NASA’s Kepler planet-hunting spacecraft was deactivated in November 2018, about ten years after it launched. The mission detected over 5,000 candidate exoplanets and 2,662 confirmed exoplanets using the transit method. But scientists are still working with all of Kepler’s data, hoping to uncover more planets in the observations.
A team of researchers have announced the discovery of one more planet in the Kepler data, and this one is nearly a twin of Jupiter.
The planet is called K2-2016-BLG-0005Lb (sorry), and it’s a whopping 17,000 light-years away. That’s almost twice as far away as the next furthest planet discovered by Kepler. Its mass is nearly identical to Jupiter’s, and it orbits its star at the same distance that Jupiter orbits the Sun. Astronomers found the world in Kepler data from 2016.
Kepler found planets using the transit timing method. But it discovered this one differently. It relied on one of Einstein’s predictions; that extremely massive objects have such powerful gravity that they can bend light. It’s called gravitational microlensing.
“The chance that a background star is affected this way by a planet is tens to hundreds of millions to one against.”
A new paper titled “Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing” presents the discovery. It’s available online at the pre-print site arxiv.org and hasn’t been peer-reviewed yet. The lead author is Ph.D. student David Specht from The University of Manchester.
Opportunities to detect exoplanets with gravitational microlensing were heightened between April and July 2016 when Kepler was looking at millions of stars toward the center of the Milky Way. In the microlensing technique, astronomers watch for the light from a background star bent by an exoplanet’s mass in the foreground. That’s not easy to do; it requires precise alignment of the background and foreground from Kepler’s point of view.
“To see the effect at all requires almost perfect alignment between the foreground planetary system and a background star,” said Dr. Eamonn Kerins, Principal Investigator for the Science and Technology Facilities Council (STFC) grant that funded this research. “The chance that a background star is affected this way by a planet is tens to hundreds of millions to one against. But there are hundreds of millions of stars towards the centre of our galaxy. So Kepler just sat and watched them for three months.”
Last year a team of researchers developed a new algorithm to search for microlensing candidates in Kepler data. Some of those same researchers are behind this new study. The researchers developed the algorithm to look for free-floating planet candidates. They found five new candidates, including one that’s “… a caustic-crossing binary event, consistent with a bound planet,” that study said.
That effort expanded the possibilities of the Kepler data, even though NASA didn’t explicitly design the mission for microlensing. “Even through a space telescope not designed for microlensing studies, this result highlights the advantages for exoplanet microlensing discovery that come from continuous, high-cadence temporal sampling that is possible from space,” the authors of the new study write.
The 2021 study “only” found one exoplanet candidate, and this new study confirms its candidacy. But in science, each planet is a data point that tells scientists something, now or in the future.
The image on the left is a Kepler image with K2-2016-BLG-0005Lb shown in a red circle. The image on the right is a Canada-France Hawaii Telescope image of the same region, with the exoplanet in a red circle. K2-2016-BLG-0005Lb is almost identical to Jupiter in terms of its mass and its distance from its star. Astronomers discovered it using data obtained in 2016 by NASA’s Kepler space telescope. The exoplanetary system is twice as distant as any seen previously by Kepler, which found over 2,700 confirmed planets before ceasing operations in 2018. Image Credit: Specht et al. 2022.Five ground-based surveys also looked at the same sky area as Kepler did from April to July 2016. Kepler saw the microlensing anomaly before they did because Kepler’s over 100 million km closer. That delay allowed researchers to get a better idea of what they saw and where they were seeing it.
“The difference in vantage point between Kepler and observers here on Earth allowed us to triangulate where along our sightline the planetary system is located,” said Dr. Kerins. Kepler’s vantage point above Earth’s atmosphere also allowed it to observe continuously.
“Kepler was also able to observe uninterrupted by weather or daylight, allowing us to determine precisely the mass of the exoplanet and its orbital distance from its host star,” Dr. Kerins said. “It is basically Jupiter’s identical twin in terms of its mass and its position from its Sun, which is about 60% of the mass of our own Sun.”
This figure from the study shows the photometric Kepler data for the detected exoplanet K2-2016-BLG-0005Lb. The caustic crossing region is clearly visible and well sampled between ?? ? ?2450000 = 7515 and 7519. Image Credit: Specht et al. 2022.This study highlights the growing importance of gravitational microlensing in exoplanet science. “Microlensing remains the principal method for detecting cool, low-mass exoplanets, including planets beyond the snow-line,” the authors write. The transit method has a built-in sampling bias: it’s more likely to detect giant planets close to large stars because the light-blocking signal is more robust. The transit method struggles to identify planets on wider orbits because it can take hundreds of years for multiple transits to occur, and astronomers need multiple transits to confirm exoplanet candidates. Gravitational microlensing doesn’t have the same limitations.
But detecting planets like 2-2016-BLG-0005Lb beyond a solar system’s snow-line is essential to build our understanding of solar system architecture and strengthen our theories of planet formation. Current thinking shows that high-mass planets form through core accretion beyond the snow line, then migrate inward towards the star. (Though some may form due to gravitational instability.) Jupiter likely did that, and though Jupiter eventually settled into its orbit beyond the snow line, other planets may not. This process explains the high numbers of Hot Jupiters in the exoplanet database.
This image shows an artist’s impression of 10 hot Jupiter exoplanets studied using the Hubble and Spitzer space telescopes. Astronomers think that about 10% of exoplanets are Hot Jupiters, but they’re detected more readily. (Colors are for illustration only.) Image Credit: By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=45642004“Simulations also indicate that lower-mass planets should exist in large numbers beyond the snow line, but that these do not typically migrate from their orbit of formation,” the authors write. “By probing the
demographics of cool, low-mass exoplanets, we can therefore test planet-formation predictions directly, without the need to consider complex migration dynamics.”
Astronomers have proven that gravitational microlensing can detect distant exoplanets, but they won’t have to rely on older Kepler data to use the technique. NASA’s Nancy Grace Roman Telescope should detect thousands of exoplanets using gravitational microlensing. One study showed it could detect over 100,000 of them.
“Roman will find planets in other poorly studied categories,” NASA says. “Microlensing is best suited to finding worlds from the habitable zone of their star and farther out. This includes ice giants, like Uranus and Neptune in our solar system,” the NASA website for the Roman Space Telescope explains. Some evidence shows that ice giants are the most common type of exoplanet in the galaxy, making our own Solar System a bit of an outlier with only two of them. “Roman will put that theory to the test and help us get a better understanding of which planetary characteristics are most prevalent.”
Roman will observe the galactic center, a region filled with stars. The more stars it looks at, the more microlensing events it’s likely to see.The ESA’s Euclid mission will also use gravitational microlensing. Its primary mission is to study dark matter, dark energy, and the expansion of the Universe. But it can also detect exoplanets. Euclid and Roman are designed to complement one another, so who knows exactly what we might learn from them.
Dr. Kerins is Deputy Lead for the ESA Euclid Exoplanet Science Working Group. “Kepler was never designed to find planets using microlensing, so in many ways, it’s amazing that it has done so. Roman and Euclid, on the other hand, will be optimized for this kind of work. They will be able to complete the planet census started by Kepler,” he said.
“We’ll learn how typical the architecture of our own solar system is. The data will also allow us to test our ideas of how planets form. This is the start of a new exciting chapter in our search for other worlds.”
