host.
To map the temperatures around the gas giant, Heather Knutson from Harvard University and her team used the Spitzer Space Telescope to observe it continuously for 33 hours. The changes in the infrared brightness of the system as the planet moved around its orbit revealed temperatures of the planet’s cloud-tops around the globe. It’s a sweltering 940 °C on the day side, and even the permanently dark side is a surprisingly hot 700 °C. Knutson’s team think that is because powerful winds whip round the globe, spreading out the heat.
Computer models suggest the wind speed must be more than 10,000 kilometres per hour – about eight times the speed of sound at sea level on Earth. “That’s definitely at the upper end of the range we thought was plausible,” says Knutson, “and far faster than a jet plane would fly.”
The windy world notched up a second claim to fame in July. Astronomers reported that Spitzer observations of HD 189733b made the first compelling case for .
Planet mass: 1.15 Jupiters
Planet orbit: 2.2 days
Distance from Earth: 63 light years
Star mass: 0.82 suns
Constellation: Vulpecula
Biggest planet on record
A mystery malaise affects many extrasolar planets – a kind of rampant obesity. Earlier this year, astronomers found the most bloated world on record, a planet less massive than Jupiter but with a volume five times bigger. With an average density less than that of a wine cork, it would float on water like a beach ball.
A team led by Georgi Mandushev from Lowell Observatory discovered the puffed-up planet in a project called the (TrES), which uses a network of small automated telescopes in the Canary Islands, Arizona and California. They look for the slight dimming of stars when planets pass in front of them, blocking out some starlight.
The periodic dimming of a star 1400 light years away , which orbits the star every three-and-a-half days. Follow-up observations using large telescopes in Hawaii and Arizona showed that the planet has just 0.84 times the mass of Jupiter. However, the amount of dimming makes sense only if the planet is about 1.7 times as wide as Jupiter.
Astronomers calculate that its average density is only about 0.2 grams per cubic centimetre – about the same as balsa wood. “It’s way lower than the density of water,” says Mandushev. “Because of the planet’s relatively weak pull on its upper atmosphere, some of the atmosphere probably escapes in a comet-like tail.”
TrES-4 is the least dense planet found to date, but several other puffed-up planets are known. Why they are so bloated is unclear. Part of the explanation is that they are very close to their parent stars and are therefore blazing hot. TrES-4, for instance, is only about 7.2 million kilometres from its star and its surface temperature is about 1300 °C.
Even accounting for the heat, TrES-4 is still far larger than theory predicts. “We just cannot explain why it’s so puffed up,” says Mandushev.
One possibility is that internal heat inflates this type of planet. Somehow their hydrogen and helium may separate, with the heavier helium sinking into the core, and this contraction could release gravitational energy as heat. However, it is not clear why this would happen to some giant planets and not others.
Mandushev adds that TrES-4’s days are numbered. Its parent star has run out of hydrogen fuel in its core and has begun to grow into a red giant. In less than a billion years, the swelling star will engulf the planet, dooming it to a fiery end.
Planet mass: 0.84 Jupiters
Planet orbit: 3.55 days
Distance from Earth: 1400 light years
Star mass: 1.22 suns
Constellation: Hercules
Crazy comet
Here’s the perfect destination for roller-coaster lovers who know no fear. circles its star every four months – except “circles” isn’t really the right word. This giant planet, almost four times as massive as Jupiter, swings wildly through its planetary system like a crazy, overgrown comet.
Its orbit, almost as elongated as that of Halley’s comet, takes it out to nearly the Earth-sun distance, then back in to just 3 per cent of that distance, perilously close to its star. HD 80606b is a gas giant planet, so there’s no surface you could stand on, but if you could float in its atmosphere as the planet hurtled inwards you would see the disc of the star zoom from the size of our sun to a whopping 900 times as big in just a few days.
“That’s just frightening,” says Udry, whose team discovered the planet in 2001. He estimates that the temperature also rockets from about 100 °C to 1700 °C as the planet rushes towards its star, and it may even sprout a giant comet-like tail.
HD 80606b has the most stretched-out orbit of any planet known. In this case the cause is the gravitational influence of a distant companion star, but astronomers have been perplexed to find that “extrasolar” worlds in general have much more elongated orbits on average than the planets in our solar system. That has come as a big surprise, because they assumed most planets would have nearly circular orbits like that of Earth.
