Editorial: 鈥�Citizen scientists, name that planet鈥�
They might be in their twilight years, but white dwarf stars are the perfect places to look for habitable planets
WELCOME to Procyon B, a nearby star that鈥檚 light years away from the sun, and not only in distance terms. Unlike the healthy star we circle, Procyon B is dim and dying. Having thrown off its outer layers, it is puny compared with the sun. And it is so dense that were you able to scoop up a spoonful of its material, it would weighs tonnes. So unlike our sun is Procyon B, in fact, that those seeking extraterrestrial life have long overlooked the star鈥檚 potential.
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But we may have been too hasty, according to , an astronomer at the University of Washington in Seattle. Though dim and diminutive, Procyon B and other white dwarf stars like it could host planets sporting mild temperatures, oceans and living critters.
With billions of white dwarfs in our galaxy alone, Agol鈥檚 work opens up a new frontier in the search for intelligent life. What鈥檚 more, even small telescopes could search the vicinity of such dying stars for potentially life-bearing worlds. 鈥淭his is something an amateur astronomer could do,鈥� says Agol.
So far, the search for alien life has focused on planets orbiting stars similar in size, temperature and brightness to the sun. That makes sense: the universe鈥檚 one known life-bearing planet orbits such a star. Since 2009, NASA鈥檚 spacecraft has been scrutinising sun-like stars in its quest for Earth鈥檚 twin. Among the 1235 candidate planets Kepler has found, 54 orbit their stars at just the right distance for water to exist in liquid form. Of those, only 5 are similar in size to the Earth, suggesting that they are rocky worlds rather than gas or ice giants, and Agol began to wonder if there was an easier way to detect small, Earth-sized planets.
Dying stars hardly evoke visions of life. White dwarfs are what most stars, including the sun one day, become after they run out of nuclear fuel. They owe their glow to leftover heat. The youngest are so hot they shine blue. As they cool, they turn white, then yellow and orange, and there they linger at a temperature similar to the sun鈥檚.
The sun has been able to nurture terrestrial life because it has been stable for billions of years. A cool white dwarf is just as stable. So if any putative planets huddle close, why can鈥檛 they develop life the way Earth did?
鈥淢y first thought was, 鈥榯hat鈥檚 crazy鈥�,鈥� says . 鈥淭hen the more I thought about it, the more sense it made.鈥�
Habitable huddle
One reason it seems so crazy is the way white dwarfs are born. After a sun-like star has burned the hydrogen fuel at its core, it balloons into a monstrous red giant more than 100 times wider than the sun. It engulfs and destroys any nearby planets, a fate that will befall Mercury and possibly Venus and Earth in about 7 billion years. Within another billion years the red giant sheds its outer layers of gas to form a glowing planetary nebula. The exposed hot, dense core is a white dwarf.
It would seem impossible for any close-in planet to survive such fireworks, but astronomers were stunned 20 years ago to discover planets around a different type of dead star known as a pulsar. Pulsars have gone through much greater drama than any white dwarf: they are born in supernova explosions. The planets probably condense from a disc of gas and dust that encircles the pulsar after it has torn a companion star to shreds.
Other clues to the possible origins of dead-star planets come from the . Many of these stars have Jupiter-sized planets orbiting close by, but these could only have formed far from the star where there is sufficient ice and rubble. Their near orbits suggest that they must have migrated or been kicked in somehow from further out. Perhaps the same processes work around a white dwarf, shepherding frigid planets towards the warmth of their dim sun. Or planets might form anew after the white dwarf has formed if an orbiting disc of gas and dust exists.
Despite their dramatic birth, white dwarfs usually exist in peace, just cooling off. Agol鈥檚 calculations show that there is a period of billions of years when a planet in the right orbit around a cool white dwarf could be habitable ().
Because cool white dwarfs are only about a ten-thousandth as bright as the sun, to stay warm a planet must huddle at one-hundredth of the distance between the Earth and the sun. Any habitable planets would complete their orbits every 4 to 32 hours, says Agol. If it were any shorter, the planet would be so close that the pull of the star鈥檚 gravity would tear it apart. At longer periods, the planet is so far out that it would freeze.
