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Published: March 20, 2012 | 9:55 am
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As a vast cloud of gas collapses under its mutual gravity and seeds the birth of a young star, the process of planetary formation begins. But far from it being a neat and tidy formation process, astronomers have uncovered a mechanism that determines why some orbits around the new star may be devoid of planets, whereas others become jammed with planets.

It may be logical to think that as a young star develops, surrounded by a protoplanetary disk — a spinning disk of dust and gas fertile for planets to grow via accretion processes — worlds of various sizes should evolve at regular intervals. What we should be left with in an “ideal” star system are worlds in regular orbits at set intervals from the parent star.

However, as exoplanetary-hunting observatories are discovering, extrasolar planets are anything but orderly.

Now, a group of astronomers think they may have found an answer to this conundrum after running a simulation of a star system, watching how the model exoplanets evolve and comparing their results with observations.

“Our results show that the final distribution of planets does not vary smoothly with distance from the star, but instead has clear ‘deserts’ — deficits of planets — and ‘pile-ups’ of planets at particular locations,” said Ilaria Pascucci, an assistant professor at the University of Arizona’s Lunar and Planetary Laboratory, who presented the results at the Lunar and Planetary Science Conference in The Woodlands, Texas on March 19.

So how are these “deserts” and “pile-ups” formed?

After a young star is born, it is surrounded by a rotating cloud. Over time, this cloud flattens out and forms a protoplanetary disk — eventually planets will grow here. However, before this happens, a tug-o-war ensues within the protoplanetary gas and dust.

The young star feeds off the protoplanetary disk, gaining mass — this mass generates a dominant gravitational field pulling more material star-wards. At the same time, the star is pumping out powerful radiation that pushes the protoplanetary disk away, effectively “putting the breaks” on the gas and dust from falling into the star. At a certain point, the inward gravitational pull balances with the outward radiation pressure, clearing a zone around the star.

This is a pretty well accepted theory on how stars form during their formative years. However, as pointed out by Pascucci’s team, the stellar radiation also has a heating effect on the protoplanetary disk that is gravitationally locked in orbit.

“The disk material that is very close to the star is very hot, but it is held in place by the star’s strong gravity,” said co-author Richard Alexander of the University of Leicester. “Further out in the disk where gravity is much weaker, the heated gas evaporates into space.”

This is known as photoevaporation and it causes mass to be ejected from the young star system. However, further away from the “evaporation zone,” the stellar radiation is too weak to cause heating, so photoevaporation only occurs in a discreet cavity within the protoplanetary disk — not too dissimilar in appearance to the spaces between Saturn’s rings. Pascucci and Alexander point out that for young sun-like stars, this cavity forms between 1-2 AU from the star. (An AU, or astronomical unit, is the average distance the Earth orbits the sun.)

This is where things get interesting.

As time goes on, giant planets form around the young star and they remain embedded within the protoplanetary material. The young planets accrete some of the protoplanetary disk material through gravity, but the material also causes drag, slowing down their orbital momentum.

As they lose momentum, they fall closer and closer to their star. The process of planetary migration pulls the giant planets toward the cavity cleared by photoevaporation. When they hit the cavity — devoid of any gas — drag suddenly stops sapping their momentum and the planets settle into the cavity.

“The planets either stop right before or behind the gap, creating a pile-up,” Pascucci said. “The local concentration of planets leaves behind regions elsewhere in the disk that are devoid of any planets. This uneven distribution is exactly what we see in many newly discovered solar systems.”

To use a crude analogy, the cavity (or gap) acts like a “speed bump” stopping the inward migration of planets. Should there be more planets in the outer reaches of the star system, they “pile-up” as they enter the cavity.

This may explain some of the weirdly distributed exoplanetary systems discovered by NASA’s Kepler space telescope. And as Kepler’s mission continues, it will become more sensitive to gas giants orbiting sun-like stars at around the 1 AU mark, providing Pascucci and Alexander with more data to compare their model with.

The research is to be published in the journal Monthly Notices of the Royal Astronomical Society.


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