Embryonic Planets: The Missing Link May Be Harboring in Stellar Disks

Alice Quillen, associate professor of astronomy at the University of Rochester, employed new Hubble Space Telescope imagery to search for baby planets, more properly called embryonic planets.

Quillen is one of the world's leading experts on the interaction between planets and stellar dust disks. She and a team of researchers at University of Rochester are studying three nearby stars that they say may hold embryonic planets within their stellar dust disks (also called circumstellar or protostellar disks).

Current planet formation theory has a "missing link" and Quillen thinks that measuring the thickness of the dust disks (circumstellar, or protostellar, disks) that surround forming stars will lead to calculations confirming and predicting the size of planets growing with the dusty disk.

Quillen explains that a gaseous disk of grit and dust usually surrounds forming stars (in the same manner as disks, rings, surround Saturn). In theory, this gritty and dust in the disk provide the raw material for planet building. The cloud of dust thins as the system ages.

But if enough dust in the disk around the star has been clumped together, the building clump--or "embryonic planet," as Quillen calls it--will knock the accumulating dust and grit into ever-more eccentric orbits (eccentric orbit: an orbit that deviates from a circle). Over time, this will cause an otherwise razor-thin circumstellar disk to appear puffed up.

Scientists have inferred the presence of nearly 250 embryonic planets in dust disks around their stars in the last decade. But Quillen's method focuses on the unique aspect of studying the protostellar disk's thickness, specifically its puffing, or clumping, thicknesses.

The conventional theory of embryonic planet formation doesn't take a disk's thickness into account. This is because until the Hubble Telescope images, astronomers had no way to measure the disk's thickness. Thus, without measurements, the largest clump size that the model could predict was only about a kilometer wide. Non-scientists and scientists alike can agree that this prediction of an embryonic planet of one kilometer wide is very far from the eventuality of a fully grown planet emerging from such disks.

But now, using Hubble images measurements can be taken of circumstellar disk puffiness or flatness. The importance of these advances lies in the attempts of scientists to decipher how our own planet came to be. And for this, they look to the forming planets of other star systems for clues. In this search, astronomers have been unable to find evidence a key stage of development in a planet's life, a step in the early period of a planet's formation when the new planet is the size of Pluto.

Using Hubble images, Quillen measured the puffiness of three nearby stars with young disks: disks that have not had a thinning of the cloud of dust and that do not exhibit an eccentric orbit and do not appear puffed up. The three stars have disks that are edge-on to Earth from the perspective of Earth based telescopes and the Hubble Telescope.

The three stars are AU Microscopii, Beta Pictoris, and Fomalhaut. Since Hubble images permit measurement, their disks can be seen to display a thicker disk than conventional models anticipate: thickness indicates clumping; clumping indicates an accumulation of grit and dust that over time causes the razor-thin disk to puff up; puffing up indicates the presence of an embryonic planet circling its star in the circumstellar disk.

"We're able to determine for the first time how large the bodies must be in a disk to scatter the dust the way we've observed," says Quillen.

Quillen used her own models of stellar dust dynamics to estimate how much mass was required to gravitationally gather the dust to the thicknesses she observed.

"Those calculations pushed us into Pluto-sized bodies," says Quillen.

Quillen is now looking for more young star systems to investigate with her model. The systems have to be young enough to still have their circumstellar disks, but old enough to be forming the embryonic planets.

"Sign of 'Embryonic Planets' Forming in Nearby Stellar Systems," University of Rochester.