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Land of the rising suns
Land of the rising suns What would life be like on a two-star planet? Marcus Chown explores the possibilities
40 | NewScientist | 23 August 2008
www.newscientist.com
MATT MURPHY/WWW.BLACKCOFFEEPROJECT.COM
●
IT IS one of the all-time classic scenes in science fiction cinema: Luke Skywalker looking on wistfully as the two suns of Tatooine set over the desert horizon in Star Wars. Isaac Asimov imagined an even stranger world in Nightfall, a short story published in 1941. In it a world permanently lit by six suns is suddenly plunged into darkness by a rare eclipse. Its inhabitants burn everything they can in a desperate attempt to restore light. Could such worlds really exist somewhere out there. And if they do, what might life on them be like? For a long time, most astronomers thought a multiple-star system would be extremely unlikely to produce a planet-sized object, let alone an inhabitable one. It’s all to do with how planets come into existence. Stars form when giant clouds of dust and gas compact, leaving some of the material drifting in the region after the star is born. In a single-star system like our own, this cosmic debris can collect in a disc around the star and over millions of years clump together to form planets. However, in a multiple-star system, the complex gravitational forces that arise when
two or more stars orbit each other disrupt this disc, meaning no clumping and no planets. Or so astronomers thought until a planet orbiting in a binary-star system was discovered in 1997 . Orbiting the star 55 Cancri, 55 Cancri b forced astronomers to look at the universe in a new light, and soon reports of planets in multiple-star systems were flooding in. Since then, four more planets have been found orbiting 55 Cancri. In fact, of the first 200 planets discovered outside our solar system, 42 were in multiple-star systems, usually binary systems. Most of the planets orbit a single star within a multiple system; a couple orbit two stars together (see diagram, page 42). So could any of them support life? In 2002 a planet was found in the inhabitable zone of one of the stars in a binary system. The two stars of the Gamma Cephei system are 28 to 30 astronomical units apart (one astronomical unit, or AU, is the average distance between the Earth and the sun), roughly equal to the distance between the sun and Uranus. Only two other systems are known where the stars are that close. The planet orbits the larger of the two stars – about 1.8 times as big as the
sun – at a distance of 2 AU, more or less the distance from Mars to the sun. The universe is littered with multiple-star systems, so the idea of life evolving on some of them certainly isn’t as far-fetched as previously thought. More than half the stars in our galaxy alone exist in multiple systems, a third of which are triple systems and some are even quadruple. Among these the variation is immense. At one extreme are binary systems with stars 3 light years apart – virtually the distance between the sun and the nearest star. At the other there are “contact binaries”, where two stars are touching. There are systems with stars of the same mass and others in which one star is 1000 times more massive than the other; the stars of some are in a circular orbit around each other and a few where the orbit is highly elliptical. Elisa Quintana of the SETI Institute in Mountain View, California, and her colleague Jack Lissauer from NASA’s Ames Research Center in Moffett Field, California, have been running computer simulations to find out what kind of binary systems might form planets and survive long enough that life
“The universe is littered with multiple-star systems. Some might be habitable”
www.newscientist.com
23 August 2008 | NewScientist | 41
STRANGE WORLDS Habitable planets like the Earth could exist in star systems with more than one star, in a variety of configurations Our solar system The habitable zone extends over a region that contains Earth and Mars
1 astronomical unit (AU) Defined as the average distance between the sun and the Earth EARTH
might have evolved there. By adjusting just a few variables, such as the mass of each star, proximity to the other stars in the system, their orbits and so forth, they soon found a bewildering array of possibilities. For example, if two stars each the mass of our sun are in a circular orbit around a point halfway between them, and are separated by at least 4 AUs, a planet-forming disc of debris extending out to 1 AU could surround both of them, implying inhabitable planets like Earth could form there.
