Q&A: Kepler mission — not just for planet research

Posted by Liz Kruesi
on Tuesday, February 24, 2009

Kepler missionNASA will launch the Kepler mission March 5. Kepler will be the first mission able to find Earth-mass and smaller planets. Its main goal is to determine how many exoplanets (terrestrial and larger) lie within (or near) the habitable zones of different types of stars. While exoplanet searches are its main goal, Kepler will perform other science studies. Because the satellite will be observing stars to look for orbiting planets, stellar astronomers will be able to use the data as well. One such astronomer is professor Steve Kawaler of Iowa State University in Ames, Iowa. I spoke to Kawaler, who's a member of the Steering Committee for the Kepler Asteroseismology Research Consortium about the upcoming mission and how it can aid his research.

Kruesi: Tell us a bit about your work with stars.

Steve Kawaler Iowa State UniversityKawaler: Well, briefly, I like to study stars at the extremes — extremes in age, size, and temperament. Ordinary stars like our Sun, while interesting and instructive as to the nature of stars, aren't exactly satisfying to those of us with short attention spans. The stars I like to study are “doing something” — for example, they are periodically varying in brightness or are in their death throes and are rapidly losing mass or otherwise changing. These rapid (or at least active) phases tell us a lot about the times in a star’s life when it is changing state — switching from one nuclear fuel to another or shutting down altogether.

Stars that rotate quickly are also interesting to me. Rapid rotation is usually a sign of youth, and this rotation creates circulation currents within the star. These currents cause some interesting mixing that shows up as chemical peculiarities on the surface, and in other ways.

One of the neat things that we can do with some stars is use their naturally occurring oscillations as probes of their interiors. This is the area of asteroseismology — using “star quakes” as seismic probes.

With naturally oscillating stars, we can get a (sometimes very) detailed peek at their internal structure. If the stars oblige by pulsating at many different periodicities, we can measure the change of their internal composition with depth, their internal rotation rates, and track internal flows driven by convection and other forms of turbulence. These sorts of probes place exciting new constraints on our models of stars — and can lead to advances in modeling that have impacts across a wide swath of astrophysics and physics in general.

Kruesi: So, how does this tie in to the upcoming Kepler mission?

Kawaler: Well, in asteroseismology we need to measure many periodicities in the light variations of stars. The more periods we see, the better. Unfortunately, studying asteroseismology from Earth is difficult because Earth’s rotation prevents us from discriminating all of the periodicities. Think about trying to listen to a complicated piece of music (Beethoven, Bob Dylan, Moby...) but only hearing every third note or so. That is what we’re up against when trying to measure stellar oscillations. We’ve tried to overcome this from Earth by using networks of telescopes across the globe (like the Whole Earth Telescope) to follow stars continuously for up to 2 to 3 weeks at a time, but we still suffer from weather problems and other technical issues.

With Kepler, however, we will have an instrument that is continuously monitoring more than 100,000 stars for 3.5 years with no significant interruptions. Combine this exquisite time sampling with the much higher precision that can be obtained from above the atmosphere, and you have, in Kepler, the ideal platform for measuring stellar oscillations for seismological studies.

Kruesi: Kepler’s main goal is to search for exo-Earths, correct? Will your research work in conjunction with this main goal?

Kawaler: Indeed, the raison-d’etre for Kepler is to use this ultra-high sensitivity to look for transits of earthlike planets around Sun-like stars. The expectation is that the project may find 100 or so earthlike planets, along with many more planets that are not earthlike. But by enlisting a team of asteroseismologists, the Kepler project will get tremendous extra value out of the data on tens of thousands of stars that don’t show planetary transits.

For those stars that are found to host planets, if they also show oscillations, we can use the oscillations to derive very precise fundamental parameters for the host star. Most importantly, we can determine the radius of an oscillating star to high precision. Knowing the star’s radius, and the duration of various features of the transits, we can therefore translate the observations into high-precision measures of the newly found planets’ properties.

Kruesi: Astronomers study stars using ground-based telescopes and also the Hubble Space Telescope. How will Kepler be any different?

Kawaler: The power of Kepler is in its consistency — it will be able to look at a single, large (about 10 square degrees) field of stars without interruption for 3.5 years. At the same time, the brightness measurements will be as precise as 1 part per million. This precision level will exceed, by two orders of magnitude (a factor of 100), the precision level that we can reach from the ground.

The Hubble Space Telescope can, in principle, reach this level of brightness precision, but the time base of the observations is always interrupted on the time scale of its orbit around the Earth (90 minutes) making a long time-series impossible. Plus, the Hubble is way too valuable a machine to dedicate to monitoring a single field of view for more than a few orbits. As exciting as the search for extrasolar Earths is, the Hubble is vital for a wide range of astronomical targets, so it is best left to those pursuits.

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