“We thought that because planets form in discs, their orbits should be circular,” says Udry. “But nature is always more surprising than we expect, and that’s good news for us.” HD 80606b’s next sizzling plunge towards its star will be in November, and NASA’s infrared has the date in its diary to watch the fireworks.
Planet mass: 3.9 Jupiters
Planet orbit: 112 days
Distance from Earth: 190 light years
Star mass: 0.9 suns
Constellation: Ursa Major
Some like it hot
One planet has surprised astronomers time and again. Discovered in 2005, has a curiously dense, heavy core. The latest news, however, is that the gas giant’s atmosphere is unbelievably hot – hotter than the surface of some stars.
HD 149026b appears to be slightly smaller than Saturn, yet much more dense, its core packed with about 70 Earth masses of heavy elements. “It sticks out like a sore thumb from all of the others in terms of its core mass and radius – it’s very peculiar,” says Travis Barman, who models planetary atmospheres at Lowell Observatory in Flagstaff, Arizona.
It looked stranger still in May this year when Spitzer observations revealed that the temperature of HD 149026b’s permanent day side is about 2000 °C, making it the . “This planet is off the temperature scale that we expect,” says team member Drake Deming from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Why so hot? The main reason is that it is close to its star, and the planet’s surface is probably very dull-coloured, so it absorbs plenty of light. If the atmosphere is also crammed with dark dust, that would help to keep the heat in. Poor spreading of heat from the day to the night side could be a factor, too. “The night side is probably colder than the day side by a sizeable amount,” says Barman.
Astronomers are not yet sure what to make of these measurements. Why might wind distribute heat efficiently on some planets and not others? Further scheduled Spitzer observations of HD 149026b at a variety of different wavelengths should shed light on this in the coming months. “In another year we’ll know a lot more about this planet,” says Deming.
Planet mass: 0.36 Jupiters
Planet orbit: 2.88 days
Distance from Earth: 256 light years
Star mass: 1.3 suns
Constellation: Hercules
The Goldilocks planet
In April, astronomers announced a new planet that was unusual in that it wasn’t. It is the closest analogue to Earth yet discovered outside our solar system. It is possibly a habitable, rocky world that enjoys balmy temperatures of 20 °C, typical of New York in June.
Udry’s team discovered the “super-Earth” using the 3.6-metre telescope at the European Southern Observatory, La Silla, Chile. “It’s the smallest, lightest planet known at this time,” Udry said earlier this year. “And it’s just at the right distance from its star for liquid water to possibly exist on its surface.”
Udry’s team had been monitoring a dim red dwarf star called Gliese 581, which was already known to have a Neptune-like gas giant in orbit. Subtle wobbles of the star suggested it also had two much lighter planets. One is about eight times as massive as the Earth and orbits once every 84 days. The other may be only five times as massive as the Earth, making it the smallest exoplanet ever found around a normal star.
The smaller planet, if rocky like the Earth, should be about 50 per cent wider. Udry’s team calculated that it has a pleasant surface temperature of about 0 °C to 40 °C. So it has been dubbed the – not too hot for life, not too cold, but just right.
The astronomers arrived at these temperatures by simply assuming that the planet reflected the same fraction of light from its parent star as the Earth or Venus. Soon afterwards, however, a team led by Werner von Bloh of the in Germany modelled the likely sources and sinks of carbon dioxide on the super-Earth. They concluded that it probably has a thick blanket of CO2 and a rampant greenhouse effect that cooks the planet into a sweltering hell far too hot for life.
There is a twist, however. Von Bloh’s team concluded that a greenhouse effect might also warm the more distant 8-Earth-mass planet, making it a cosy haven for life.
Whatever the answer, Gliese 581 has triggered lively debates about the multitude of factors that affect a planet’s habitability, among them the planet’s evolution and chemistry. “In the end, only observations can really give the answer,” says Udry.
All three planets circling Gliese 581 should be visible to the European Space Agency’s three Darwin spacecraft, due for launch in 2015. The mission will probe these worlds for signs of life. “For more than 2000 years we have been asking ourselves whether there’s life elsewhere in the universe,” says Udry. “Now we are at a special time in history where science can give us the answer.”
