Earth twins more often than expected

One of five sun-like stars could have rocky planets

Earth twins around sun-like planets - representation of an artist. © NASA / JPL-Caltech / R.Hurt (SSC-Caltech)
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Are planetary systems with Earth-like planets rare or have we not yet discovered the others? Probably the latter is true, as astronomers have found out using NASA's Spitzer Space Telescope. In the journal "Astrophysical Journal Letters" they report that sun-like stars in the Milky Way are far more common than they were thought to be. This has also increased the potential for life in space.

An international team of astronomers, led by Michael Meyer of the University of Arizona, studied six groups of stars of different ages, each with a mass approximately equal to that of the Sun. The age of the stars ranged from three to ten, from ten to thirty, from thirty to one hundred, from 100 to 300 million years, and from three hundred million to one billion, and from one to three billion years.

Dust temperature as a signal

"We wanted to study the evolution of gas and dust around sun-like stars and compare the results to what we assume from the early days of the solar system, " explains Meyer. Our sun is about 4.6 billion years old. The observation tool used was the Spitzer Space Telescope, which can image and measure cosmic dust and gas in the infrared range.

The temperature of the dust depends on how far it circles away from the central star. Therefore, researchers can determine from the wavelengths of the infrared light emitted by the dust, and thus from the temperature, whether material is at a distance from the star that would allow the formation of rocky planets. An indication of this would be a 24 micron wavelength, which corresponds to a temperature between minus 24 degrees and 140 degrees.

"We found that about 10 to 20 percent of the stars in each of the four younger groups have a 24-micron emission, " explains Meyer. "With the stars older than 300 million years, however, we rarely see warm dust. This also corresponds to the timescales that we have for the development of the solar system. "Display

Warm dust as a result of rock formation

In a separate study, Scott Kenyon of the Smithsonian Astrophysical Observatory at the University of Utah's Cambridge, Massachusetts, and Ben Bromley had developed planetary formation models consistent with these observations. They also predict warm dust in the wavelength range around 24 microns, as this occurs when smaller celestial bodies collide and resulting in larger bodies such as asteroids, minor planets and moons.

"Our work indicates that the warm dust that Meyer and his colleagues discovered is a natural consequence of the formation of rocky planets, " says Kenyon. "As Spitzer has observed, we also predict a higher proportion of dust emission from younger stars."

Depending on the interpretation between 20 and 60 percent

However, according to the researchers, there are quite different ways to interpret the sharpener data. One variant assumes that the dust disks are preserved by 20 percent of the young stars as these stars age. This would mean in the extrapolation that at least one of five sun-like stars could potentially form rocky planets.

But there is another way of interpreting it: "An optimistic scenario would suggest that the largest, most massive dust disks start the collision process first and form their planets quickly, " explains Meyer. This is what we now see in the recent stars. Their dust disks live intensely and die young. But the smaller, less massive slices start later. Their planet formation is retarded, as there are fewer particles in them that can collide with each other.

If this interpretation is correct, then as many as 62 percent of the observed stars may have formed or are still forming planets. The right answer is probably somewhere between the pessimistic case of 20 percent and the optimistic estimate of more than 60 percent, "said Meyer.

Space mission and giant telescope could confirm data

A test of which interpretation is correct could be NASA's Kepler mission, which will launch next year. It is supposed to search for the tiny changes in light that a planet passing in front of its star triggers. Also, the Giant Magellan Telescope (GMT), a telescope of several interconnected 8.4 meters mirrors, which is to be built by 2016 in Chile, could provide more information here. Its resolution will be that of a single massive mirror of 24.5 meters.

"Identifying a terrestrial planet using spectroscopic and optical techniques is extremely difficult, " explains Phil Hinz, astronomer at the University of Arizona. But if earthly planets really should be so common, then GMT could capture the first image of another Earth. This is an exciting prospect

(University of Arizona, 18.02.2008 - NPO)