Many “Earths” out there, scientists say after new planet find
and World Science staff
Astronomers say they have found the smallest planet ever detected around a normal star outside our solar system, and that the find indicates there are many Earth-like planets out there.
The planet weighs five times as much as Earth and orbits a relatively cool star, known as a red dwarf, every decade, astronomers said. It would thus be the latest in a series of discoveries of increasingly Earth-like planets outside the Solar System, since others found to date are even larger compared to Earth.
Also, with the finding, astronomers have now identified planets on both sides of the “habitable zones” of distant stars. A habitable zone is a narrow region around a star whose temperature is such that liquid water can exist, so that presumably life can form.
Most planets found around stars other than the Sun—known as extrasolar planets—to date have been in the hotter-than-habitable zone. The newfound world, by contrast, is too cold, astronomers said. That suggests the discovery of a planet lying in the lucky middle might not be far off, they added.
“It’s encouraging that we now have examples of planets on both sides of the habitable zone,” said Scott Tremaine of Princeton University in Princeton, N.J., one of the astronomers who announced the latest finding.
The distance between the planet and its star is about three times greater than that between the Earth and the Sun, the researchers found. That would put it somewhere between Mars and Jupiter in terms of distance from its host star.
The large orbit and its dim parent star make its likely surface temperature a frigid minus 364 degrees Fahrenheit (minus 220 degrees Celsius), the astronomers calculated. This temperature is similar to that of Pluto, though it is is about one-tenth closer to its star than Pluto is to the Sun.
The planet, identified with the help of an armada of telescopes, has been designated OGLE-2005-BLG-390Lb.
“This finding means that Earth-mass planets are not that uncommon,” said Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md., and a founding member of the Probing Lensing Anomalies Network team that helped detect the new planet. “If we found one, there must be more.”
The findings are to be published Jan. 26 in the research journal Nature.
The discovery also supports theories for how our solar system formed, the astronomers said. “The favored theory proposes that planets were created from material accreting [clumping up] around a star,” explained Princeton’s Bohdan Paczynski, a member of the Optical Gravitational Lensing Experiment, a group that also helped identify the object.
“Around red dwarfs, the theory predicts Earth- and Neptune-sized planets to be more common than Jupiter-sized planets. The planets would be located between 0.1 and 10 times the Earth-Sun distance from their stars.”
Astronomers identified the planet indirectly, with a technique called gravitational microlensing. The method exploits stars’ random motions. If one star passes precisely in front of another star, the gravity of the foreground (“lens”) star bends the light from the background (“source”) star.
The foreground star, therefore, acts like a huge lens, brightening the background star. A planetary companion around the foreground star can produce additional brightening of the background star. This can thus reveal the planet, which is otherwise too faint to be seen by telescopes.
The higher the mass of the “lensing” star, the longer the microlensing event lasts. So, while a microlensing event due to a star lasts many days, the extra brightening due to a planet lasts a few hours to a couple of days. In the case of the newly found planet, the extra brightening lasted only about 12 hours, researchers said.
Also using microlensing, astronomers determined the planet’s mass. This method, however, reveals nothing about an object’s makeup. Astronomers said the planet probably consists of ice and rock and is a giant version of “terrestrial” planets like Earth and Mars.
The object orbits the most common star in our Milky Way Galaxy, a red dwarf five times less massive than the Sun, they added. The pair is located about 20,000 light-years away in the constellation Sagittarius, not far from the central bulge of our galaxy.
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