In today’s world, one of the more useful gifts for a scientist to have is the ability to save money. Enter the Jet Propulsion Laboratory’s Wesley Traub, who copes with problems like NASA’s indefinite hold on Terrestrial Planet Finder with a low-cost alternative of his own. Last year, Traub and crew experimented with the Solar Bolometric Imager, an observatory lofted by a balloon to altitudes of 35 kilometers and more. Their study of air distortions at those altitudes convinced Traub that the balloon’s movements through the stratosphere would not distort received images, and that led to speculation about doing exoplanet science close to home.
A balloon-based TPF? Hardly, but Traub does talk about imaging perhaps twenty exoplanets, according to a recent story in New Scientist. The method: A coronagraph teamed with a one to two-meter mirror. The so-called Planetscope weighs in at $10 million, making it a bargain when compared to space-based observatories, and cheap enough to tempt experimentation, even though a full-blown space mission would obviously offer far higher levels of performance.
Traub’s presentation to the American Astronomical Society’s annual meeting in January explained how Planetscope’s coronagraph could block the glare of the stars being investigated while allowing light from their planets to be detected, opening up the possibility of spectrographic studies of distant atmospheres. With both TPF and Darwin coping with budgetary issues, not to mention technological questions in need of resolution, Planetscope could become a useful stop-gap, just as coming observatories will open up new options from the ground.
Not nearly as inexpensive but nonetheless well below some Terrestrial Planet Finder estimates is another mission Traub has championed called Small Prototype Planet Finding Interferometer (SPPFI). Here we’re talking about a passively cooled two-telescope space interferometer operating at the L2 point in near- to mid-infrared wavelengths. The team investigating this one believes that it can be used to study the atmospheres of non-transiting exoplanets and perhaps taken still further:
Clearly such a mission concept has sufficient sensitivity to detect and characterize a broad range of extrasolar planets. If the telescopes are somewhat larger than has been discussed in some of the exisiting mission concepts (e.g., 1-2 m) and are somewhat cooler (e.g., < 60K) so that the interferometer can operate at longer wavelengths, it is possible for the SPPFI system to detect earth-like planets around the nearest stars. This is especially important now that there is an increasing belief that lower mass planets are very common, based on the detection of the 5.5 Earth mass planet using the microlensing approach...
A mission like this one comes with a price tag of $600 to $800 million and offers the opportunity to study not just exoplanets but the debris disks around stars we’ll later want to look at with missions like Darwin and TPF. We’re clearly ready for these next steps. Radial velocity methods have steadily improved, to the point where we can now find planets not only of Saturn mass but that of Neptune and Uranus, with improvements expected in the near future. Microlensing and transit studies both offer the chance to spot smaller, rocky worlds. Given our budgetary constraints, concepts that can take us to the next level and set the table for the breakthrough observatories we all hope for have to be affordable.
Which is why work like what Traub’s team is doing deserves your attention. Think affordability. Right now NASA has funded nineteen teams for studies for future observatories, a total of $12 million in fiscal 2008 and 2009 that includes a concept Centauri Dreams has always admired both for its technology and its budget, Webster Cash’s New Worlds Observer. Also under scrutiny is a study of direct imaging of giant planets around nearby stars using 2-meter class optical space telescopes. Study results for these latter mission ideas are expected in March of 2009.
For more on the work of Traub’s team, see Danchi et al., “Towards a Small Prototype Planet Finding Interferometer: The next step in planet finding and characterization in the infrared,” a white paper for the Exoplanetary Task Force. On 2-meter class optical space telescopes, see especially Stapelfeldt et al, “First Steps in Direct Imaging of Planetary Systems Like our Own: The Science Potential of 2-m Class Optical Space Telescopes,” also submitted to the AAAC Exoplanet Task Force (abstract). We’ll follow all these mission concepts as they make their way through the system.