Australasian Science: Australia's authority on science since 1938

Astronomers Witness Planet Birth

By Stephen Luntz

An international team has for the first time photographed a planet in the process of formation. The finding confirms long-standing theories of how solar systems evolve, and opens the door to a range of exciting further research.

The standard theory of planetary formation holds that the gas left over after a star begins to shine gradually condenses into larger and larger objects that become planets. Until now, however, we have not been able to witness this occurring.

Prof Peter Tuthill of the University of Sydney’s School of Physics was part of a team that reasoned that planets in the process of formation would be quite hot, making them visible in certain wavelengths.

T Chamaeleontis is one of what Tuthill calls “a handful” of stars known to have protoplanetary disks. It is thought to be just 6–7 million years old. Planets should theoretically carve out gaps in these disks as they form.

“There are two ways to find a gap in a disk,” Tuthill says. “Subsequently we confirmed the presence of an inner and outer disk using interferometry, but initially it was assumed from missing energy.”

Inner parts of the disk produce short wave radiation while the outer parts produce long wave radiation. A shortage of radiation at certain frequencies suggested that some material was missing.

Although it is possible to capture an image of a planet at this stage of formation, it’s far from easy. Tuthill designed the instrument that recorded the tiny mote of light sitting in the disk’s gap, which was announced in Astronomy and Astrophysics.

So far the object has only been detected at one wavelength and confirmed at another. As a result we don’t know how hot it is, let alone how large. “Someone with a planetary model might tell you how hot it should be,” Tuthill says. However, since part of the aim of the research is to one day be able to test these models, the figure would have to be considered uncertain.

While the masses of other exoplanets have been determined using Doppler analysis of their effects on the parent star’s motions, Tuthill says this is “very difficult with this class of star”.

The unnamed planet’s orbital radius is estimated at a billion kilometres, a little further out than Jupiter is from the Sun.

Although planetary formation from circumstellar disks is very much the dominant theory, there are competing explanations for how this happens. Tuthill says we don’t yet know enough to shed light on any outstanding questions, but says “it’s a major thing to have confirmed cloud collapse”. He hopes that the confirmation of more planets at a similar stage of life will shed more light on the details of planetary formation.

“As with biology, studying newborns teaches us a lot,” says Dr Michael Ireland, also of the University of Sydney. “We get to see their initial properties before they get scrambled up and influenced by their environment.”