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November 2010 - Issue #11

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How a trail of darkness leads to a planet born

Introduction by Julian Cribb

View articles in related topics: Astrophysics/Astronomy

Complex supercomputer models of galactic dust and gas are helping to spot newborn planets.

An international team of astronomers has laid the groundwork for the next astronomy sensation: watching a far distant planet in the act of being born. Their work is refining the study of the disks of dust that enfold newborn planets, creating a clearer view of cosmic birth.

Swinburne University of Technology’s Associate Professor Sarah Maddison and colleagues Dr Laure Fouchet of Berne University, Switzerland, and Dr Jean-François Gonzalez of Lyon University, France, have developed a technique for studying the vast disk of dust and gas surrounding an infant star. Their technique will allow astronomers using the latest and most powerful millimetre wavelength telescopes to watch as young planets take shape and create that star’s solar system – albeit about 500 years ago in Earth time.

This remarkable observation centres on gaps, which form (like grooves in an old LP vinyl record) in this protoplanetary disk of dust. These gaps can indicate the presence of either accumulating grains of dust that have grown beyond the size of the observing telescope (and thus rendered ‘invisible’) or the ‘wake’ left by an unseen planet and its gravity.

In other words, because a planet cannot be seen – the dust obscures all optical light and they do not emit enough radiation to be detected at a millimetre wavelength – a new planet’s existence has to be inferred from the behaviour of the dust (and gas) around it; much as a ship’s presence might be inferred, by an observer flying high above, from its wake on the ocean.

Millimetre telescopes are used because they are able to detect the cool dust that makes up these disks, as distinct from the hot emissions from the young star itself, which is observable with optical telescopes (and even the naked eye that can see starlight).

So in the search for young planets forming in the dust and gas still present after the birth of a star, astronomers need a way to better identify the presence of unseen planets and not waste time studying something that turns out to just be an accumulation of dust.

Astronomers know that there are more than 450 planets orbiting other stars outside our solar system, and most are quite old. The main interest is to try to detect very young planets, still forming.

This study of the dust and gas disk surrounding a young star, and the gravity gaps potentially carved by invisible planets, will now play an important support role to the development of a powerful new telescope being built in Chile called ALMA (the Atacama Large Millimetre Array).

Associate Professor Maddison’s team has been investigating what is happening in the dust and gas disk, so that astronomers can better gauge what is creating the observable gaps. “But rather than hunt for the planet itself, our aim is to investigate the formation and structure of a gap in the dust layer,” she explains.

These dust disks last only a relatively short time – a few million years out of a star’s lifetime of about 10 billion years – as its planets accrete to form its solar system.

To help spot newborn planets the team has assembled a complex supercomputer model of the dust and also the gas surrounding a new star. This model can simulate what is happening in a protoplanetary disk of dust: “We can add a virtual planet to our model and watch how the dust and gas disks behave in response,” she says.
“This gives us a recognisable ‘signature’, which astronomers can then compare to what they are observing in real planetary disks.
“It helps them to distinguish between gaps due to dust grain growth or, more excitingly, gaps caused by young planets in the process of forming.”

The subtlety of the new technique is based on the fact that both gas and dust in a solar disk respond differently to the presence of a massive body.

By combining the two signatures they give off around a gap, the team’s technique can say more definitively whether a young planet is the cause.

“What separates this from previous work is that a planet has a different effect on the dust than on the gas in the disk,” Associate Professor Maddison says. “Previous models just looked at the gas, but it turns out that planets have a stronger effect on the dust phase than on the gas phase.

“This is great news, because ALMA probes the dust. Our simulations now show we’re much more likely to detect planetary gaps than we previously thought.”

She says this will let astronomers know what mass planets they can detect and out to what distance. “We’ll be able to catalogue possible disk/planet systems that ALMA can detect … everyone will be wanting to get their hands on the telescope, to look at their favourite disk and hunt for planets.”

Associate Professor Maddison says that one of the exciting aspects of the development is that the simulations being developed are able to predict what observers will ultimately see. “We had that experience in 2005 when our simulation predicted the mass and location of a planet in the dust disk of the young star, Formalhaut.

“When a planet was reported in 2008, very close to our prediction, it was a great buzz! We hope this work will be similar.”

The team’s new technique will take astronomers for the first time across the threshold of the birthing suite of the planets, assisting them to observe the fascinating processes as a planet similar to our own Earth gradually assembles itself from tiny grains of matter too small to see to become a giant object potentially capable of sustaining life.

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