Saturday, August 31, 2013

Detecting Exoplanets Using Nanosatellites

ExoplanetSat is a proposed miniature space telescope that is designed to seek out Earth-like planets orbiting nearby bright Sun-like stars using the transit method. When a planet happens to pass in front of its host star, it dims its host star by a small amount which depends on the planet-to-star area ratio. ExoplanetSat is comprised of 3 CubeSat units joined together with total dimensions of just 10 cm × 10 cm × 34 cm. CubeSats are a class of nanosatellites developed at Stanford University and California Polytechnic State University to facilitate low-cost access to space. A single CubeSat unit measures 10 cm on a side and has a mass on the order of 1 kg. By adopting the standardized CubeSat form factor, ExoplanetSat can be built at a much lower cost than a larger or non-standardized configuration. The compactness of ExoplanetSat allows it to piggyback the launch of large spacecraft to orbit which further reduces cost.

 Figure 1: Artist’s impression of an Earth-like planet. Credit: Bryan Kolb

ExoplanetSat will monitor a single Sun-like star for the presence of transiting planets. If a transiting planet is detected, the star will become a high-priory target for follow-on observations. A transiting planet offers a unique opportunity for its atmospheric constituents to be characterized. During a transit event, some of the starlight passes through the planet’s atmosphere and picks up spectral features to create a planetary transmission spectrum. By studying the planetary transmission spectrum, the planet’s atmospheric constituents can be determined. Although this technique allows the atmospheres of Earth-like planets to be studied, it only works for stars that are bright enough. Hence, ExoplanetSat will target the nearest and brightest Sun-like stars. Once a single ExoplanetSat prototype has been flown successfully, multiple copies can be made and launched to allow for the monitoring of multiple target stars simultaneously for transiting planets.

The brightest Sun-like stars are too widely separated in the sky for a single telescope’s field-of-view to continuously monitor. Because of that, the ultimate goal is a fleet of ExoplanetSats, with each ExoplanetSat monitoring a single Sun-like star. ExoplanetSat will be place in a low-inclination low Earth orbit (LEO) at approximately 650 km altitude. It will observe its target star only during orbital night (when in Earth’s shadow) due to thermal, power and lighting constraints during orbital day. Since an Earth-like planet takes over 10 hours to transit a Sun-like star, the observational cadence from LEO (~ 30 minutes of orbital night followed by ~ 60 minutes of orbital day) is sufficient to detect a transit event. During orbital dawn, ExoplanetSat will reorient to point its solar panels toward the Sun. Likewise, during orbital dusk (just before orbital night); ExoplanetSat will slew to re-acquire its target star.

Figure 2: Baseline configuration of the ExoplanetSat.

ExoplanetSat uses an optical payload which consists of a commercially available single lens reflex (SLR) camera lens and a composite focal plane array made up of a single CCD detector surrounded by multiple CMOS detectors. The CCD performs the photometric function of accurately monitoring the brightness of its target star for transiting planets. The CMOS detectors track the centroids of surrounding “guide” stars and keep the telescope precisely pointed at the target star. To detect the tiny dimming caused by a transiting Earth-size planet, ExoplanetSat will need a photometric precision of better than 10 ppm. This will be a challenging goal to reach within the constraints of a CubeSat. If the 10 ppm photometric requirement cannot be met, then the science objective may be adjusted to focus on detecting larger planets or Earth-size planets around dimmer stars.

Smith, Matthew W. et al., “ExoplanetSat: detecting transiting exoplanets using a low-cost CubeSat platform”, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave