A new observing program designed to study planets around small, cool stars is in the works. TEDI, the TripleSpec – Exoplanet Discovery Instrument, saw first light on the 200-inch Hale Telescope just before Christmas, and is now in its commissioning phase, with an observing program scheduled to begin this spring. And for those who occasionally wonder why we seldom discuss stars like Barnard’s Star or Proxima Centauri in terms of the planet hunt, read on. For TEDI is the kind of program that should be able to survey not just M dwarfs but L and T class stars as well, opening exciting possibilities for discovery.
Planets around Proxima Centauri? Perhaps, and extending all the way down to T-class brown dwarfs makes things interesting as well. But finding such planets is a challenge with conventional radial velocity methods. Here’s why: Radial velocity searches are generally conducted in the optical band, and work well with stars, like the Sun, that are bright at these wavelengths. The small, cool stars TEDI is going after are much brighter in the near infrared than the optical. TEDI is the first instrument specifically designed to study the spectral shifts that flag radial velocity changes — the effects caused by an orbiting planet — in the infrared.
The TEDI program leverages the power of Externally Dispersed Interferometry (EDI), which boosts the power of spectrographs by inserting a small interferometer in the beam. EDI methods can be used on existing spectrographs, creating features in the processed light that extract useful data and boost the resolution of the spectrograph by factors of 2 to 6 times. The Exoplanet Tracker team at the University of Florida used EDI methods to detect a planet around HD 102195, the first confirmed EDI find.
The TEDI project at the 200-inch Mt. Palomar instrument uses EDI coupled with an infrared spectrograph called TripleSpec, built at Cornell University. Developed with the Cornell team and researchers from the University of California, Berkeley and Lawrence Livermore National Lab, TEDI’s expected result is described on the EDI site:
The interferometer will add fringes to the spectrum which will greatly increase the sensitivity to Doppler shifts caused by a planet tugging on the star. The infrared (0.8 – 2.5 micron) sensitivity of Triplespec will allow detections of planets around small cool stars that emit too little visible light to be included in current Doppler searches.
This is exciting stuff because of the ubiquity of smaller stars in our galaxy. M dwarfs alone are thought to account for 75 percent or more of all stars in the Milky Way. That makes understanding the kind of planets that form around them crucial in working our way through various planet formation theories. Perhaps just as significant is the fact that low-mass stars should more readily yield up information about smaller planets once we’re able to refine the methods for measuring their Doppler shifts. Extending radial velocity methods to these stars could allow us to begin taking the measure of stars like Proxima Centauri and its ilk.
Unrelated to TEDI but perhaps of interest is the fact that one reason the Project Daedalus team chose Barnard’s Star as its destination for an interstellar probe was the belief that the star had planets. Astronomer Peter Van de Kamp (Sproul Observatory, Swarthmore College) took 25 years of data on Barnard’s Star before announcing he had detected a wobble that indicated a planet of 1.6 Jupiter masses orbiting at 4.4 AU. As many as three planets were ultimately posited, but John Hershey, also at Swarthmore, was able to show that problems with the Swarthmore instrument were the actual reason for the mistaken detection.
With the radial velocity game much further refined, we can assume no such errors with TEDI. The plan is to observe 100 M dwarfs, 25 L dwarfs and 10 T dwarfs. More on the program and the needed tests for getting it up to speed is available in Edelstein et al., “TEDI: the TripleSpec Exoplanet Discovery Instrument,” slated to appear in SPIE Volume 6693, Techniques and Instrumentation for Detection of Exoplanets III (available online). Thanks to Mike Wirth for valuable pointers on this story.