SUN
1AU
The distance from a star at which water is neither permanently gaseous nor frozen. Planets orbiting in this zone may contain life as we know it
MARS
Distant binary systems When the two stars are separated by a large distance, planet-forming habitable zones can exist around each. For example, two sun-like stars would have to be separated by at least 4 AU for this to occur
The Goldilocks factor The planet-forming disc is less stable if the stars have different masses or if they are in an elliptical orbit. In the latter case, that’s because of the gravitational tug of the second star as it swings in close. Still, that doesn’t apply to all elliptical systems, Quintana and Lissauer found. The discs around two sun-like stars in an elliptical orbit 15 AU at their furthest apart and 5 AU at their closest would remain stable, for example. If two stars orbit each other too closely, a disc can’t coalesce around each star, but it could envelop both stars together, a so-called circumbinary formation, as seen with 55 Cancri b. The search for planets then needs to be narrowed down to the star’s habitable zone. This is defined as the region where liquid water, generally considered to be an essential ingredient for life, can exist on the surface of a planet or moon. The inner edge of the zone is where it is just cool enough for water to condense, the outer edge where it is just warm enough to prevent water from permanently freezing, and its depth and distance from the star depend on how much heat the star radiates. In our solar system, only Earth and Mars are in the habitable zone. Margaret Turnbull, currently at the Global Science Institute in Antigo, Wisconsin, and Jill Tarter at the SETI Institute are compiling a list of candidate stars that are most likely to have planets that are not only in the habitable zone, but have been stable for as long as Earth – long enough for intelligent life to have evolved, in other words. Using an existing catalogue of 118,218 nearby stars, they first narrowed down their selection to star systems in which a planet could have a stable orbit in the habitable zone. Then they looked for stars whose radiation level had not varied by more than 3 per cent over a period of at least 3 billion years, like the sun. 42 | NewScientist | 23 August 2008
Habitable zone
4AU or more
Circumbinary and ternary systems If two stars are very close together, any planets that form will orbit both. Again, for sun-like stars, the distance between the stars would have to be much less than 4 AU. A third planet in the system at a greater distance could also have its own habitable zone
Less than 4AU
4AU or more
So far, they have identified 17,129 stars with plausible habitable zones, of which 2200 are known or suspected to be in multiple star systems. Nineteen of the binaries have stable circumbinary habitable zones. The nearest star system to the sun and hence the easiest to study, the Alpha Centauri triple system, just fails to meet Turnbull and Tarter’s criteria, though others say life could exist there nevertheless. It consists of two stars, Alpha Centauri A and B, orbited at a distance of about 10,000 AU by Alpha Centauri C, better known as Proxima Centauri, because it is believed to be the closest star to the sun. The two central stars are about 0.9 and 1.1 times the mass of the sun, while C is much smaller, at about 0.1 solar masses. No planets have yet been detected in the system, and though the possibility of finding one remains, it’s unlikely to be habitable, Turnbull
and Tarter say. The orbit of Alpha Centauri A and B is very elliptical, with their separation varying between 11 AU and 36 AU. Any planet in the vicinity would experience a 3 per cent change in the amount of stellar radiation, which corresponds to a change in average global temperature of between 15 and 23 ˚C. “This is a bigger fluctuation than the Earth is believed to have experienced in its 4.6-billionyear history,” says Quintana. On Earth, the changing surface temperature caused by the location of the sun drives the circulation of weather systems and the ocean currents. So a planet in a multiplestar system, where the surface temperature undergoes wilder fluctuations, might experience more violent weather events, such as hurricanes. “Life would have to arise and evolve against this more turbulent backdrop,” says Quintana. www.newscientist.com
However, Lewis Dartnell, an astrobiologist at University College London, thinks such a seething planet may indeed harbour life, particularly primitive microorganisms. Bacteria living 3 kilometres down in the rocky crust or around hydrothermal vents under an ocean don’t care what the weather is like on the surface. “As long as there are places on a planet where there is liquid water, bacteria will be happy,” he says. Planets in binary systems with greatly separated stars will experience much less of a temperature variation. But even at 80 AU – the distance from the sun to the edge of the solar system – a second star would appear more than 150 times brighter than the full moon. It would shine in daylight, and at night it would banish darkness and even a view of the distant stars, with implications of course for the evolution of any nocturnal life. Living in a world with two suns would have other implications, too. For example, if the stars were different colours, they might affect photosynthesis, Dartnell says, with some organisms exploiting the light wavelengths from one star and other organisms tuning in to the light of the second. If a multiple-star system had a comet cloud wrapped around its outer reaches, like the sun’s Oort cloud, a companion star would probably shake it up periodically. Any planet in the habitable zone might suffer serious
