and relatedThe next time you look up at the night sky and find yourself marveling at the number of stars overhead, know that you are only seeing part of the magnificent bounty that our galaxy holds. Most of those Milky Way stars are not isolated orbs. Rather an average star has at least one planetary companion, invisible to the naked eye and in most cases as yet unseen by telescopes, according to a new analysis.
That extrasolar planets should be even more common than stars, which themselves seem innumerable, lends support to the hope that somewhere up in the night sky, circling one of those stars, is a world like Earth where life may have had a chance to take root, and maybe even have evolved into an intelligent form.
The analysis of planetary frequency in the Milky Way appeared in the January 12 issue of Nature. (Scientific American is part of Nature Publishing Group.) The researchers, led by astronomer Arnaud Cassan of the Paris Institute of Astrophysics at University Pierre and Marie Curie, used a small sample of planetary discoveries to infer the size of the overall planetary population. Extrapolating from a few known planets and the relatively low probability that each of those planets should be detectable from Earth, the researchers found that each star is home to an average of 1.6 planets.
The process is a bit like estimating the average number of children in a typical family by peering into a handful of random homes, counting the number of children in view, and estimating how many more are at school or otherwise out of sight. As such, the planetary demographics are still rudimentary; given the small-number of statistics, the actual average could be closer to one planet per star, or it could be well over two planets per star. But the general ubiquity of extrasolar planets, which other astronomical campaigns have also suggested in recent years, seems unassailable.
"This is not a surprise, but it's a really interesting thing to know," says astronomer Scott Gaudi of The Ohio State University, who did not contribute to the new research. Perhaps most encouraging is the finding by Cassan and his colleagues that the frequency of planets rises as the mass of those planets decreases. Large planets akin to Jupiter are relatively rare, midsize planets such as Neptune are present around roughly 50 percent of stars, and small planets just five to 10 times the mass of Earth are even more numerous than that. "Planets are common, and low-mass planets are as common as dirt in some sense," Gaudi says.
Cassan based the galactic census on a planet-finding method called gravitational microlensing. Using the Warsaw University Telescope in Chile, astronomers monitor roughly 200 million stars to look for the sudden and anomalous amplification in the light from any one of them. That brightening can be caused by another star passing in front of the background star, with the gravitational field of the intervening star acting like a lens to focus the light of the background star toward Earth. Such alignments are rare, but by monitoring so many stars for years on end, the campaign, known as the Optical Gravitational Lensing Experiment (OGLE), has recorded thousands of microlensing events.
The brightening and subsequent dimming of the background star due to microlensing does not always follow a smooth bell curve, however. In about a dozen cases identified by OGLE and by the similar Microlensing Observations in Astrophysics (MOA) experiment based at Mount John University Observatory in New Zealand, irregularities in the lensing signal point to a planet orbiting the foreground star and distorting the symmetry of the lens. The duration of a deviation from the bell curve indicates the suspected planet's mass.
Microlensing has its downside—the planetary signals are ephemeral, lasting only as long as the background star and the planet-hosting star remain in alignment (typically about a month). But it has one critical advantage over other planet-hunting techniques: it is sensitive to bodies not especially close to their parent stars. More prolific planet-search methods, including the technique employed by NASA's Kepler spacecraft, which detects periodic variations in starlight caused by orbiting planets eclipsing their stars, have the most success detecting planets that orbit very close to their host stars and hence complete an orbit very quickly.
"Microlensing can probe planets of all masses for a very large range of orbital separations," from about 0.5 times to 10 times the Earth–sun distance, Cassan says. He notes that the abundance estimates can only increase with exploration of a larger range of orbital distances and planetary masses. "Our results are given for masses between five Earths and 10 Jupiter masses," Cassan says. "If there are other planets farther or closer in, the average number of planets per star would increase accordingly."
