Exoplanets

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Recent studies, reports, and peer-reviewed publications are listed below, along with the originally accepted key projects and recent community science studies.

SIM Lite ExoEarth Discovery Space 2009 February, Catanzarite, J.

ExoEarth Detection Limits 2010 May, Gorjian, V. This graphic illustrates SIM Lite detection limits for exoEarths. The light blue bars display the limits of the habitable zones for nearby stars. Minimum distance limits for different exoplanet masses of are presented

Key Projects

Discovery of Planetary Systems A search for new planets around nearby stars, which also will study the stars where scientists currently think planets have been found. Dr. Geoffrey W. Marcy University of California, Berkeley http://astron.berkeley.edu/~gmarcy/

Summary      Full proposal (PDF)      SIM book Theme I      Catalog of Nearby Exoplanets

Extrasolar Planets Interferometric Survey A search for planets using a large sample of stars. This study addresses one of SIM's primary science goals: taking a census of planetary systems around nearby stars. Dr. Michael Shao JPL (Science Team Chair)

Summary      Full proposal (PDF)      SIM book Theme I

The Search for Young Planetary Systems and the Evolution of Young Stars A study of the early stages of the formation of planetary systems around young stars that will provide new insight into how planets like Earth might have formed. Dr. Charles A. Beichman California Institute of Technology http://spider.ipac.caltech.edu/staff/chas/

Summary      Full proposal (PDF)      SIM book Theme I

Stellar, Remnant, Planetary, and Dark-Object Masses from Astrometric Micro-lensing A novel technique of micro-lensing will be used to make exceptionally precise measurements of the masses of stars and a variety of other astronomical sources. Micro-lensing involves changes to a star's appearance that occur due to gravity from a nearby object. Dr. Andrew P. Gould Ohio State University, Columbus

Summary      Full proposal (PDF)      SIM book Theme III

Detection and Characterization of Resonant Planetary Systems

Eric Ford (U. Florida)

The combination of SIM Lite and long-term, high-precision radial velocity (RV) observations will provide a unique tool to precisely measure planet masses and orbital elements, enabling precision dynamical modeling. Since some (but not all) planet formation models predict that many low-mass planets may be found in mean-motion resonances (MMRs), measuring the frequency of such planets will test planet formation models. For technical reasons, detecting and characterizing such planetary systems may be significantly more challenging than in the case of nonresonant planetary systems. We propose to explore the sensitivity of combined SIM Lite and RV data for detecting planets in or near MMRs. We will study how the number and time span of observations affect the detection probability and the precision of orbital elements for resonant planetary systems. We will identify when it is essential to include mutual planetary interactions and pay particular attention to identifying what types of planetary systems and observing strategies would be able to distinguish systems “in resonance” from those “near resonance.” Our results will help to inform the design of SIM Lite planet searches and could lay the foundation for a future SIM Lite observing proposal to determine the frequency of resonant planetary systems as a function of mass and orbital period. Ultimately, we aim to improve the capability of SIM Lite observations to test, and perhaps distinguish, between models of planet formation, migration, and eccentricity excitation.

Measuring the Astrometric Signature of Transiting Planets

B. Scott Gaudi (Ohio State U.)

When a planet with radius Rp transits in front of its parent star with radius R*, the flux of the star decreases by a fractional amount r 2 = (Rp / R*)2, while the stellar photocenter shifts by r 2 ?*, where ?* is the angular radius of the star. For the nearest transiting planets, this shift is of order ?as and so is within the reach of SIM Lite. Measurement of the astrometric shift during transit yields the angular radius of the star, which, when combined with the stellar density determined from the photometric light curve and the stellar parallax, yields the radius and mass of the star. This astrometric shift also allows one to determine the planet’s (three-dimensional) orbit, including the direction of the vector normal to its orbital plane, which is useful for a number of applications. I propose to perform an in-depth study of the astrometric signature of transiting planets as applied to SIM Lite, and in particular fully explore the feasibility of detecting this signature, considering all practical aspects, including mission scheduling and pointing constraints. In addition, I will consider the astrometric signature of eclipsing binaries.

Search for Planets Orbiting White Dwarfs

John Subasavage (Georgia State U.)