impacts, again implying a harsh environment for the evolution of life. However, Quintana points out that while comet impacts could be hazardous to life, they could also be beneficial, as comets can deliver water-rich material to otherwise dry terrestrial planets. Quintana and Lissauer reckon asteroid belts surrounding the central stars of the Alpha Centauri system would probably be too hot for a comet to contain water. However, a recent study by Jeremy Wertheimer and Greg Laughlin of the University of California, Santa Cruz, considered the possibility that Alpha Centauri A and B are surrounded by a mutual comet cloud and that this might be cool enough for a comet to contain water. What’s more they suggest Proxima Centauri might be close enough to perturb it. If this is the case, Proxima could nudge water-rich comets into collision courses with planets forming around Alpha Centauri A and B. “No one knows how likely the scenario is,” says Quintana. “However, the existence of habitable planets in the Alpha Centauri system cannot yet be entirely ruled out.” Another problem faced by planets in multiple-star systems is that they will be vulnerable to changes in each of the stars. All in all, on a geological timescale, the living environment on such planets will be a lot more changeable than on Earth. Dartnell points out that this may not be bad for the
MATT MURPHY/WWW.BLACKCOFFEEPROJECT.COM
“The living environment on planets in multiple-star systems will be harsher than on Earth” evolution of life. There is some evidence to say that low levels of environmental stress are bad for life, as well as too much. It might be advantageous for life to be knocked about a bit to secure its continued existence, he says. Quintana and a group of astronomers at the University of California, Santa Cruz, have a telescope in Chile dedicated to the search for Earth-like planets in the habitable zones of Alpha Centauri A and B. “The project has the potential to detect planets as small 1.8 Earths within four years,” she says. The chances seem better than ever that life might exist on a planet in a multiple-star system, and researchers are beginning to imagine what life on such planets might actually be like. Vegetation on Earth is predominantly green because of the wavelength of light emitted by the sun. On a planet with different coloured stars, plant life might be predominantly red or blue. Even a world of permanent light, like that imagined by Asimov in Nightfall, could be possible. “Alien worlds just got more alien than anyone imagined,” says Quintana. ● Marcus Chown is the author of Quantum Theory Cannot Hurt You (Faber, September 2008) Further reading: Terrestrial planet formation in binary star systems by Elisa Quintana and Jack Lissauer (www.arxiv.org/abs/0705.3444) Formation and detectability of terrestrial planets
http://space.newscientist.com/channel/astronomy/astrobiology www.newscientist.com
23 August 2008 | NewScientist | 43
40 | NewScientist | 23 August 2008
www.newscientist.com
MATT MURPHY/WWW.BLACKCOFFEEPROJECT.COM
●
IT IS one of the all-time classic scenes in science fiction cinema: Luke Skywalker looking on wistfully as the two suns of Tatooine set over the desert horizon in Star Wars. Isaac Asimov imagined an even stranger world in Nightfall, a short story published in 1941. In it a world permanently lit by six suns is suddenly plunged into darkness by a rare eclipse. Its inhabitants burn everything they can in a desperate attempt to restore light. Could such worlds really exist somewhere out there. And if they do, what might life on them be like? For a long time, most astronomers thought a multiple-star system would be extremely unlikely to produce a planet-sized object, let alone an inhabitable one. It’s all to do with how planets come into existence. Stars form when giant clouds of dust and gas compact, leaving some of the material drifting in the region after the star is born. In a single-star system like our own, this cosmic debris can collect in a disc around the star and over millions of years clump together to form planets. However, in a multiple-star system, the complex gravitational forces that arise when
two or more stars orbit each other disrupt this disc, meaning no clumping and no planets. Or so astronomers thought until a planet orbiting in a binary-star system was discovered in 1997 . Orbiting the star 55 Cancri, 55 Cancri b forced astronomers to look at the universe in a new light, and soon reports of planets in multiple-star systems were flooding in. Since then, four more planets have been found orbiting 55 Cancri. In fact, of the first 200 planets discovered outside our solar system, 42 were in multiple-star systems, usually binary systems. Most of the planets orbit a single star within a multiple system; a couple orbit two stars together (see diagram, page 42). So could any of them support life? In 2002 a planet was found in the inhabitable zone of one of the stars in a binary system. The two stars of the Gamma Cephei system are 28 to 30 astronomical units apart (one astronomical unit, or AU, is the average distance between the Earth and the sun), roughly equal to the distance between the sun and Uranus. Only two other systems are known where the stars are that close. The planet orbits the larger of the two stars – about 1.8 times as big as the
