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TEDI: Looking for Planets Around Small Stars

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.

Comments on this entry are closed.

  • andy March 10, 2008, 8:47

    A number of exoplanet host stars have distant red dwarf companions, including the multiple planet hosts 55 Cancri and Upsilon Andromedae, and it would be good to know whether these companion stars also host planets. I wonder if such stars are on the target list for TEDI. According to Eugenio at systemic the proximity of the brighter star should not be a problem for these systems.

  • dad2059 March 10, 2008, 12:39

    It’s amazing what improved computer and software technology has on older optical equipment. Quite the cost saving measure.

  • ljk March 11, 2008, 0:00

    CHARA Array Measurements of the Angular Diameters of Exoplanet Host Stars

    Authors: Ellyn K. Baines, Harold A. McAlister, Theo A. ten Brummelaar, Nils H. Turner, Judit Sturmann, Laszlo Sturmann, P. J. Goldfinger, Stephen T. Ridgway

    (Submitted on 10 Mar 2008)

    Abstract: We have measured the angular diameters for a sample of 24 exoplanet host stars using Georgia State University’s CHARA Array interferometer. We use these improved angular diameters together with Hipparcos parallax measurements to derive linear radii and to estimate the stars’ evolutionary states.

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.1411v1 [astro-ph]

    Submission history

    From: Ellyn Baines [view email]

    [v1] Mon, 10 Mar 2008 13:29:40 GMT (62kb)


  • Administrator March 11, 2008, 7:18

    andy, re target stars for TEDI, Jerry Edelstein was kind enough to send the following in response to my question on that point:

    “We are still straddling the line between commissioning and new science which means we will spend time looking at both stars that are known to have no planets, ones that are already known to have them, and with what little time remains, a few unknowns. We need to be sure that our gizmo doesn’t claim false positives or negatives.

    “Your objects [Proxima Centauri and Barnard’s Star] don’t happen to be in our next few-day run. We will develop our science target list as we understand the instrument capabilities and future observing allocations. These types of treasure hunts do not typically publicly release their plans, but I understand you special interest in these very nearby objects.”

    More on all this as things develop.

  • James M. Essig March 11, 2008, 7:39

    Hi Folks;

    I am particularly intreagued about the prospect of finding planets around small stars such as low end mass range red dwarfs. These stars can live upto 10 EXP 15 years and thus can support civiliztions for that length of time. This is a huge time for civilizations to evolve including any future human civiliztions or colonies around such stars.

    Much of the interstellar gas throughout the universe has never been processed into stars and much of the gas produced in supernovas and novas is still hydrogen rich. Thus, there is a lot of gas yet to be incorporated into stars some of which no doubt will be red dwarfs.

    It would be interesting if we as a civiliztion could figure out how to cause interstellar gas clouds to condense into red dwarfs through applications of computational fluid dynamics, finite element analysis, and the like using perhaps very large thermonuclear devices precisely placed so as to perturb interstellar gas clouds into forming protostars associated with red dwarf formations.

    Perhaps utilizing the study of chaos in the fractal sense could be done to most effeciently cause the collapse of such interstellar gas clouds based on sensitivity of the gas cloud evolution on initial condidtions. Thus, there are potentially a huge amount of future red dwarf stellar planetary system homes for humanity.

    If some how, some aspects of the steady state theory are correct such as the gradual creation of hydrogen atoms in the vacuum of space even though the Big Bang Theory might be correct and may have started the existence of space time and mattergy within our universe, we might have all the more hydrogen during epochs of cosmic duration to produce red dwarfs for future human and ETI habitats. Just imagine how advanced we as human beings could become spiritually, intellectually, emotionally, socially, etc., with 10 EXP 15 or more years of evolution, both natural and anthropocentrically artificially based. I can imagine all of the pretty or beautiful female genotypes that would come into existence as the population of humanity soared to over 10 EXP 20 to 10 EXP 30 plus in the comming millions of years.



  • Ronald March 12, 2008, 5:11

    @James: intriguing indeed, although I am just a simple man and hardly worry about anything beyond the next gigayear :-)

    Between 0.5 and 1 gy the earth is expected to leave the CHZ of our sun on the inside. Now that concerns me ;-)