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"As
soon as the Sun set, I opened the camera shutter and began collecting
starlight...This became a personal quest - me against the star. Interview with Dr. Debra Fischer, Planet-Hunter Dr. Debra Fischer began hunting planets orbiting around other
stars theWoman Astronomer: Dr. Fischer, you have a very exciting career searching for planets orbiting around other stars. How did you get into this area of research? Dr. Debra Fischer: It is some combination of curiosity and luck. Curiosity led me through my course of study in science and physics to pursue a graduate degree in astrophysics at UC Santa Cruz. But, the luck part was really important too. As I was finishing grad school, my former thesis advisor in physics at SF State, Geoff Marcy, and colleague Paul Butler had just confirmed the detection by Michele Mayor and Didier Queloz of a planet orbiting 51 Peg. Paul was heading off to the Anglo Australian Telescope and he and Geoff were starting a program at Keck Observatory. So, I joined as a postdoctoral fellow, managing the Lick Observatory program. theWoman Astronomer: To the best of my knowledge, extrasolar planets are mainly found by using one of two methods, either measuring the wobble created by the planet’s gravitational tug on the parent star or by measuring the reduction of light from the star as the planet transits in front of it. Which method do you use and why? Dr. Debra Fischer: There are a couple of other techniques that are starting to reach maturity now - notably microlensing. However, the first technique is the one you describe, called the "Doppler" technique. We monitor the velocities of stars on our samples, searching for tiny gravitational tugs from the orbiting planets. My experience is with this technique, and that's what I've used. The Doppler method has detected most of the ~250 known exoplanets. However, when we find a good candidate, we pass the information to our friends who can measure the reduction of light from transiting planets. The best candidates are the ones that orbit extremely close to their suns - these planets are most likely to exhibit transits. theWoman Astronomer: Where is the closest extrasolar planet to Earth? The farthest? Have any extrasolar planets been directly imaged? Dr. Debra Fischer: The closest star with a detected planet (in fact with three planets) is GJ 876. The most distant detected planets come from deep photometric searches and are more than 3000 light years away. Their host stars are so faint that it is difficult to do any other follow-up science with them. Imaging planets is the new Holy Grail and bonafide images with several pixels on the planet probably a decade or more away. theWoman Astronomer: What was the first planet you discovered? Would you describe what such a discovery meant to you? Dr. Debra Fischer: The first planet I discovered at Lick Observatory was orbiting the star HD 217107. At the same time, I found another one orbiting the star HD 195019. These were among the 300 stars that I had added to the program at Lick so it was incredible to have that success. I felt like I was adding to the sum of human knowledge - an inheritance for my children and the children of the world. theWoman Astronomer: There have been over 200 extrasolar planets discovered to date. How many have you discovered and do you have a favorite? If so, what is special about this particular planet? Have you given it a working name other than its official designation? Dr. Debra Fischer: In 1998, there was one star on the Lick program that was particularly special: Upsilon Andromeda. Geoff and Paul had discovered a planet there that was incredibly close to the star. It took only four days for this giant planet, about 250 times the mass of our Earth, to complete one circle around Upsilon Andromeda. However, they could see that there was something odd about the velocities - an upward trend that suggested the presence of a second, more distant planet. I was under orders to be vigilant in collecting data on this star, and I took this very seriously. As the outer planet was completing its orbit, I would work during the day in my office and dash up to the Observatory - a 2-hour, winding drive. We were losing Upsilon Andromeda - from our perspective, it was moving behind the Sun (of course it is really the Earth that was moving around the Sun). So, I would take all the calibration images before sunset and set the telescope on the star. As soon as the Sun set, I opened the camera shutter and began collecting starlight that the instrument turns into a spectrum. This became a personal quest - me against the star. I would immediately run the analysis and plot up the next velocity point. With the data in hand, and Upsilon Andromeda finally behind the Sun, my next job was to create a mathematical model that described the velocity measurements. At the same time a team at Harvard, led by Bob Noyes, called Geoff Marcy and said that they had data also showing the second planet and they decided to write a joint paper. But, I was struggling with the 2-planet model and feeling quite dejected because I couldn't get a good fit. To look at the noise in my fit, I subtracted my best 2-planet model from the real data and couldn't believe my eyes. A clear, coherent wiggle remained, looking for all the world like another planet. I changed my model to include three planets and found a perfect fit. The three planet model also worked better on the data from the Harvard team. The result was picked up in the newspapers. Perhaps the best part of the story is that I then received a letter from a 4th grade class in Moscow, Idaho. They had been studying astronomy, and their teacher told them about the discovery. The students wondered if we had named the planets yet - of course, they had suggestions. The "little" planet in the system was the one found by Geoff and Paul in 1996 and the students wanted to call this 250-earth mass planet "Dinky." The next planet (the unexpected third planet I had modeled) was two times the mass of Jupiter so they suggested "Twopiter" as a name. And the outer planet that caused me to chase the star as it fell behind the Sun was four times the mass of our Jupiter (more than 1200 Earth masses) so they recommended "Fourpiter." The names were so clever that they stuck! The thing I loved about this discovery, beyond the fact that I finally succeeded in modeling it, was that it was the first star where more than one planet had been found. It showed us that gravitational domains (kind of like personal space for people) were pushed close together, just like the planets in our solar system. It was our first clue that