sun – at a distance of 2 AU, more or less the distance from Mars to the sun. The universe is littered with multiple-star systems, so the idea of life evolving on some of them certainly isn’t as far-fetched as previously thought. More than half the stars in our galaxy alone exist in multiple systems, a third of which are triple systems and some are even quadruple. Among these the variation is immense. At one extreme are binary systems with stars 3 light years apart – virtually the distance between the sun and the nearest star. At the other there are “contact binaries”, where two stars are touching. There are systems with stars of the same mass and others in which one star is 1000 times more massive than the other; the stars of some are in a circular orbit around each other and a few where the orbit is highly elliptical. Elisa Quintana of the SETI Institute in Mountain View, California, and her colleague Jack Lissauer from NASA’s Ames Research Center in Moffett Field, California, have been running computer simulations to find out what kind of binary systems might form planets and survive long enough that life
“The universe is littered with multiple-star systems. Some might be habitable”
www.newscientist.com
23 August 2008 | NewScientist | 41
STRANGE WORLDS Habitable planets like the Earth could exist in star systems with more than one star, in a variety of configurations Our solar system The habitable zone extends over a region that contains Earth and Mars
1 astronomical unit (AU) Defined as the average distance between the sun and the Earth EARTH
might have evolved there. By adjusting just a few variables, such as the mass of each star, proximity to the other stars in the system, their orbits and so forth, they soon found a bewildering array of possibilities. For example, if two stars each the mass of our sun are in a circular orbit around a point halfway between them, and are separated by at least 4 AUs, a planet-forming disc of debris extending out to 1 AU could surround both of them, implying inhabitable planets like Earth could form there.
SUN
1AU
The distance from a star at which water is neither permanently gaseous nor frozen. Planets orbiting in this zone may contain life as we know it
MARS
Distant binary systems When the two stars are separated by a large distance, planet-forming habitable zones can exist around each. For example, two sun-like stars would have to be separated by at least 4 AU for this to occur
The Goldilocks factor The planet-forming disc is less stable if the stars have different masses or if they are in an elliptical orbit. In the latter case, that’s because of the gravitational tug of the second star as it swings in close. Still, that doesn’t apply to all elliptical systems, Quintana and Lissauer found. The discs around two sun-like stars in an elliptical orbit 15 AU at their furthest apart and 5 AU at their closest would remain stable, for example. If two stars orbit each other too closely, a disc can’t coalesce around each star, but it could envelop both stars together, a so-called circumbinary formation, as seen with 55 Cancri b. The search for planets then needs to be narrowed down to the star’s habitable zone. This is defined as the region where liquid water, generally considered to be an essential ingredient for life, can exist on the surface of a planet or moon. The inner edge of the zone is where it is just cool enough for water to condense, the outer edge where it is just warm enough to prevent water from permanently freezing, and its depth and distance from the star depend on how much heat the star radiates. In our solar system, only Earth and Mars are in the habitable zone. Margaret Turnbull, currently at the Global Science Institute in Antigo, Wisconsin, and Jill Tarter at the SETI Institute are compiling a list of candidate stars that are most likely to have planets that are not only in the habitable zone, but have been stable for as long as Earth – long enough for intelligent life to have evolved, in other words. Using an existing catalogue of 118,218 nearby stars, they first narrowed down their selection to star systems in which a planet could have a stable orbit in the habitable zone. Then they looked for stars whose radiation level had not varied by more than 3 per cent over a period of at least 3 billion years, like the sun. 42 | NewScientist | 23 August 2008
Habitable zone
4AU or more
Circumbinary and ternary systems If two stars are very close together, any planets that form will orbit both. Again, for sun-like stars, the distance between the stars would have to be much less than 4 AU. A third planet in the system at a greater distance could also have its own habitable zone
Less than 4AU
4AU or more
So far, they have identified 17,129 stars with plausible habitable zones, of which 2200 are known or suspected to be in multiple star systems. Nineteen of the binaries have stable circumbinary habitable zones. The nearest star system to the sun and hence the easiest to study, the Alpha Centauri triple system, just fails to meet Turnbull and Tarter’s criteria, though others say life could exist there nevertheless. It consists of two stars, Alpha Centauri A and B, orbited at a distance of about 10,000 AU by Alpha Centauri C, better known as Proxima Centauri, because it is believed to be the closest star to the sun. The two central stars are about 0.9 and 1.1 times the mass of the sun, while C is much smaller, at about 0.1 solar masses. No planets have yet been detected in the system, and though the possibility of finding one remains, it’s unlikely to be habitable, Turnbull