Planet masses: 16 Earths, 5 Earths, 8 Earths
Planet orbits: 5.4 days, 13 days, 84 days
Distance from Earth: 20.5 light years
Star mass: 0.3 suns
Constellation: Libra
Feeling the pulse
The very first planets found beyond the sun knocked astronomers off balance and remain some of the oddest to date. That’s because they circle the collapsed remains of a once magnificent star – a superdense pulsar only about 15 kilometres wide. Its planets would view the stellar corpse as a dim point-like dot that emits little visible light.
The pulsar formed when a massive star exploded in a supernova and left behind a neutron star – a dense, collapsed core. As the neutron star rotates, once every 6 milliseconds, the powerful radio beams it emits sweep across Earth, so astronomers pick up regular radio “beeps”.
Ironically, it was discovered only because a telescope was out of action for repairs. Alex Wolszczan at Cornell University in Ithaca, New York, wanted to survey a large area of sky for new pulsars, but it would have taken many hundreds of hours, and no radio telescope could spare the time. By chance, however, engineers were repairing cracks in the giant in Puerto Rico. This meant the telescope’s targeting ability was restricted, making it useless for most purposes. But it was perfect for Wolszczan’s survey, just letting the heavens drift by as the radio dish turned with the Earth. “All of a sudden, I had all the time in the world,” says Wolszczan, now at Pennsylvania State University.
During the survey, in 1990, Wolszczan discovered a pulsar called PSR B1257+12 and quickly noticed that there were curious changes in the timing of its pulses. After painstaking measurements and analysis, he deduced that three planets in eccentric orbits are making the pulsar wobble slightly, disturbing its rhythm.
It took him several years to convince everyone. “At some level, people didn’t want to believe it,” says Wolszczan. “They wanted the first discovery to be a Jupiter-like planet around a sun-like star.”
The clinching evidence came in 1994. Other scientists predicted that if Wolszczan’s planets were real, their mutual gravity should alter their orbits over time in a measurable way. Further observations by Wolszczan proved that these predictions were .
The masses of the planets are about 0.02, 4.3 and 3.9 times the Earth’s mass, while the sizes of their orbits are similar in proportion to those of Mercury, Venus and the Earth. “It’s just like a carbon copy of the inner solar system, scaled down by a factor of 2. Isn’t that amazing?” says Wolszczan.
What these planets are actually like depends on how they formed. Maybe metal-rich debris from the supernova formed an orbiting disc around the neutron star. In the extreme case that the 3.9-Earth-mass planet was made of iron, it would be roughly twice as dense as Earth and its surface gravity would be about three times what we are used to. “Standing on a planet like that would be very, very hard work,” says Wolszczan.
With a big rotating iron core, the planet would also have a very strong magnetic field. Although the pulsar wouldn’t provide much daylight, Wolszczan suspects the planet has a colourful glow. Pulsars emit dense streams of energetic particles moving at close to the speed of light, so the sky would glow with magnificent auroras. “These would probably be so bright that you could read a book by them,” says Wolszczan.
The high-energy particles would also gradually erode the planet’s surface, generating a low-level haze.
Wolszczan’s discovery of the first extrasolar planets has made him a celebrity in his native Poland. Along with Nicolaus Copernicus, Pope John Paul II and others, Wolszczan has featured on a series of celebrating the past millennium.
Planet masses: 1.6 moons, 4.3 and 3.9 Earths
Planet orbits: 25 days, 66 days, 98 days
Distance from Earth: 980 light years
Star mass: 1.35 suns
Constellation: Virgo
Future worlds
So what kind of weird exoplanets will turn up in future? There are already hints that a planet might inhabit a quadruple star system 152 million light years away in the Crater constellation. The system comprises two close pairs of stars on an elongated orbit, with concentric rings of dust around one pair – suggesting that the gap between the rings may have been cleared out by the gravity of a planet. Aliens on that world would see the spectacular sunrises of its two orange parent stars, and the two companion stars would waltz brightly through the night sky.
At the Santorini meeting, Jack Lissauer from NASA’s Ames Research Center in Moffett Field, California, wondered whether superdense planets made of lead might exist. At the opposite extreme, there may be fluffy cold hydrogen worlds, more like clouds than planets as we know them, hanging precariously together at huge distances from their stars. Maybe there are even planets circling black holes.
Inevitably, there must also be weird worlds that no one has even dreamed of yet. “It’s just amazing the poor rate of prediction that we have,” says Lissauer. “Nature really is much more imaginative than scientists.” With their poor track record for second-guessing nature, astronomers would do well to stay away from the bookies for now.
