Interview with Neville Woolf: Part II

SIRTF will be able to detect and characterize circumstellar disks around nearby stars, providing key information about the formation of ‘extrasolar’ planetary systems.
Credit: NASA/JPL

Astrobiology Magazine: You mentioned earlier [in Part One of this interview] that you would be using several different techniques in an attempt to detect planets around other stars. One of the research areas your team will be focusing on in that search is dust around young stars. Can you describe that work?

Neville Woolf: Well, we’ve got a few different handles on that. One is that the Space Infrared Telescope Facility (SIRTF) is hopefully going to be launched in August. And we have a link to the work there. We’re part of the team that will be collecting all of the historic data from it and trying to put it together. In particular, there will be a study of stars of a variety of ages near to the Sun, and a search to find out what exists in the way of dust around them, whether the dust shows signs of being chopped off in places, as it would be if planets were there and sweeping out the dust.

AM: Is this like the gaps in the rings of Saturn?

NW: It is very much like the gaps in the rings of Saturn. The Cassini division [in the rings] is caused by Saturn’s satellite Mimas, and in the same way a dust disc around a star will have gaps cause by resonances with planet orbits.

And then there is another part of it, which will be done from the Kitt Peak Observatory, where they will be looking for spectral features. The gas shows up well in the places where the dust has been swept out. And so we’ll be using that as a tool for looking for gaps in the rings. Most of the dust-disk work will be concerned with gaps.

Saturn’s satellite, Mimas.

AM: Once you find the gap, will you be able to detect a planet in the gap, or are the stars that you’re studying too far away for that?

NW: We’re not quite sure at the present time how the pieces will fit together. Sometimes it will be possible, sometimes it won’t. Sometimes we’ll have to change wavelengths to look for gaps where there is a chance of seeing planets. To put together a complete picture of a planetary system, you need to learn about planets that are close in, planets that are further out, and so on. It’s really a mixture of techniques that have to be used.

But we really don’t have any feel about the planetary systems that have been detected to date. How far out from the star do planets go? Are there planets that are somewhat lower mass than the ones that we’re seeing? Even lower mass planets would cause gaps in rings and might still be bright enough to be seen on their own. But they don’t cause much effect in the radial-velocity spectroscopy, which is the current way of detection. So the new work will be a way of extending our current knowledge.

AM: Will this study help in understanding the evolution of planetary systems?

NW: I would expect so. In general, the dust one sees in a planetary system has not persisted for very long. It’s made by breaking up of small objects, like asteroids, and they break up because they’re stirred up to collide with each other by giant planets, which then also shape the orbits of the dust residue. The dust does not last long. I hope the work will help to understand how planetary systems evolve with time.

We’ve been trapped with so little and fragmentary knowledge. We’ve felt like the blind men trying to decide what an elephant is, one feeling the side and another the trunk and another yet the tail. And we’ve got to try to put the pieces together. And so we are hoping that our study will start this process of putting the pieces together.

AM: You also plan to search for interstellar molecules that may be relevant to the origin of life.

The Arecibo radio telescope is currently the largest single-dish telescope in the world used in radio astronomy.
Credit: NAIC – Arecibo Observatory, David Parker / Science Photo Library

NW: [That is] the third piece [of our research]. There seems to be a process that collects together the dust and molecules of space and turns them into comets and brings them into planetary systems. And while it’s nice that the material does come in, it’s only possible to determine what it is either when it’s out in space or when you end up with a solid lump on the ground [as a meteorite]. And, of course, being part of a planetary system that’s almost five billion years old, quite a lot has happened to the stuff [that has fallen to Earth] by now.

So we’d like to look out into space with radio telescopes to try to determine what is out there. We think that sugars are particularly important. And the reason for that is they don’t tend to form very well under acidic conditions. Yet they are needed for early life, and we think that early life formed under acidic conditions. So it might be helpful to life starting if the sugars were available early, and life only had to learn a little bit later how to make sugars. So we think it would be very interesting to find whether sugars are formed in space and whether they enter the solar system in comets.

There has been already an indication of a first sugar in space. It hasn’t been one of the key biologically interesting ones, but we think as we go on, improve identifications, increase identifications, that it will be possible to find many, many more complex organic molecules out there in space [that would have been] collected and come into the early Earth.

The problem with looking for sugars in the radio spectrum is that there are a huge number of unidentified spectral lines. So you need laboratory studies of all the wavelengths where a given substance will emit spectral lines. And you have to observe many lines with high wavelength precision to decide whether a particular molecule has been detected. And then you can finally straighten out exactly what you’re seeing, what materials are out there. So it will be a combination of observational work with radio telescopes and lab work to identify what spectral lines are associated with what particular molecules.

AM: Will you be looking in any particular region of space?

The constellation Orion.
Credit: Weber State University

NW: There are some regions in space which are particularly dense concentrations of interstellar molecules. There are some roughly in the direction of the galactic center, but there are other regions like Orion where you can see things, too. And, certainly, a lot of the study of the molecules will be in the directions of the dense, dark clouds where these molecules are found most easily.

AM: What is the procedure you use in the lab work?

NW: In the lab work you have to get the molecules into a vacuum and excite them so that they will radiate all of their spectral lines. You then detect all of these spectral lines, [measure] their various strengths and [see] how the wavelengths are related. And that allows you to then go back and look at the big table that you’re creating of unidentified spectral lines in various places. The reason that progress hasn’t gone further has been mainly that there hasn’t been the appropriate lab work to combine with the observations from the telescopes.

AM: Once you have constructed your library of spectra, do you use primarily intuition or computer searches to match them up with the telescope observations?

NW: It has to be a massive computer search program. One of the good features about it is that the spectral lines are extremely sharp and therefore can be measured to very high precision, but the concern is that [you have to] match all the lines. You may need a simultaneous detection of, say, 20 spectral lines from a complex molecule to be sure you have found it.

AM: And I assume it’s complicated by the fact that you usually detect lots of molecules at once.

NW: You have lots and lots of them, and this is why it’s a great untangling process. But we think the work is important because the material that came to Earth is one ingredient of the way that life was able to get started. And until we know what came to Earth, we can’t ask some key questions.

Editor’s note: The University of Arizona NAI lead team will also conduct a winter school for each of the next five years. The school will invite graduate students – biologists, geologists and chemists, as well as astronomers – to spend the winter semester in Tucson working collaboratively with the astronomers on the NAI team and researchers in a broader range of astrobiological fields.