planet formation was truly a robust process. theWoman Astronomer: Most of the extrasolar planets discovered so far have been massive, many times the size of Jupiter and Saturn, with orbits very close to their parent star. What does this tell us about solar system dynamics? What are the implications for our own solar system? Dr. Debra Fischer: Most of the planets that have been found have been big ones - not many times the mass of Jupiter, but typically Saturn (1/3 Jupiter) to 1-2 Jupiters in mass. The fact that there are not many planets that are "super Jupiters" tells us that the protoplanetary disk, where planets are born, generally give rise to the types of planets in our solar system. In the same way that a 15-pound baby is rare, planets that are several times the mass of Jupiter are exceedingly rare. As we look harder, we see that the lowest mass planets are the most common. We find that planets in multi-planet systems are "packed" together to the edge of gravitational stability. This is true in our solar system as well. Simulations show that in about 4 billion years, some of the small planets (Mercury and then Pluto) are likely to be ejected from the gravitational jostling of the other planets. But, of course, the IAU already ejected Pluto as a planet! theWoman Astronomer: The first extrasolar planet with possible Earth-like properties was recently discovered orbiting Gliese 581. With over 200 discoveries since 1995, why did it take so long? What does this planet tell us about our own solar system? Dr. Debra Fischer: The Doppler technique relies on the ability of the planet to move the star around. The smaller the planet, and the further it resides from the star, the weaker the tug. Our measurement precision is now 1 m/s. For stars that are waltzing with partner planets at speeds faster than 1 or 2 meters per second, we can now detect the perturbing planet. But Earth exerts only a 0.1 m/s tug on the Sun. We are a long way from being able to detect that! I've hit upon this idea that planets in our solar system and (in some cases) other solar systems are pushed as close together as they can be and still remain in stable orbits. That's because I think this is a fundamental characteristic, inherited by the way planets form in their disks. This is speculation - I could certainly be wrong - but my gut instinct is that this is probably right. If right, take the (correct) statement that about 10% of the stars have detected "Jupiters." What about the other 90%? In systems where we just detect one "Jupiter," what else is there? How would our own solar system look to us if we lived on a planet orbiting another star? Answering my last question first, our technique might have detected Jupiter, but would never have detected any of the other planets in our solar system. If planets are typically stacked up, then it suggests that (undetectable) Earths are common. The detection of GJ 581 (b,c, and d!) are scratching down to the low mass regime and supports the idea that rocky planets are common. theWoman Astronomer: What is the future of extrasolar planet hunting? Dr. Debra Fischer: It's hard to say. Given support from funding agencies, we could continue to improve our precision and find lower mass planets. The perfect compliment to our work is a space born mission to do "astrometry," measuring wobbles of stars in the plane of the sky with unimaginable, unprecedented, but completely attainable precision. These techniques (astrometry plus Doppler) will work together and they would find Earths around the closest stars in "habitable zones," where liquid water could exist. But, in an era where NASA is planning a return to the Moon and manned missions to Mars, funding for science may be cut to the bone. It's tough for scientists who invest the prime decades of their careers to have missions pulled from under them. Yet, this happens regularly. The Stratospheric Observatory For Infrared Astronomy (SOFIA), the Space Interferometry Mission (SIM), the Keck Interferometers, the Terrestrial Planet Finder (TPF) are some examples that I've seen struggling to stay upright in the current political winds. To be fair, most missions run over cost. Once a mission is selected by NASA, contractors may raise their prices or the engineers encounter unexpected problems. This creates a nightmare for the management of these projects by agencies like NASA. Nevertheless, the system is very political, changing with 4-year election cycles, and that's a tough way to do science. theWoman Astronomer: Would you please describe a typical day in the life of an extrasolar planet hunter? What do you find most rewarding in this career? Are there any disappointments? Dr. Debra Fischer: Maybe what I like best is that there isn't a typical day.
What don't I like? As you could probably tell from my last answer, I find the politics of science to be distressing, not for myself, but for my colleagues who are more affected and less able to speak out. theWoman Astronomer: If someone wanted to pursue a career in planet hunting, what would they need to do? Are there particular areas of study they should pursue? What degree programs are best suited for a career in planet hunting? Dr. Debra Fischer: To be a planet-hunter or an astronomer, you need to take as much science as possible. Undergrads should take mathematics, biology, chemistry, physics, astronomy. But, don't neglect philosophy, languages, literature, music and any other classes that interest you - they'll help you think "outside the box." There is no question that most graduate programs prefer students with undergraduate degrees in physics, and you'll need the Physics GRE (in addition to the regular GRE) to apply to doctoral programs. If you need to strengthen your background, consider a Masters program in Physics - SF State has a terrific program and great success at helping students move on to top rank institutions. So, the next step is obviously grad school where the terminal degree is a
Ph.D. You should aim to establish your expertise - grad students are often first
author on 2 - 5 papers by the time they graduate. theWoman Astronomer: Dr. Fischer, thank you for your time. You are truly an amazing woman astronomer! |
Updated 01.01.2008 |
as a graduate student at the University of California, Santa Cruz. She
is currently a professor of astronomy at San Francisco State University and
continues her quest for worlds around distant stars. She was featured in Timothy
Ferris' recent PBS film, "Seeing in the Dark." Dr. Fischer is an
amazing astronomer and an incredible woman. She took some time from her busy
schedule to answer a few questions about her research and her life as a woman
astronomer. 