Elevating exoplanet research to its own division within NASA’s science directorate could enable more funding for future missions and cooperation with other aspects of the agency, like human spaceflight to service proposed starshade missions.

Future exoplanet missions: NASA and the world (part 2)

by Philip Horzempa
Monday, October 15, 2012

[Editor’s Note: Part 1, covering several other potential exoplanet missions, was published last week.]

Euclid and WFIRST

There are two dark energy missions that will obtain data on the abundance of exoplanets. The first of these, Euclid, is an ESA mission that has been approved and funded, with a launch targeted for 2020. It will feature a one-meter aperture telescope that will image distant galaxies in the infrared in order to detect the influence of dark energy. As a secondary goal, Euclid will conduct several observing runs looking for the signature of exoplanet microlensing events, making it the first space mission to utilize this approach. Microlensing refers to the fact that as a planet passes in front of a distant star, its gravity will bend that star’s light, as a consequence of Einstein’s general theory of relativity. This bending effect causes a temporary brightening of the distant star, acting as a virtual lens. Given the small mass of planets, the brightening is minimal but detectable. A space telescope that can stare at one patch of the Milky Way for weeks or months is the best way to observe these fleeting and unpredictable events. The importance of microlensing is that it can detect all types of exoplanets at all distances from their parent star. A mission such as Euclid should be able, as a result, to acquire valuable statistics on the architectures of solar systems.

In addition, as Euclid is looking for microlensing events, it will also be able to detect transits like those observed by Kepler. Euclid will observe in the near-infrared, and so is optimized to look for planets around cooler M-class dwarf stars. However, Euclid’s target star field is crowded and the target stars are faint, making transit detection much more difficult than with Kepler. Still, this capability will add to the database of exoplanet abundance.

The Wide-Field Infrared Survey Telescope (WFIRST) is NASA’s version of a microlensing mission. It has languished with almost no funding due to the fiscal effect of the James Webb Space Telescope (JWST). However, a recent development may pull it out of limbo. Earlier this year, the NRO presented NASA with a couple of gifts: two large mirrors polished and qualified for a space mission! Apparently, they were “leftovers” from a cancelled reconnaissance project. Within the past few months, NASA has requested proposals to use these mirrors in a mission. One of the prime candidates would be the WFIRST project. In addition, WFIRST may include an internal coronagraph that would block much of a star’s light and allow study of its surroundings. This, in turn, would enable it to measure the density of dust clouds in other solar systems. Such observations are valuable precursor data for direct-imaging missions, such as New Worlds. However, this mission may not fly until years after the launches of JWST and a formation-flying starshade. Since a precursor must fly before the mission it supports, the exoplanet community is looking for other, near-term, methods to obtain this data.

Small missions

There are a number of smaller exoplanet missions in the pipeline (see “Exoplanet explorers”, The Space Review, January 12, 2012). Of these candidates, the Transiting Exoplanet Survey Satellite (TESS) transit-detection mission seems to have the best chance for approval as it is in its second round of proposal review. TESS will complement Kepler’s transit search, expanding it to the whole sky as opposed to Kepler’s narrow target field. Like Kepler, it will help in assembling a census of solar system architectures in the galaxy. TESS will not use a high-precision photometer like Kepler but instead a bank of nine cameras. This results in a cheaper mission, but one that is still able to monitor the brightest two million stars in the sky, including many close stars. Kepler’s search is narrow and deep, while TESS’s will be wide and shallow. TESS’s design now allows for detection of Earth-sized planets around nearby stars. So, if we are fortunate and Nature has provided a nearby Earth analog that transits its star, we may “land” the Holy Grail of exoplanet research soon. However, the odds are that such a world will not be so favorably aligned, and will only be detected by the New Worlds or NEAT missions. ESA has a proposed mission, PLATO (PLAnetary Transits and Oscillations of stars), that would also observe the sky with a bank of cameras. However, it would also be capable of Kepler’s high-precision photometry of its target stars. PLATO was passed over in ESA’s most recent M-Class selection, but is a top candidate for the next selection opportunity later this decade.

