Fomalhaut exoplanet discovery Q&A with NASA scientist

Posted by Daniel Pendick
on Thursday, November 13, 2008

Fomalhaut bThe discovery and optical imaging of Fomalhaut b, a planet orbiting the nearby star Fomalhaut, has wider implications for exoplanet science. I talked to NASA scientist Marc Kuchner about it. Kuchner works in the Exoplanets and Stellar Astrophysics Laboratory at Goddard Space Flight Center in Greenbelt, Maryland. He uses computer models to study the effect planets have on interplanetary dust.

Kuchner (pictured at left below) and Christopher Stark, a graduate physics student at University of Maryland, recently published a study showing that a planet nearly as small as Mars could carve rings and other structures in interplanetary dust that telescopes on Earth could detect — even if the planet is too small to see directly. Here’s what Kuchner had to say:

Pendick: Tell us a little about your work on exoplanets.

Marc KuchnerKuchner: I do computer modeling and high precision observations of planetary systems. My primary focus is predicting how planetary systems will appear in images, like the ones reported in the press releases this week, and coming up with new ways to image them myself. So you could say I'm one of the main competitors of the teams reporting this week's discoveries. My theoretical predictions include the shapes of debris disks and the compositions of planets. My observational work has focused on new techniques that create the high contrast and high angular resolution needed to image planetary systems directly: coronography and interferometry.

Pendick: What was your first reaction to hearing that your colleagues had imaged an actual exoplanet?

Kuchner: There have been some other images of objects that may well be exoplanets. But this is the first time we have witnessed a planet orbiting a star. That's powerful!

I think that this discovery, along with some others soon to be announced, marks the beginning of a new era — a time when we can look at other planetary systems and see extrasolar versions of our own solar system, like looking in the mirror.

New York Times columnist Thomas Friedman, commenting on the progress of globalization, and the remarkable way our lives are connected to those across the ocean, titled his book The world is flat. Now we are seeing new worlds across another kind of ocean, the space between our solar system and other planetary systems. I'm starting to think, "The universe is flat.”

Pendick: Can you expand a little on those “other images of objects that may well be exoplanets”? Which do you mean?

Kuchner: There have been several good planet candidates detected by direct imaging. Some of them have turned out to be a bit more massive than what most people consider to be the upper limit of planethood. The best candidate before Fomalhaut b is probably 2M1207b, an object with a mass of about 4 Jupiter masses orbiting the brown dwarf star 2M1207 in the constellation Centaurus, about 170 light-years from Earth.

Pendick: We recently covered a study in the magazine claiming that the Gemini North Telescope imaged an 8-Jupiter-mass planet. Should I consider that a really big planet or just a really small star?

Kuchner: With that mass, you would have to call it planet, not a small star. But in general, it’s hard to determine the mass of a planet in a direct image like that, just as you can’t tell the weight of a carton just by looking at it. And unlike the Fomalhaut planet, the Gemini planet candidate was not yet observed to orbit its star. There’s a chance it might just be a background object.

Also, in between those two categories (star and planet), there is a class of objects called brown dwarfs that are like stars in some ways and like planets in other ways. Some planet candidates have turned out to brown dwarfs.

Pendick: The other thing I’m wondering about is possible parallels or differences between the Fomalhaut discovery and your computer modeling work on detecting small planets — perhaps too small for direct imaging — and their effect on interplanetary dust.

It seems that large structures, such as sharp-edged dust rings, could help astronomers target other solar systems for closer study so that we may image and study the actual planets. The dust structures then become a sort of giant billboard saying, “Planet this way!” Is that the common message of your modeling work and the Fomalhaut discovery?

Kuchner: Yes, that's exactly it. Dust rings are the signposts of planetary systems. They are much brighter than planets, and when we see them, we know that there's something interesting going on. Sometimes they even seem to point to right where the planets are located.

Pendick: If rings point the way, how do we observe rings? Does it always work best at the longer wavelengths? For Fomalhaut, it seems they first observed the ring in submillimeter wavelengths. How else will we be able to find the rings? Infrared? Other?

Kuchner: For these kinds of rings, it's often easier to spot them at longer wavelengths, say, the mid-infrared or submillimeter, where they shine in thermal radiation, and where the star they orbit is relatively faint. To see them in visible light, you need to block out the light from the star somehow (for example, using a coronagraph, as Kalas and his colleagues did when they imaged the planet orbiting Fomalhaut).

There are a few new telescopes coming online in the next decade that should be fantastic for finding and imaging these rings: James Webb Space Telescope (JWST), which works in the infrared, and Atacama Large Millimeter Array (ALMA), operating in the submillimeter range (between infrared and radio wavelengths).

I'm hoping we'll also be able to fly the Terrestrial Planet Finder (TPF), which will use the latest in coronagraph technology to block out the starlight. It will allow us to look for rings much closer to the star, in the habitable zone where we might see earthlike planets.

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