I’ve got one hell of a soft spot for exoplanet research, especially new and interesting ways to find planets. And this latest one, well, it just might change everything. Here's why.
In a paper published on arXiv and accepted for publication in the journal Astronomy and Astrophysics, researchers led by Helen Giles from the University of Geneva found a planet 1,800 light-years from Earth called (deep breath) EPIC248847494b. The planet itself is very interesting, and we’ll come to that later, but it’s how they found it that is super exciting.
We have a number of ways to find planets, such as looking for the wobble in a star caused by the gravitational tug of a planet, known as the radial velocity method. The most common way, however, is the transit method. This involves watching for the dip in a star’s light as a planet passes in front.
This is by far the most successful method, turning up thousands of planets, mostly by NASA’s Kepler telescope. One problem, however, is that you need to see three transits of a planet to confirm it exists and to rule out other possibilities like the signal being a fault with the telescope.
For planets on a short orbit, from days to weeks, this is relatively easy. For planets on a longer orbit, however, such as those on an Earth-like orbit of one year, this is particularly tricky, because you need to observe the star for a long time. Consequently, we’ve found more short orbit planets than long ones.
Short orbit planets are generally too close to their star, and thus too hot, to host life – except for those that orbit cooler dwarf stars, although their errant bursts of radiation might be a problem. Planets in habitable zones around more Sun-like stars, where temperatures are just right for water to exist, tend to be on longer orbits.
That’s why this latest paper is so exciting. The researchers were able to confirm that a planet on a long orbit existed from just a single transit, dubbed a monotransit planet. They did this by first spotting the single transit in data from the second run of the Kepler telescope – K2 – before using the radial velocity method to follow up and find the planet.
“Typically to just ‘find’ a transiting exoplanet, at least three transits are needed and most automated transit finding software has this built in,” Giles told IFLScience. “But my technique for discovering this transit was to look through thousands of light curves by eye.”
And this could have huge implications for finding more planets in the future, including ones that are like Earth.
A handful of other monotransit planet candidates have been found so far. Most, however, are on short orbits. EPIC248847494b is unique in that it takes 10 years to orbit its star, making it the longest period monotransit planet ever found. Using the traditional transit method, it would have taken us at least 30 years to confirm its existence.
Giles and her team, however, performed follow-up radial velocity measurements with the Swiss Euler telescope in Chile, plugging in information on the star from ESA's Gaia mission. By looking through data from Kepler manually, they were able to spot the candidate planet and then study it.
They found that the planet is likely a gas giant about 1.1 times the size of Jupiter, orbiting about 4.5 AU (astronomical units, 1 AU is the Earth-Sun distance) from its star, a sub-giant. This places it right in the expected habitable zone of that star.
Its transit is also incredibly long, lasting a whopping 54 hours. Earth’s transit seen from afar would only last about 13 hours. So you would need to observe the planet for a long time in the first place to even see the transit.
“The transit method has always struggled to find long period planets, so that is one reason why this is an exciting discovery,” said Giles. “This opens a new window for planet hunting.”
This could prove incredibly important for the Transiting Exoplanet Survey Satellite (TESS), launched in April 2018. Using the transit method and observing about 85 percent of the sky, TESS is expected to find more than 20,000 planets.
It’ll only be looking at each region of the sky for 27 days, however, meaning it’ll only see three transits for planets on short orbits of less than a week. For other planets on longer orbits, such as ones like Earth, techniques like that employed by Giles will be absolutely crucial.
“Theoretically – yes!” Giles said, when asked if her technique could help us find more Earth-like planets. “In the case of another Earth, we can start chasing it after a single transit.”
Speaking to IFLScience, Dr John Mather, senior project scientist for NASA's James Webb Space Telescope (JWST) who was not involved in the paper, agreed that the technique used to find the planet was an exciting one for future exoplanet astronomy. “As they say, we will have many 'monotransits' to think about and will have to develop methods to deal with them,” he said.
“Earth-like planets will also be candidates from monotransit observations. If any of them seem very interesting, we will certainly want to find a way to confirm them by other means, and that will take a pretty big effort.”
As for EPIC248847494b itself, well, it’s not just here to make up numbers. While the planet is likely a gas giant, all of the gas giants in our Solar System have moons. If this planet does too, called exomoons, then its location in the habitable zone might just be a great place for life.
“This is an excellent candidate for attempting to detect exomoons which may well be habitable,” the team wrote in their paper. We haven’t found any exomoons anywhere yet, but it's pretty likely they're out there, and this might just be a good – if tricky – place to look.
René Heller from the Max Planck Institute for Solar System Research in Germany, also not involved in the paper, told IFLScience that the planet “could potentially host large, Mars-sized moons”. But these would be “very, very tough” to find, as the planet only transits its star once every ten years, and there are still uncertainties in its orbit.
At the very least, we have a great new way to hunt for exoplanets. In the search for potentially habitable worlds in orbits like our own, EPIC248847494b might just be the first of thousands of exciting monotransit planets.