Another of the smaller missions, the EXoplanetary Circumstellar Environments and Disk Explorer, or EXCEDE, has as its goal the measurement of the reflected light from the clouds of very fine dust in other solar systems. In our system, this is known as zodiacal light, and exozodiacal light in other systems. The intensity is crucial to direct imaging missions, as exozodiacal brightness, much like a fog bank, may interfere with capturing photons from actual exoplanets. There is hope that EXCEDE can be funded and launched to ascertain dust levels in other solar systems by the end of this decade. However, considering the budget squeeze, an even cheaper, but simpler, version of EXCEDE may be ready sooner. Zodiac-II is a balloon-borne telescope that could obtain exozodiacal measurements for a limited number of star systems more cheaply than an orbital mission such as EXCEDE. Its results could support the New Worlds mission, allowing the starshade project to target the most promising star systems.

Another balloon-borne telescope mission is the Big Balloon Exoplanet Nulling Interferometer (BigBENI). This project would fly a pair of one-meter telescopes to high altitude. Its goal is to test the concept of a nulling interferometer. This method combines the light from two telescopes in such a way that the light waves from a star are cancelled, or nulled, out (wave crests canceling out wave troughs). With the intense light of a star zeroed-out, the more feeble light of an orbiting planet can then be detected. This idea has been proposed over the years, but never tested in space. BigBENI is a low-cost, low-risk way of accomplishing those tests.

An ongoing project that is using data from a non-exoplanet mission (much as Euclid will) is the STEREO TRansiting Exoplanet and Stellar Survey, or STRESS. The twin STEREO (Solar TErrestrial RElations Observatory) spacecraft were launched into solar orbits in 2006 to observe the Sun. As a byproduct, imagers on the probes also provide wide-angle images of the sky. About one million stars are in the field of view. The images can be used to monitor those stars, looking for telltale dips in brightness that may be a sign of planetary transits. The data are not as robust as from Kepler, but sufficient to indicate the presence of Jupiter- or Saturn-mass planets.

There are also a few less well-known missions that could contribute to the search for exoplanets. The first of these is a US Navy mission, the Joint Milli-Arcsecond Pathfinder Survey (JMAPS). Unfortunately, it was recently cancelled because of DOD budget cutbacks. Its mission was to provide data to update the star charts that were produced by the Hipparcos spacecraft 20 years ago. Its primary purpose was to support the mission of the United States Naval Observatory to provide accurate star maps for national defense needs. However, one of the byproducts of the JMAPS data would have been astrometric measurements of nearby star systems that may have shown the existence of Jupiter-class planets. It would have complimented the Gaia mission since these nearby, brighter stars would saturate Gaia’s CCDs.

However, while JMAPS has been cancelled, its legacy lives on. Its astrometry camera may be used by the New Worlds starshade mission to zero-in on the position of the JWST, allowing the shade to occult its target star. That camera, however, needs to be developed and that is where a program named OPTIIX (Optical Testbed and Integration on ISS experiment) comes to the rescue. The JMAPS camera is being provided to the OPTIIX team in exchange for that team’s effort to raise its technology readiness level. Progress in this field can come in small, and unexpected, developments such as this multi-project cooperation. OpTIIX is an audacious project that plans to assemble a segmented telescope using the ISS as its testbed. Its goal is to demonstrate that a large optical system can be robotically assembled in space cheaply and at low risk. The pieces should arrive at the ISS in 2015, pending sufficient funding. If the OpTIIX project can show that large, segmented space telescopes are affordable in the short-term, then this will make future exoplanet characterization missions seem more plausible. Large apertures will be necessary at some time in order to obtain better spectra and better images of exoplanets.

