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Recent observations and new computer modeling has strengthened the growing expectation that a bounty of fledgling planets, perhaps even some the size of Earth, exist around newborn stars. New search methods and more powerful telescopes could soon allow astronomers to find these nascent worlds.
Around a young, nearby and common sort of star, astronomers last week announced they'd found a leftover disk of dusty debris from which planets may be forming.
And in separate work, theorists modeled how stellar dust disks might be probed for just-born Earth-sized worlds using NASA's new Spitzer Space Telescope. Meanwhile, as the number and types of stars known to have disks grows, researchers expect to begin doing serious comparisons to learn about the range of scenarios that might be involved in building planets.
Garden variety
The latest discovery involves a dim star, called an M-type, that is 33 light-years away. It is the closest and youngest star known to have such a circumstellar disk, as the plane of material is called.
The star is catalogued as AU Microscopium and referred to as AU Mic.
Some 85 percent of all stars in the galaxy are M-type. AU Mic is half as massive as the Sun and one-tenth as luminous. It is about 10 million years old, an infant in star years.
"Gas that once surrounded the star has mostly disappeared, perhaps by forming gas-giant planets," explained study leader Paul Kalas of the University of California at Berkeley. "The system has become a planet preschool."
Immature gas giant planets, assuming they are there, would still be dissipating heat generated by their gravitational contraction, Kalas told SPACE.com . Meanwhile, comets and asteroids circling the star are likely colliding, creating the dust that's been observed. Some of these might gather to form rocky, Earth-sized planets.
This is how planets are thought to have formed in our solar system, which is 4.6 billion years old. But the details of planet formation are not known, making the newly spotted, developing system an interesting case study.
Other types of stars have similar disks. In 2002, for example, astronomers made a strong case -- based on dust clumping -- for a
Saturn-sized planet around Fomalhaut, the 17th brightest star in the sky and one that is more than twice as massive than our Sun. AU Mic's disk is interesting because it is such a common type of star.
"For the first time we have discovered that a nearby M-star, AU Mic, is surrounded by a dust disk that indicates the existence of asteroids and comets orbiting the star," Kalas said. "In principle, small bodies such as asteroids and comets can join to form even larger objects such as planets."
Possible planet
Other M-type stars are known to harbor giant planets, which have been found by studies that look for wobbles in stars indicating the gravitational tug of an orbiting world. AU Mic is much younger than other planet-harboring M-type stars, so Kalas is eager to study the system further this summer to learn how newborn planets interact with disk material.
If there is a planet around AU Mic, it might be found with the indirect wobble method. Or, because the system is so close, a planet's infrared radiation might be sorted out by the Hubble Space Telescope, yielding a direct detection. Finally, because the disk happens to be edge-on as seen from Earth, a planet within the disk might pass directly in front of the star, becoming evident by its shadow.
The discovery of the disk, one of just a handful known, was detailed last Thursday in the online edition of the journal Science. The observations were made with the University of Hawaii's 2.2-meter telescope atop Mauna Kea in Hawaii.
Kalas' team has already found other evidence for a planet in the system.
In a companion analysis that will be published in the Astrophysical Journal, the researchers report a dust-free hole in the disk extending out to about the same distance from the star as Uranus is from the Sun.
"The dust missing from the inner regions of AU Mic is the telltale sign of an orbiting planet," said Michael Liu of the University of Hawaii. "The planet sweeps away any dust in the inner regions, keeping the dust in the outer region at bay."
More observations of the dust are planned for August using the
Spitzer Space Telescope, which specializes in infrared observations.Broader understanding
Understanding how planets form around AU Mic could provide an important comparison to the process around another star called Beta Pictoris, which also has a dust disk.
Beta Pictoris is 65 light-years away. It's roughly the same age as AU Mic but much bigger, about 2.5 times the mass of the Sun. Astronomers have
seen dust clumps in the star's disk that suggest a solar system like our own might be forming.Yet Kalas says the dynamics of the larger star's environment should be quite different.
Beta Pictoris "is so luminous that ices have a very short lifetime, and any small grains produced by asteroid and comet collisions are pushed out of the system immediately," Kalas said. "With a benign radiation environment, the AU Mic system would probably retain a larger fraction of ices, such as water ice. It would also retain a larger fraction of small dust grains because there isn't enough radiation to push it out of the system."
The overarching goal for all these sorts of studies is to find the first Earth-like planet around another star. Most experts consider it likely that many such worlds exist, but they cannot yet be detected.
More than 100 planets have been found around fairly nearby, mostly sun-like and mature stars. But the planets are all giants, much like Jupiter. In some systems, the giant planets are far enough from their host stars that, mathematically speaking, Earth-sized planets could survive on inner orbits.
How to find Earths
Recent computer modeling by another team suggests how dust disks could be probed for signs of nascent Earth-sized planets.
Scott Kenyon of the Smithsonian Astrophysical Observatory and Benjamin Bromley from the University of Utah seeded virtual circumstellar disks with a billion objects that are 0.6 miles (1 kilometer) in diameter -- objects called planetesimals. Then they let the scenario run forward in time and watched what happened at distances similar to the orbit of Venus out to a ways beyond Earth's path.
Their results were published in the Feb. 20 issue of the Astrophysical Journal Letters.
They found that initial collisions between objects are low-speed, allowing mergers. Within 20,000 years, objects as large as the Moon develop.
Then things change. The gravity of the larger "protoplanets" begins to slingshot smaller objects, so that collisions are less friendly. Chaotic smashups create concentrations of dust.
"The dust forms right where the planet is forming, at the same distance from its star," said Kenyon.
Importantly, dust in a Venus-like orbit would be hotter than dust in an Earth-like orbit. The Spitzer telescope should be able to detect dust peaks that point to rocky planet formation around other stars, Bromley said.
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