What are Exoplanets?
Exoplanets are planets orbiting stars other than our Sun. The first confirmed detections date back just over 20 years. So far, nearly 2000 confirmed exoplanets have been found and several thousand more candidate exoplanets have been detected. It’s now thought that most stars are orbited by planets. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected indirectly by looking for the dip in brightness as a planet passes in front of the star (the transit method) or looking for a wobble in a star’s position caused by the gravitational tug of an orbiting planet (the radial-velocity method).
Many of the exoplanets we’ve found so far are quite different to those in our solar system: Hot-Jupiters are giant planets that orbit very close to their star; super-Earth’s are rocky planets up to ten times the mass of Earth. Perhaps most intriguingly, the quest goes on to find Earth-like planets in the habitable zone – not too hot and not too cold – that might be able to support life.
However, we know very little about the exoplanets we’ve found to date beyond their mass, density and distance from their star. Twinkle will be the first mission dedicated to helping us understand what these distant worlds are like.
How Twinkle will work
From a vantage point in orbit 700 km above the Earth, Twinkle will observe more than 100 planets orbiting distant stars. When an exoplanet passes in front of the star that it orbits, a tiny amount of starlight is filtered through the molecules and clouds in the planet’s atmosphere. Twinkle will pick out this light from the background starlight and split it into a spectrum. Different types of molecules absorb light at specific wavelengths, embedding a pattern of characteristic spikes or ‘fingerprints’ within the spectrum. By analysing this pattern, we can deduce which gases are present on the planet.
Twinkle’s instrument will analyse light in the visible and near-infrared wavelengths (0.5 to 5 micrometers). This will allow Twinkle to detect a wide range of molecules, including water vapour, carbon dioxide, carbon monoxide, hydrogen sulphide and exotic metallic compounds. Twinkle will detect organic molecules such as methane, acetylene, ethylene and ethane. It will also be sensitive to precursors to amino acids (the building blocks of life) e.g. ammonia and hydrogen cyanide.
Science with Twinkle
Gases present in a planet’s atmosphere hold clues to whether it formed in its current orbit or whether it has migrated due to collisions, tidal forces or the gravitational influence of other planets. The make-up, evolution, chemistry and physical processes driving an exoplanet’s atmosphere are strongly affected by the distance from its parent star. Atmospheres of small, Earth-like planets can evolve quite dramatically from their initial composition through loss of lighter molecules, impacts with comets or asteroids, volcanic activity or biological processes. Atmospheric composition is therefore a tracer of an exoplanet’s history as well as whether it might be habitable – or even host life.
By measuring the visible light reflected by an exoplanet and the infrared heat that it emits, Twinkle will work out the planet’s energy balance, its temperature and whether clouds are present or absent in the atmosphere. For very large planets orbiting very bright stars, Twinkle will even be able to obtain 2-D maps of temperature and clouds. With repeated observations over the lifetime of the mission, this will tell us about climate and weather on those planets.
Why do we need Twinkle?
Twinkle will be the first mission specifically designed with the unique capabilities required for characterising exoplanet atmospheres. Spectroscopy of exoplanets’ atmospheres has been pioneered with the Hubble and Spitzer space telescopes, both now nearing the end of their missions.
Other current and planned exoplanet space missions (e.g. Kepler, TESS, CHEOPS, Gaia and PLATO) and ground-based facilities (e.g. HARPS or Super-WASP) are designed to find new planets, rather than characterise their atmospheres. Thus, Twinkle will fill a current gap in facilities suitable for this challenging area of science.
Twinkle is using tried-and-tested technology to pioneer a new era of low-cost commercial missions for astronomy research. Surrey Satellite Technology Ltd (SSTL) will construct the spacecraft using the SSTL-300 platform it has developed for high-resolution Earth imaging. Twinkle will use off-the-shelf components and will reuse existing software to maximise cost, effectiveness and reliability. The spacecraft will have a high level of autonomy to minimise operational costs.
Twinkle will be built to operate for a minimum of three years, with the possibility of an extended lifetime of five years or more.
Funding and timeline
Twinkle is planned for launch by 2019. The payload study is already underway and will be completed by the end of December this year. Preliminary work and the instrument study are funded through a grant from the European Research Council and UK universities. Funding for the overall mission will come from a combination of public and private sources.