Besides the usual players in the search for exoplanets, there are signs of interest from other countries. Japan’s space agency has an ongoing program to launch astrometric satellites into space. The JASMINE (Japan Astrometry Satellite Mission for INfrared Exploration) project will begin with the launch of Small-Jasmine in 2014. It will conduct a mission similar in scope to JMAPS, but will search in the infrared. This will allow it to look at galactic bulge stars. Being an astrometric telescope, it may be able to detect Jupiter-class planets around nearby stars, and, like the abandoned plan for JMAPS, it will complement Gaia by observing brighter stars. Small-JASMINE will be Japan’s pathfinder in this arena and will be followed by JASMINE, a more capable craft that will also observe in the infrared.

China’s space program is growing in capability every year, and now there are signs that it is interested in launching an exoplanet mission. It has invested in several significant ground-based telescope projects, has hosted an exoplanet conference, and will soon launch its first X-ray space observatory. In addition, it has plans to launch a solar polar mission in 2019. That would include a flyby of Jupiter in order to adjust its solar orbit. At an exoplanet workshop last year, China revealed that it is planning to build the SAT, the Satellite Astrometry Telescope. Not much is known about its design, but indications are that it will be capable of exoplanet searches, possibly using an interferometer. In Russia, Roscosmos has published their outline for space projects for the coming decade. Funding for the Russian space program is erratic, so it is difficult to determine the reality of this outline. However, the Osiris (Astrometria) project is still on their to-do list. It is an ambitious undertaking, considering their track record, but Osiris is proposed to be a high-accuracy interferometric astrometry craft, perhaps resembling NASA’s Space Interferometry Mission (SIM). Its current launch date is 2018, but this may be optimistic considering the current chaos in Russia’s deep space program.

Complementary approaches

So, we have two main missions in the pipeline—Gaia and the New Worlds starshade—with a series of smaller, yet significant, missions being flown by NASA and ESA. One can envision a synergy between the New Worlds starshade and NEAT astrometry missions. Whichever spacecraft flies first will find Earth analog candidates that will need to be confirmed. New Worlds and NEAT can provide this crossover verification. At one time, NASA had planned a robust exoplanet program that went by the name of Navigator. It included high-profile missions such as SIM and Terrestrial Planet Finder (TPF) that would have performed search and verification of exoplanet candidates. However, that program has been completely eliminated. What has risen from that chaos are two missions, from two space agencies, that will carry forward the goals of the Navigator program: New Worlds and NEAT.

One can imagine a scenario in which New Worlds has seen an extrasolar “Earth,” but at the threshold of detection because of the interference from the glare of that system’s exozodiacal light. In that case, with the star system identified by New Worlds, NEAT could search for the wobble that would verify that planet’s existence. Since an astrometry mission is virtually immune to the interference of exozodiacal light since it observes the parent star itself, the NEAT spacecraft could get an accurate mass and orbit. One can also imagine the reverse order of discovery. With NEAT’s identification of an “Earth” candidate, the New Worlds starshade could set its sights on the system to verify its existence and to obtain spectra. In addition, a transit mission such as TESS may have a marginal detection of an “Earth” caused by the noise of its parent star. One of the discoveries of the Kepler mission has been that many Sun-like stars are quite unlike the Sun in that they have large variations in brightness. In that case, New Worlds or NEAT could be called upon for verification. This shows the value of a multi-prong approach to the effort.

It appears that a division of labor is forming on the two sides of the Atlantic. Beginning with Gaia and continuing with NEAT, Europe is developing an expertise in using astrometry in its future exoplanet efforts. On this side of the ocean, NASA is now set to focus on direct imaging. As NASA begins this new effort, it is useful to review what occurred to its predecessor, SIM. I described much of this saga in an earlier article (“SIM and the ‘ready, aim, aim’ syndrome”, The Space Review, October 18, 2010). However, there are a few “new” details to this story. The fate of SIM serves as a warning to the teams now working on future exoplanet missions.

page 2: the lessons of SIM >>