Frequently asked questions
Twinkle’s instruments are optimised for studying atmospheric features in bright exoplanets, such as hot-Jupiters and super-Earths orbiting close in to their star.
Twinkle’s instruments will not be able to produce full spectra for planets at habitable temperatures. However, for targets close to Earth where the conditions are right, simulations suggests that we will be able to obtain a handful of data points in the infrared light for small, rocky planets. This means that as well as delivering the spectral signatures of bright exoplanets, Twinkle can also help identify targets of interest for further observation by larger telescopes in the future.
Exoplanets are planets orbiting stars other than our Sun. The first confirmed detections date back just over 20 years. So far, over 3500 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.
Most space mission names fall into three categories: acronyms of longer titles that describe what the mission is doing e.g. OSIRIS-Rex (Origins Spectral Interpretation Resource Identification Security – Regolith Explorer); names of famous astronomers, physicists or philosophers (e.g. Cassini-Huygens); or names that relate to Greek mythology (e.g. Juno).
As our mission is slightly different from most, we wanted to be creative with the name. We wanted to call it something that was easy to remember and didn’t require background knowledge of science, history or culture: a name that would be accessible to everyone, of all ages.
Twinkle seemed the perfect name, firstly, because when an exoplanet passes in front of its host star, the tiny drop in light makes the star appear to Twinkle. Secondly, because “Twinkle, Twinkle Little Star” is one of the most well-known songs in the English language, familiar to babies and great-grandparents, and the words of the song are highly appropriate to Twinkle’s mission:
“How I wonder what you are.” The second line sums up perfectly the starting point for any scientific discovery – wondering what something is, how it works and how it came to be there.
“Up above the world so high, like a diamond in the sky.” Some of the most exotic exoplanets found recently may possibly be made actually of diamond!
There are many ways to find an exoplanet, some unexpectedly simple for such a complex field as astronomy, and some slightly more abstract.
The seemingly simplest way to find a new planet is just to look for them through telescopes, both ground and satellite based. However ‘direct imaging’ isn’t the easiest method, as exoplanets are very small and dark compared to their stars. Looking for an exoplanet without knowing where it is can be like looking for one needle in several different haystacks. Direct imaging is a growing field but so far only less than 50 of the roughly 3500 exoplanets discovered in the last 20 years have been found in this way. (See SPHERE, GPI and SCExAO)
This brings us to more subtle and successful methods such as radial velocity. When a planet orbits a star, it can cause the star to ‘wobble’ a little as the planet’s gravity acts upon it. By looking at large portions of the sky and searching for these wobbling stars (more specifically at the Doppler shift in the light emitted by these stars), it is possible to identify the stars that are likely to have planets. (See HARPS and ESPRESSO)
The next method involves looking for transiting planets. Some exoplanets orbit their star in a plane that is aligned with our line of sight: as the planet travels between its star and earth, it blocks off the light from its star, telling us that the planet is there. By looking for subtle changes in light, the planets can be found. (See WASP, Kepler, CoRot and HAT)
The transit technique is currently the most successful way of finding exoplanets, and this is good news for Twinkle! We will be looking at the light from a star shining through a planet’s atmosphere, and for us to do this, a planet needs to have an orbit that makes it observable with the transit technique. Twinkle will focus on ~100 of the brightest exoplanets already discovered.
When light shines on molecules it cause them to resonate, or vibrate. Each compound or molecule resonates at different frequencies depending on which atoms it is made from, and how these atoms are bonded together. Most importantly, each and every molecule resonates most with specific frequencies that correspond to particular wavelengths in the electromagnetic spectrum. Molecules preferentially absorb certain wavelengths from the incoming light and re-emit radiation at others, which means that each molecule creates an individual ‘fingerprint’ of spiky peaks and troughs across the spectrum.
When an exoplanet transits its star, the starlight we observe will have traveled though any atmosphere and clouds surrounding the planet, and will carry the fingerprints of the molecules present.
Twinkle will use a pair of spectrometers to split the starlight into a spectrum. As well as the violet-red we see in a rainbow of visible light, Twinkle will be able to see infrared ‘colours’ too. Varying intensity of these ‘colours’ tell us which spectral signatures are present – and, therefore, the composition of the planet’s atmosphere.
Twinkle will be the first mission specifically designed with the unique capabilities required for characterising exoplanet atmospheres. International space agencies are not planning a mission dedicated to this type of observation for at least a decade. 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. Twinkle will fill a current gap in facilities suitable for this challenging area of science.