On 18 April from Space Launch Complex 40 at Cape Canaveral a Falcon 9 lifted off carrying NASA’s Transiting Exoplanet Survey Satellite. This satellite will conduct a two year survey to look for planets around the nearest stars to us. As the name suggests, this satellite will watch the nearest stras for anything that transits them, giving away valuable information on possible planets orbiting these stars. Scientists expect that this mission will provide a treasure trove of data on exoplanets. Based on the probablility of what previous surveys have turned up, scientists working on TESS are expecting to find around 300 Earth sized or bigger rocky planets. The data on these planets will then be used to figure out what to do next and see which planets will be candidates for further study.
The TESS mission will examine the entire sky by breaking it up into 26 tiles of 24 X 96 degrees. The cameras on the satellite will stare at each tile for 27 days to look for any characteristic dip in brightness that may indicate the transit of a planet between the star and the satellite. The previous mission that surveyed for exoplanets was the Kepler mission and the follow on K2 survey, these activities looked at significantly dimmer stars which made follow up work using ground based telescopes a bit more problematic. TESS will look at much brighter stars but cover the entire sky, Kepler only looked at a portion of the sky. Kepler was designed to stare at a fixed area of space of about 150,000 stars but suffered a failure in it’s stability mechanism which led to a change in mission, known as the K2 mission. The mission was very successful in greatly increasing our understanding of the variety of exoplanets and directly led to the estimate of about how many planets there might be in the galaxy. Some of these estimates suggest that there could be as many as 40 billion rocky planets with about 11 billion of them orbiting stars similar to our own Sun. With the help of Kepler there are around 3000 exoplanets now identified.
A lot of information can be discovered from examining a transit of a planet in front of its star. This has occured on Earth for centuries as astronomers have observed Venus and Mercury transits many times. In fact the reason that the Royal Navy sent James Cook and the crew of the Endeavour to the South Pacific back in 1769 was to observe the transit of Venus and later to New Zealand to observe the transit of Mercury. By doing this, early astronomers could work out how big these planets were and study the nature of their orbits. An interesting aside was the debate in the 19th century about a proposed planet that orbited inside of Mercury, known as Vulcan. Despite many observations looking for transits across the face of the Sun, none conclusively identified another planet. This planet was supposed to have existed to explain the variabilities in the observed orbit of Mercury, and these variations were eventually explained by Einstein’s theory of general relativity. This is similar to what TESS will do, except that the planets it is studying are considerably further away.
As a planet passes in front of a star in the field of view of TESS, a noticable dimming of the light output of the star can be measured. This measurement combined with knowledge of the star itself enables astronomers to determine the size of the planet and details of the orbit. This helps to figure out what conditions might be like on the planet such as temperature etc. Importantly, what can then happen is that the details for the star can be forwarded to ground based telescopes to make follow up observations such as details about the planet’s mass and even hints about the atmospheres of these alien worlds.