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SIM Lite will search for Earth-mass planets in the habitable zones around the nearest 60 solar-type stars — a capability unmatched by any existing instrument or instruments currently in development.

This graphic illustrates the sensitivity of different methods for discovering and analyzing exoplanets plotted with mass sensitivity of the technique vs. exoplanet orbital period. Maximum sensitivity limits are presented for Gaia (brown curve) and radial velocity (RV) searches to 1M? are plotted (grey curve). For SIM Lite, the sensitivity for finding a habitable exo-Earth is presented (red and pink curves) for both the easiest and most difficult stars and a median star among its best 60 target stars.

Plotted in the background are planets from Ida and Lin simulations, many of the known planets discovered by RV measurements, and solar system planets from Venus through Neptune. SIM Lite can discover habitable worlds in a large volume of Mass-Period space for F-G-K stars.

SIM Lite and Gaia

SIM Lite and Gaia are both astrometric missions. Gaia is an all-sky survey mission currently under development by the European Space Agency. Are both needed? The answer is emphatically yes, for two reasons. First, the advent of microarcsecond-level astrometric precision opens up a wide array of topics in astrophysics for which astrometry can now play a major role. Far from being a specialist technique, astrometry is once again becoming a fundamental tool for astronomy. Second, these missions are complementary in a way that every astronomer appreciates: Gaia is a broad-survey instrument and will fly first. SIM Lite is a powerful, sensitive, pointed instrument that will build on the results from Gaia.

Is there science overlap between SIM Lite and Gaia? The simple answer is — surprisingly little (see figure). This is because the SIM Lite science program is designed to complement, not duplicate, Gaia science. In general, Gaia will pursue those programs for which the science is derived from measurements of an ensemble of a very large number of targets. SIM Lite will focus on science that requires the highest precision on individually selected targets. Many examples can be found in this book. Two of these are the search for Earth-like planets orbiting the closest Sun-like stars and probing the Galactic potential by measuring the trajectories of individual hypervelocity stars.

The Kepler-SIM Connection

The Kepler mission, launched on March 6, 2009, is the first step in NASA's quest for finding Earth-like planets with space-based telescopes. Kepler will monitor 100,000 stars for the telltale dimming that will occur when a planet, Earth-size or larger, passes in front of its parent star and blocks out a small portion of its star's light. The depth and duration of that transit will give information about the orbital period of the planet and its size. Kepler must see a minimum of three transits in the 3.5 year lifetime of the mission to verify that the planet is real. Kepler must examine a large number of stars because, to be visible to Kepler as a transiting system, the orbital plane of planet must be within a degree or two of exactly edge-on. By studying such a large number of stars over 3.5 years, Kepler will provide the statistics of how common Earth-like planets are around stars similar to our Sun.

The second step in finding Earth-like planets after Kepler is the SIM Lite Astrometric Observatory, currently in development Phase B. Unlike Kepler, SIM Lite will be monitoring stars for the astrometric shift induced by the orbit of a planet on its parent star. The advantage of this method over the transit method is that it doesn't rely on the chance alignment of telescope, planet, and star. Also, the shift in the parent star's position in the sky is the cumulative effect of all the planets in orbit around it, not just the one that happened to be transiting it. This means that SIM Lite will be able to measure the masses and orbits of planets which complete more than one revolution during the five year mission. The disadvantage, and why Kepler is needed, is that the astrometric technique can only be applied to one star at a time and the stars have to be relatively close to Earth. The statistics supplied by Kepler set the overall number of stars that SIM Lite will need to observe. If Kepler determines that Earth-like planets are common, then SIM Lite will concentrate its efforts on looking for Earths around every suitable nearby star, confident that there is a high probability that such planets will be found. If Earth-like planets are rare, then to expand the sample and the probability of discovery, SIM Lite will have to monitor a much larger number of stars which will include many that are further away. Greater distances make finding Earth-like planets more difficult and the search will most likely reveal only planets that are 2-3 times more massive than the Earth.

The difference in measurement technique allows SIM Lite to address additional questions about the planetary systems found by Kepler. For the stars around which Kepler finds Earth-like planets, SIM Lite can detect Jupiter-sized planets that may not be lined up edge-on so as to produce a transit signal. At the average distance of the stars being surveyed by Kepler, 270 pc, SIM Lite will be sensitive to planet masses down to 3 Jupiters at 0.1 year orbital duration, 1 Jupiter at 0.6 year, and 0.3 Jupiter at 3.6 year orbits. This information will help to establish what type of solar systems these Earth-like planets inhabit. By accumulating statistics on these multiple planet systems we can get a sense of the stability of such systems. If there are a lot of multi-planet systems, then those systems must be stable; if not, then multi-planet systems must not be very stable, implying that our own solar system is a rarity.

Finally, assuming that the typical Kepler target is a G2V star at V = 12 and 270 pc, Kepler is expected to find approximately 30 Earth-like planets. Based on the time required to achieve a 3 µas differential accuracy, SIM Lite can follow-up and study all those discoveries by devoting just 1% of its 5 year mission lifetime.

The combined efforts of the Kepler mission and a follow-on mission like SIM Lite pave the way for the next phase of studying Earth-like planets around stars: spectroscopy. If SIM Lite shows that a planet is (1) rocky and (2) has enough mass to hold onto an atmosphere and that (3) it orbits in the habitable zone of its parent star, a future space-based mission can take a spectrum of that planet and its atmosphere to see if there might be signs of life!

The job of later missions will not be to search for Earth-like planets but to follow-up on those that have already been found by SIM Lite and tell us the properties of those planets by taking images and spectra. At the moment, several follow-up missions are in the study phase, but the capabilities that will be necessary to characterize planets found by missions like SIM Lite are already well known. It is that final goal of getting the chemical signatures of life on an alien world that is the driving force behind the planet searches that NASA has undertaken.