The conclusion that smaller planets occur more often than bigger ones reinforces what Kepler has shown for planets that orbit close to their stars. The spacecraft is designed to locate worlds similar to our own—small, rocky planets at temperate, Earth-like distances from their host stars. That hunt is still underway, but early results from the mission have revealed that smallish planets—those just a bit bigger than Earth—are common in the hotter, close-in orbits to which Kepler is already sensitive.
"Kepler has already been finding that small planets are actually quite ubiquitous around stars," Gaudi says. "That bodes well for our goal of eventually finding an Earth-size planet in the habitable zone. All signs are pointing to the low-mass planets being common, so I think there's a good chance that we'll find a system like that in the coming years."
Not really surprising (when you're dealing with billions of stars, billions of planets become a statistica certainty from what we know now), but still nice to know.Milky Way could contain billions of habitable planets like Earth, says study
Habitable planets may be in orbit around billions of stars in the Milky Way, a long-term study has suggested.
Astronomers came to the conclusion after a six-year star survey which suggests planets on which humans could live are commonplace in our galaxy.
Scientists estimate as many as 10 billion stars in the Milky Way may host planets in the habitable - or 'Goldilocks' - zone.
This is the orbital band within which conditions are not too cold and not too hot but 'just right' to allow surface liquid water and, potentially, life.
The discovery raises the possibility of a universe teeming with life, as depicted in popular sci-fi movies and TV series such as Star Wars and Star Trek.
However, scientists stress that just because a planet has conditions suitable for life it does not follow that life has evolved there.
Over the past 16 years, astronomers have made more than 700 confirmed detections of 'exoplanets' orbiting distant stars.
The vast majority have been Jupiter-like gas giants or scalding hot planets hugging close to their stars. Both offer little hope of finding life.
In those cases astronomers relied on spotting tiny 'wobbles' in the host star caused by a planet's gravitational pull, or the minute dimming of starlight as a planet crossed in front of its star.
Both techniques are not suited to finding small rocky planets like the Earth in the habitable zone.
The new survey employed a radically different method called 'gravitational microlensing'.
This involves a foreground star's gravity acting like a 'magnifying glass' to bend and amplify light from a background star.
If there is a planet orbiting the foreground star, a small extra 'bump' might be seen in the light signal.
The technique just happens to be most sensitive to planets a mid-distance away from the star - in other words, those in the 'habitable zone'.
However, very special conditions are needed to detect planets by gravitational microlensing. The background and foreground stars have to be lined up, and an additional chance alignment of the planet's orbit is also needed.
Despite these obstacles, analysis of six years' worth of microlensing data from telescopes around the world uncovered an unexpected number of exoplanets.
'In a six-year period from 2002 to 2007 we observed 500 stars at high resolution,' said Danish astronomer Dr Uffe Grae Jorgensen, head of Astrophysics and Planetary Science at the University of Copenhagen.
'In 10 of the stars we directly see the lens effect of a planet, and for the others we could use statistical arguments to determine how many planets the stars had on average.'
The results are published today in the journal Nature.
Combined with exoplanet findings using different detection methods, they suggest around 10 billion stars out of the 100 billion that fill the Milky Way have habitable zone planets.
The findings showed that planets orbiting stars were 'more the rule than the exception' and billions of them may be habitable, said Dr Jorgensen.
However it was quite another thing to jump to the conclusion that life had arisen on large numbers of these worlds as it had on Earth.
Life as we know it on Earth had developed as a result of 'many unique events', Dr Jorgensen pointed out.
But he added: 'Perhaps other coincidences in other solar systems have led to entirely different and exciting new forms of life.'
Dr Martin Dominik, who led a British team from the University of St Andrews involved in the research, said: 'We do not know yet where all the planets are, how big or small, dense or fluffy they are, or whether they are home to life or not, but our latest results tell us that while we may not see all the planets, wherever in the sky we look, they are there.'
PS. I call dibs on the habitable ones.
-- Asuka