Astronomy Now Online


Planck and Herschel Exclusive Interviews

Bill Dent

(UK Astronomy Technology Centre)

Principal Investigator for the 'Gas in Protoplanetary Systems'

Herschel Open Time Project

Artist impression of a protoplanetary disc: a very young star encircled by a disc of gas and dust, the raw materials from which rocky planets such as Earth are thought to form. Image: NASA/JPL-Caltech.

Based on the summary I've read, you plan to study the transition between gas-rich and gas-poor protoplanetary systems by observing several hundred different objects. Could you briefly summarise the main differences between these two scenarios and explain what the differences are in terms of the planets that these two extremes could form?

Just to make clear, we're not looking at the young planets themselves, but instead the regions and environment in which they will be forming. The so-called "protoplanetary systems" that we're looking at are systems around young stars which are likely to contain planets in the process of formation. The whole question about planet formation is not well understood - we know that they form from disc-shaped clouds of gas and dust around young stars. They initially are made mostly of gases like molecular hydrogen, carbon monoxide, water, etc, and contain very small amounts (about 1%) of dust particles. But the dust particles are important - they are thought to be the ¨seeds¨ from which all planets form (the Earth is really just a big dust particle!). We don't understand how these initially gassy clouds evolve (over a few million years) to form planetary systems that we know about. A planetary system contains perhaps a couple of giant planets (like Jupiter), some rocky Earth-like planets, and asteroids and dust; there is very little (if any) of the gas left. Planetary systems are mostly dry and dusty. So we are trying to understand how the fraction of gas in these systems drops from 99% to a negligible amount over a few million years. This evolution is important to understand, as the amount of gas left in the system affects the planets that can form there - for example, a gas-rich system can easily form gas giant planets like Jupiter-Saturn. A dusty, gas-poor system clearly can’t - there just isn´t enough gas left. However, in these dusty systems, rocky planets like Earth could still be forming.

What properties will you measure for each system and what do you expect this to tell you about how protoplanetary gas eventually forms planets?

The transition from gas-rich to gas-poor systems, which we will be studying with Herschel, is particularly interesting. During this period, Earths-like planets can still be forming inside them, and the amount of so-called ¨volatile¨ materials (particularly water) on these planets could be affected by the environment where they form. In other words, if there's still a lot of water remaining in the protoplanetary disks, there could be abundant water on the rocky planets. Herschel will, for the first time, allow us to make a detailed study of these systems and measure the amount of water in the planet formation zones. With Herschel there are three main species we will be observing: Water (in particular, warm water, at around 100-400K), Oxygen and Carbon. The abundance of all of these affect the formation, evolution and the eventual makeup of the young planets.

The summary says you plan to observe clusters in the age range of 1-30 million years. What different features would you expect to see in the circumstellar discs across this age range?

Most stars in systems which are 1 million years old still have their surrounding gas-rich disks around them. They can have 10 or even 100 jupiters worth of gas and dust where planets can easily form. By 30 million years, it's all over - the gas has gone, the planets have formed. There are just small amounts rocky, dusty, dry debris left behind.

Are you expecting to uncover new planets as part of the survey?

No - we are looking at whole planetary systems and the planet-forming environment. Looking for young planets in these systems will need the high angular resolution of ALMA (due to come online in a few years).

Overall, how do you think Herschel will advance this particular area of research?

Herschel is unique in that it can study organic materials like water, oxygen and carbon. The Earth's atmosphere (being full of water and oxygen) blocks the emission from these, so we absolutely need to go into space to see them. We've just not had the capability to observe these in the past. Apart from just being above the Earth´s atmosphere, the other things that Herschel brings is sensitivity and spatial resolution. In other words, the instruments on Herschel need to be sensitive enough to detect the emission from the discs. And the telescope is large enough to allow us to detect the gas from the discs, and separate it from any other contaminating emission (such as from other gas clouds nearby in the sky). By the end of the Herschel mission, we should have a much better understanding of how gas clouds around young stars evolve to form planetary systems like our own.