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.
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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.
Reference:
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