A major goal in the study of exoplanets is the detection and characterization of Earth-like planets in the Habitable Zones of nearby stars. However, direct imaging of exoplanets is extremely difficult because a star is typically billions of times brighter than the orbiting planet and from a distance of several light-years, an exoplanet will appear just tens of milli-arcseconds from its host star. This makes it almost impossible to directly image an exoplanet since it will be lost in the star’s overwhelming glare. A proposed mission known as the New Worlds Observer (NWO) will allow the direct detection and characterization of Earth-like planets around stars. NWO consists of a 50 meter diameter starshade placed 80,000 kilometres in front of a 4 meter aperture space telescope. The starshade suppresses the starlight by a factor of several billion and allows planets orbiting the star to be imaged by the telescope.
The starshade is attached to a
spacecraft whose main purpose is to move the starshade to block different
target stars and maintain precise alignment with the telescope during
observations. To align with each target star, the starshade is expected to
travel thousands of kilometres. For such retargeting manoeuvrers between target
stars, the spacecraft uses a solar-powered ion propulsion system to move the
starshade. This method of propulsion is selected to allow for the largest
number of target stars with a given amount of propellant mass. During retargeting
manoeuvrers by the starshade which can account for up to 70 percent of the telescope observing time, the telescope will be
dedicated to general astrophysics. Typical starshade travel time between target
stars is 5 to 10 days and typical observation durations at each target star
ranges from 24 hours for imaging to 14 days for detailed spectroscopic
observations. The
planned mission duration for NWO is 5 years with the goal for an extended
mission of an additional five years. Both the sunshade and telescope will be
placed at the Sun-Earth L2 point, which is a low-acceleration environment in
space. To prevent sunlight from illuminating the telescope-facing side of the
starshade, the starshade will be tilted so that the telescope-facing side will
always remain dark.
NWO can observe an entire planetary
system around a target star at once. After suppressing light from a target
star, NWO can image every planet from the Habitable Zone outward in a span of
just several hours. Although the primary goal of NWO is to detect and
characterize Earth-like planets in the Habitable Zones of nearby stars, it can
also image and characterize planets beyond the Habitable Zone such as
Jupiter-like and Neptune-like planets in long-period orbits. As a planet
rotates, photometric observations by NWO will show variations in colour and
intensity as different surface features rotate in and out of the telescope’s field-of-view.
For Earth-like planets, this allows for the detection of surface features such
as oceans, continents, polar ice-caps and clouds.
Spectroscopic observations of an
Earth-like planet in the Habitable Zone of a target star can quickly reveal
information about the planet’s atmospheric composition, surface conditions and
even the presence of life. The spectrum of an Earth-analogue exoplanet observed
by NWO for just several hours will reveal a brightening at short wavelengths
that is indicative of Rayleigh scattering which accounts for the blue sky we
see here on Earth. At longer wavelengths, the presence of molecular oxygen will
produce two moderate-strength absorption features at the 0.76 and 1.26
micrometer wavelengths. Molecular oxygen is a key biosignature since it is
chemically reactive in the Earth’s atmosphere and must be continuously
replenished by the biosphere. The presence of water also produces six strong absorption
features (0.72, 0.82, 0.94, 1.13, 1.41 and 1.88 micrometers) which get
dramatically stronger towards longer wavelengths. NWO will allow astronomers to
easily detect Earth-sized planets and image entire solar systems. This mission
may prove to be the quickest and most affordable path to the discovery of life
on other planets.