Tuesday, January 29, 2013

Characterizing Habitable Earth-like Planets

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.