WHAT I REALLY WANT TO KNOW IS…
How can we detect slow-orbiting planets around other stars?
Helen Giles is employing a new technique to quickly discover hard-to-spot worlds that take many years to orbit their stars
Most of the thousands of exoplanets discovered in recent years have been hot Jupiters. The reason for this is not because they are more common but because they are much easier to find and quicker to confirm.
Most exoplanets have been found using the transit method, where they pass in front of the star, producing a measurable dip in the brightness of the starlight. Massive planets in rapid orbits produce more noticeable dips thanks to their sheer size.
NASA’s Kepler space telescope also helped us find a number of planets with longer orbital periods, but that was simply because Kepler observed stars for four years during its initial mission. Otherwise, these slower-moving planets have so far mostly been discovered using radial velocity, where a ground-based spectroscopy instrument measures the wobble in a star’s light. Astronomers have been conducting searches with radial velocity a lot longer than we’ve been doing transit photometry.
The eye of the beholder
Computer algorithms are used to detect signals in Kepler data that indicate the existence of an exoplanet. I found that applying human judgement allowed me to find a candidate for an exoplanet quite unlike the majority of discoveries, which had the longest period of any planet spotted by Kepler.
My discovery came about when I was scanning the data from K2 (the follow-up mission from Kepler) trying to find any sort of exoplanet – I didn’t really mind what kind of planet or what its period was. The computer code ranks potential exoplanet candidates from the best looking to the worst, and I checked every single one by eye. This planet stuck out like a sore thumb.
It was ranked very low among the possibilities, since only one potential transit had been observed by K2. But as soon as I applied my human eye to look at the data, I saw a beautifully-shaped dip in the star’s light-curve and thought, “Okay, that looks interesting, let’s investigate further.” Transits of most detected exoplanets typically last between two to 16 hours, and it is standard practice to observe at least three passes before the exoplanet is confirmed. My exoplanet’s transit lasted 54 hours – almost two and a half days. Having to wait three orbits to confirm it would take over 30 years at that rate. So instead my team at the University of Geneva used other observations of the star to learn more about it and its potential planet. The parent star is known by its catalogue number EPIC248847494. We first used recently released data from ESA’s Gaia mission to determine that it lies about 1,500 lightyears from us. That information, together with the length of the transit, told us that the planet’s distance from its star must be around 4.5 times that of the Earth from the Sun, as well as the length of its ‘year’. Importantly, we needed to check that this orbiting object was a planet and not another star. We turned to the Swiss 1.2m Leonhard Euler Telescope at La Silla in Chile to measure the star’s radial velocity, or, more simply, its wobble. This showed that the newly discovered companion is less than 13 times as massive as Jupiter, so too small to be a star. Officially this discovery is still just a great candidate for an exoplanet, but a very convincing one! To get confirmation using radial velocity will take a full orbit of 10 years, though we should get some estimation in around half that time. This demonstrates a useful technique for finding more slow-orbiting exoplanets. Some might already lurk in archived data; finding them would be a good citizen science project. But the main focus will be on future data collected by the likes of NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched in April. TESS will only observe each bit of sky for 27 days, so it should detect a lot of single transits. We look forward to discovering many more exoplanets. In theory, we could detect a planet as small as Earth, if its star is fairly stable and bright.