Once launched, SIM Lite will be the most precise astrometric instrument ever developed. These capabilities are vital to exoplanetary studies, in particular, for low-mass, Earth-like planets. I propose to use SIM Lite to observe a sample of ~25 nearby white dwarfs in hopes of detecting planetary companions with masses in the 10 M? range. Because of the nature of white dwarfs’ spectral signatures (a few broad, if any, absorption lines), current radial velocity planet hunting techniques are not viable. Astrometry is currently the only technique capable of detecting low-mass planets around white dwarfs, and SIM Lite is the best-suited astrometric instrument to do so. One advantage of white dwarfs is that they have lost a significant amount of mass during their evolution so that an astrometric signature is amplified when compared to an identical system around the more-massive progenitor. Planetary detections around white dwarfs would better enable us to probe planetary formation theory as well as planetary evolution theory in conjunction with stellar evolution. Because astrometric signatures are inversely related to distance, the closer the system, the larger the signature (all else being equal). Since most stars will eventually end their lives as white dwarfs, these objects are plentiful and on average closer to the Sun than are more-rare objects. Thus, a number of white dwarfs are close enough to the Sun to permit low-mass planetary signature detections. Given that white dwarfs are the remnants of main-sequence dwarfs with spectral classes from B to K (thus far), we could better understand planetary formation over a broader range of objects than those currently investigated using
radial velocity techniques (primarily F, G, and K stars).

Detecting Terrestrial Mass Planets Around M-Dwarfs

Angelle Tanner (Santa Barbara Applied Research)

In the past few years, there have been public claims that SIM Lite is unnecessary as a terrestrial planet search tool since radial velocity studies will be able to reach sensitivities of 10 cm/s. This is adequate to detect terrestrial planets in the habitable zones of M and K dwarfs. However, it has not been demonstrated that the RV technique will be sensitive to terrestrial planets at these separations under the different sources of stellar jitter inherent to M dwarfs — granulation, star spots, flares, and p-mode oscillations. Therefore, we have designed a study to investigate the astrophysical jitter inherent to potential SIM Lite M dwarf targets using spacecraft, including Convection, Rotation, and Planetary Transits (CoRot), Hubble Space Telescope, Spitzer Space Telescope, Microvariability and Oscillations of Stars (MOST), and ground-based, ultraprecise photometric data. The goal of the study will be to present a thorough comparison of the sensitivity to terrestrial planets using either SIM Lite or 10 cm/s radial velocity measurements with realistic noise sources. Since the exoplanet taskforce has recently placed M dwarfs as high-priority targets, the results of this study can be used to guide near-term planet search programs as well as promote SIM Lite.

Planets in Binary Systems: A Catalog of Wide, Low-Mass Binaries for SIM

Keivan Stassun (Vanderbilt U.)

A critical piece of SIM Lite exoplanet science will be to determine the frequency and nature of planets in binary star systems. Among the most scientifically interesting of these will be wide, low-mass binaries, in which planetary orbits about one or both stars are stable and where the detection of planets in the habitable zone is most feasible. We are assembling the largest catalog to date of wide (typical orbital separations ~3000 AU), low-mass (typical spectral type ~M0) binaries. The binaries in our sample are in a range of brightness easily amenable to study with SIM Lite. Importantly, our sample includes a broad diversity of stellar subpopulations that is of considerable interest for determining the frequency of planets in different regimes of parameter space: stellar mass ratios, metallicity, age, activity, and dynamical history. Finally, to explore the value-added stellar science made possible with our sample, we will study the extent to which multiple observations of these binaries with SIM Lite’s exquisite astrometric precision will permit the determination of orbital parameters and dynamical stellar masses with which to test stellar evolutionary models.

Searching for Solar System Giant Analogs

Robert Olling (U. Maryland)

SIM Lite astrometry, in combination with Hipparcos data and other astrometric observations from the 20th century, can uncover giant extrasolar planets with masses exceeding Jupiter’s with orbital periods up to about 200 years. At a mission time of less than 2.5 hr per star, SIM Lite could survey 500 to 1000 nearby stars to determine the frequency of very-long-period, massive extrasolar planets. Detection of planetary companions can be achieved for systems with periods up to about 1000 years. Current theories of planet formation predict that migration moves the outer cutoff of giant planets from the ~200-yr to the ~600-yr regime. Our proposed survey will probe giant planets and light brown dwarfs with these periods and can help constrain planetary migration models. We propose to study how well period and mass estimation will work in the long-period regime, with a major focus on the effects of elliptical orbits. In addition to the Hipparcos data, astrometric catalogs dating back as far as 90 years are expected to significantly constrain the presence of high-mass companions such as brown dwarfs. We will also investigate alternate observing strategies that could reduce the required mission time by a factor up to about four.