One reason exoplanets are so fascinating is the possibility that they may harbor life, but the definition of habitable used by astronomers is so broad that it could include planets that obviously aren't. To help zero in on the more likely candidates, a British-built satellite called Twinkle will look at the atmospheres of exoplanets to seek more definite signs of life, as well as clues as to the chemistry, formation and evolution of exoplanets.
According to NASA, there are 1,813 confirmed exoplanets with thousands more candidates under consideration discovered by the Kepler Space Telescope and others using the transit method. That is, the light coming from particular stars was measured over time and any dips in their light curves that may indicate a planet passing in front of them were recorded and measured.
The technique has been remarkably effective, but the information that it returns about the planets is pretty basic. Scientists can deduce the mass and density of a planet, determine if its rocky or gaseous, its distance from its star, its orbital period, and estimate its surface temperature. Unfortunately, those estimates are very rough. According to the definition of habitable as used currently, Earth certainly fits it, but so do Venus and Mars because they also fall inside the Goldilocks zone where liquid water can exist on a planet's surface.
But to make a real determination, scientists need more data – especially about the atmosphere. Venus, for example, is in the habitable zone, but its superdense carbon dioxide atmosphere rains sulfuric acid and is hot enough to melt lead. Mars, on the other hand, has a vestigial atmosphere of carbon dioxide that lets in deadly radiation and is so dry that it makes the Atacama desert look like a rain forest.
This is where the £50 million (US$76.2 million) Twinkle mission comes in. Scheduled to launch in four years, it's being built by a team led by University College London and Surrey Satellite Technology Ltd. The latter is using cost-effective design and off-the-shelf-parts to build the orbiter at a tenth the cost of a comparable satellite. In an overview presented at the Royal Astronomical Society on February 6, the team laid out a design for an extremely stable unmanned orbiter that can study the atmosphere of exoplanets.
Twinkle uses the transit method, but for a different purpose. Pioneered by the Hubble and Spitzer Space Telescopes, Twinkle's technique is to look at the light as the planet passes in front of its star, but instead of measuring the dip in light, the orbiter records the spectrograph of the star's light as it passes through the planet's atmosphere. This allows the spacecraft to make a record of the gases that make it up, and especially the presence of water vapor and oxygen, which would be key indicators that life is present.
According to the team, the atmosphere doesn't just indicate whether or not life could exist on a planet, it also helps scientist to understand the history of a planet, such as whether it evolved in its present orbit or was moved there from another place in the star system. For example, a loss of light molecules could indicate impacts by comets, asteroids, or protoplanets.
When launched, Twinkle will be set in a polar low-Earth orbit on a three to five-year mission, during which it will study 100 exoplanets in our galaxy. These will not only include Earth-type planets, but also super-Earths and hot Jupiter-sized giant planets orbiting a variety of stars. The team says that if a planet is orbiting a particularly bright star, it may be possible to make maps of the planet's temperature and cloud cover.
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