and Tarter say. The orbit of Alpha Centauri A and B is very elliptical, with their separation varying between 11 AU and 36 AU. Any planet in the vicinity would experience a 3 per cent change in the amount of stellar radiation, which corresponds to a change in average global temperature of between 15 and 23 ˚C. “This is a bigger fluctuation than the Earth is believed to have experienced in its 4.6-billionyear history,” says Quintana. On Earth, the changing surface temperature caused by the location of the sun drives the circulation of weather systems and the ocean currents. So a planet in a multiplestar system, where the surface temperature undergoes wilder fluctuations, might experience more violent weather events, such as hurricanes. “Life would have to arise and evolve against this more turbulent backdrop,” says Quintana. www.newscientist.com
However, Lewis Dartnell, an astrobiologist at University College London, thinks such a seething planet may indeed harbour life, particularly primitive microorganisms. Bacteria living 3 kilometres down in the rocky crust or around hydrothermal vents under an ocean don’t care what the weather is like on the surface. “As long as there are places on a planet where there is liquid water, bacteria will be happy,” he says. Planets in binary systems with greatly separated stars will experience much less of a temperature variation. But even at 80 AU – the distance from the sun to the edge of the solar system – a second star would appear more than 150 times brighter than the full moon. It would shine in daylight, and at night it would banish darkness and even a view of the distant stars, with implications of course for the evolution of any nocturnal life. Living in a world with two suns would have other implications, too. For example, if the stars were different colours, they might affect photosynthesis, Dartnell says, with some organisms exploiting the light wavelengths from one star and other organisms tuning in to the light of the second. If a multiple-star system had a comet cloud wrapped around its outer reaches, like the sun’s Oort cloud, a companion star would probably shake it up periodically. Any planet in the habitable zone might suffer serious
impacts, again implying a harsh environment for the evolution of life. However, Quintana points out that while comet impacts could be hazardous to life, they could also be beneficial, as comets can deliver water-rich material to otherwise dry terrestrial planets. Quintana and Lissauer reckon asteroid belts surrounding the central stars of the Alpha Centauri system would probably be too hot for a comet to contain water. However, a recent study by Jeremy Wertheimer and Greg Laughlin of the University of California, Santa Cruz, considered the possibility that Alpha Centauri A and B are surrounded by a mutual comet cloud and that this might be cool enough for a comet to contain water. What’s more they suggest Proxima Centauri might be close enough to perturb it. If this is the case, Proxima could nudge water-rich comets into collision courses with planets forming around Alpha Centauri A and B. “No one knows how likely the scenario is,” says Quintana. “However, the existence of habitable planets in the Alpha Centauri system cannot yet be entirely ruled out.” Another problem faced by planets in multiple-star systems is that they will be vulnerable to changes in each of the stars. All in all, on a geological timescale, the living environment on such planets will be a lot more changeable than on Earth. Dartnell points out that this may not be bad for the
MATT MURPHY/WWW.BLACKCOFFEEPROJECT.COM
“The living environment on planets in multiple-star systems will be harsher than on Earth” evolution of life. There is some evidence to say that low levels of environmental stress are bad for life, as well as too much. It might be advantageous for life to be knocked about a bit to secure its continued existence, he says. Quintana and a group of astronomers at the University of California, Santa Cruz, have a telescope in Chile dedicated to the search for Earth-like planets in the habitable zones of Alpha Centauri A and B. “The project has the potential to detect planets as small 1.8 Earths within four years,” she says. The chances seem better than ever that life might exist on a planet in a multiple-star system, and researchers are beginning to imagine what life on such planets might actually be like. Vegetation on Earth is predominantly green because of the wavelength of light emitted by the sun. On a planet with different coloured stars, plant life might be predominantly red or blue. Even a world of permanent light, like that imagined by Asimov in Nightfall, could be possible. “Alien worlds just got more alien than anyone imagined,” says Quintana. ● Marcus Chown is the author of Quantum Theory Cannot Hurt You (Faber, September 2008) Further reading: Terrestrial planet formation in binary star systems by Elisa Quintana and Jack Lissauer (www.arxiv.org/abs/0705.3444) Formation and detectability of terrestrial planets
http://space.newscientist.com/channel/astronomy/astrobiology www.newscientist.com
23 August 2008 | NewScientist | 43