With the European Space Agency’s DARWIN at least a decade off and funding for the Terrestrial Planet Finder as problematic as ever, what would be a suitable interim mission to extend our exoplanet exploration program? COROT is already flying, with the possibility of detecting large terrestrial planets in close orbits, while Kepler should be able to detect Earth-mass planets in Earth-like orbits by 2013 or earlier. All of which is promising, but we’re still missing key elements of the puzzle.
Take those transiting exoplanets COROT and Kepler track. We’ll retrieve a wealth of data, but we probably won’t be able to get the kind of spectroscopic information we’d like to see from a more advanced mission. Similarly, ground-based telescopes using adaptive optics, and future space missions like the James Webb Space Telescope should show us hot gas giants, but we’ll be unlikely to see planets like our own Jupiter and Saturn, cooler worlds in more distant orbits.
The solution? A team of NASA researchers including Karl Stapelfeldt and Wesley Traub at the Jet Propulsion Laboratory make a strong case for a 2-meter optical space telescope equipped with the latest in coronagraph technology to reveal details of planetary systems that would otherwise be lost in the central star’s glare. A smaller space telescope could be flown at a cost much reduced over the kind of budgets the full-scale Terrestrial Planet Finder would have required.
Among its benefits:
- Measuring the color, taking the spectra and providing astrometric measurements of outer gas giant planets in nine nearby systems known to host planets. We should be able to analyze their atmospheres and measure the depth of their uppermost cloud decks. Factors such as the planets’ albedo and the presence of ring systems should be accessible, as will be planetary size.
- The discovery of new gas giants in the kind of five to ten AU orbit Jupiter and Saturn occupy in our own system. Thirty stars within 25 parsecs that are already known to host close-in planets (detected through radial velocity methods) would be prime candidates for study. Ultimately, such an instrument has the potential to find outer Jovian worlds around as many as 200 nearby stars.
- A 2-meter class instrument in space should be able to resolve rings, warps and asymmetries caused by planetary movement within circumstellar dust disks. With a contrast a thousand times sharper than Hubble’s, such an instrument could be sensitive enough to detect exosystem analogues to our own tenuous Kuiper Belt.
I save the best for last. The researchers believe their instrument may have the ability to detect Earth-sized planets in the habitable zones of five to ten of the brightest, nearest stars. Studying their photons would allow a basic spectral characterization, doubtless motivating intense studies by later, more powerful instruments (and, of course, tuning their target lists).
A mission like this is currently missing from the catalog of upcoming observatories, doubtless because of over-optimism in early assessments about both the growing power of adaptive optics on ground-based telescopes and the possibility of refitting the Hubble instrument with an advanced coronagraph, an idea since abandoned. “Our community can either resign itself to waiting out JWST,” the authors write, “or look for ways to achieve signiﬁcant new exoplanet science, sooner, through more modest projects.”
The paper is Stapelfeldt, “First Steps in Direct Imaging of Planetary Systems Like our Own: The Science Potential of 2-m Class Optical Space Telescopes,” a white paper submitted to the AAAC Exoplanet Task Force (Spring, 2007), available online.