No one has ever found a planet around a white dwarf. Yet astronomers are enthusiastic about Agol鈥檚 idea because the diminutive size of white dwarfs mean such planets should be easy to find. When an Earth-sized planet passes in front of a star as large as the sun, it blocks so little of its light that only an expensive mission, such as Kepler, can detect the eclipse. But white dwarfs are as small as Earth, so a passing planet may well block all the star鈥檚 light from reaching us. The planet鈥檚 presence would look like a total solar eclipse every day, through any backyard telescope that could see the white dwarf. 鈥淭here are enough amateurs out there with telescopes that could be perfect for this sort of project,鈥� says Agol.
With billions of white dwarfs in the Milky Way, there are plenty of places to look. The nearest one, Sirius B, is probably too hot to have habitable planets and Procyon B, 11.4 light years away, is too close to a bright star for most amateur telescopes to detect it. But other white dwarfs fit the bill: Van Maanen鈥檚 star is just 14 light years from Earth and is bright enough for a small telescope to observe.
White dwarfs further away are dimmer 鈥� and that means larger telescopes are needed. Agol says telescopes with mirrors between half-a-metre and a metre across are ideal. That鈥檚 too big for most amateurs to have at home, but many keen star-gazers belong to astronomy clubs with access to such telescopes.
Daily eclipses
The more people who look, the better. Even if close-in white-dwarf planets abound, only about 1 in 100 is going to be situated so that it can stop starlight from reaching Earth. Agol therefore wants to search thousands of white dwarfs. 鈥淚鈥檝e thought about trying to coordinate amateur astronomers around the world,鈥� he says.
鈥淓ven amateur astronomers could search these dying stars for life-bearing worlds鈥�
Even so, it could be a long slog. With the nearest and brightest white dwarfs scattered across the sky, a single telescope usually monitors just one at a time. Still, , an astronomer at the University of Hawaii in Honolulu, hopes to build an array of eight 25-centimetre telescopes called that will scan the entire visible sky twice a night, principally to give early warning of an asteroid strike. 鈥淎tlas will get a look at 10,000 or 20,000 white dwarfs every night,鈥� he says. In the best possible scenario, in which every white dwarf has a planet, Atlas should start seeing eclipses within a week or two. Tonry says the cost of his proposed project is relatively modest, at $3 million.
Far more ambitious and expensive is the Survey Telescope, an 8-metre telescope to be built in Chile later in the decade. Designed to observe most of the sky, it will also yield evidence of close-in white-dwarf planets, if they exist.
However they are found, planet-induced eclipses of white dwarfs will be brief, lasting just a couple of minutes. Confirming the presence of a planet will be easy because it goes around its star in a matter of hours. In contrast, eclipsing planets like Earth, with one-year periods, take much longer to confirm.
If Agol is right, we have neglected a type of star that may support life throughout the galaxy. And we may not be alone in our folly. Any extraterrestrial astronomers living on a white dwarf planet looking for signs of life in the universe are likely to be examining other white dwarfs. By doing so, they鈥檒l be missing the sun and the Earth. But who can blame them? After all, detecting little planets around big stars like the sun is hard. It鈥檚 a lot easier to catch such a world orbiting a white dwarf.
Aliens of the twilight zone
Life on a planet orbiting a tiny white dwarf would be quite different from life on Earth. Since any life-bearing planet in a white-dwarf system would orbit close to its star, the gravitational forces would be immense. 鈥淭hey would cause the planet to rotate at the same rate it orbits the star,鈥� says astronomer Eric Agol at the University of Washington in Seattle. That means the same side of the planet would always face the star: the sun would never set on the hot, day side, whereas the night side would be freezing and forever in darkness.
The star鈥檚 pull would also keep the planet upright on its axis and its orbit circular, so there would be no seasons. There would be no moon because the star鈥檚 gravity would have torn any satellites away. A year would last only about as long as Earth鈥檚 day, so you鈥檇 need some other unit to reckon lengthy periods of time.
Not everything would differ from Earth, though. The white dwarf sun would look much like our own, even down to its colour. And though a white dwarf is about one-hundredth the size of our sun, it would appear the same size as ours in the sky because it would be 100 times closer.
鈥淚f the air had the same composition as Earth鈥檚 air, then the sky would look blue, just like our skies,鈥� says Agol. 鈥淏ut if you lived on the day-night boundary, you鈥檇 see a permanent sunset.鈥�
For aliens living in the twilight zone, that would mean a perpetually red sun.
