For the first time, astronomers have directly observed a planet in the making.
The baby planet circles a very young, sun-like star located in a giant cloud of molecular gas 430 light-years from Earth in the constellation Taurus.
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Astronomers had previously noted a hefty gap in the disk of gas and dust surrounding the star, known as LkCa 15. They suspected the gravitational pull of an evolving planet had cleared out an orbital zone, similar to how some moons circling Saturn create gaps in its rings.
Now, a new series of observations adds key details of the planet-in-the-making, showing for the first time how it is feeding on hydrogen gas.
"This discovery has far-reaching implications for our understanding of the planet-forming process and of the properties of young planets," Princeton University astrophysicist Zhaohuan Zhu wrote in a commentary in this week's Nature.
The research also reveals that the planet, called LkCa 15 b, seems to have one or two siblings.
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"You have your star, and then there's this gap as you move out from the star. In the images, you see a few points of light in that gap. Those would be the planet candidates," astronomy graduate student Stephanie Sallum, with the University of Arizona, told Discovery News.
Because LkCa 15 is so far away, the span between the star and its planets appears mind-bogglingly close to Earth-based telescopes, like the width of a pin from a kilometer (0.62 miles) away.
With adaptive optics that corrects for atmospheric distortions and a new masking technique, Sallum and colleagues used the Large Binocular Telescope in Arizona to peer directly into the gap in LkCa 15's dust disk. There they found telltale chemical fingerprints of superheated, 17,500-degree Fahrenheit hydrogen gas falling deeply into the cavity containing LkCa 15 b, located about 16 times farther away from its parent star than Earth is to the sun.
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The researchers were unable to detect similar emissions from the suspected sibling planets.
"It could just be that we're not seeing the light because it's being blocked by dust. Another option is that accretion may not be something that happens steadily. There could be variable accretion where you're seeing much more hydrogen falling onto one planet right now, but then in the future it might be the other one is accreting more strongly, or something variable like that," Sallum said.
Astronomers have lots of time for follow-on studies. Computer models show it takes a few million years for a planet to form. That may sound long by human standards, but it's puzzlingly short compared to the lifetimes of stars.
One of the key questions raised by the emerging LkCa 15 system is how such a giant planet can form and still be growing around a star that is just 2 million years old.
The research is published in this week's Nature.