When the TESS (Transiting Exoplanet Survey Satellite) mission was launched in 2018, NASA predicted that the space telescope would discover thousands of planets — including a handful of worlds that may be habitable to aliens. TESS targets stars 30 to 300 light-years away and about 30 to 100 times brighter than the Kepler Spacecraft targets, which are 300 to 3,000 light-years away. Now, scientists using data from TESS and retired Spitzer Space Telescope have announced the discovery of a planet about as large as Neptune that orbits the young nearby star AU Microscopii –still surrounded by a disk of debris left over from its formation– in just over a week.
The system, known as AU Mic for short, is located 31.9 light-years away in the southern constellation Microscopium. provides a one-of-kind laboratory for studying how planets and their atmospheres form, evolve and interact with their stars. Red dwarfs cannot be easily observed because of their low luminosity contain the bulk of our galaxy’s planet population reports NASA’s Hubble Site, which could number tens of billions of worlds.
“AU Mic is a young, nearby M red-dwarf star. It’s surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, it has a planet with a direct size measurement. There is no other known system that checks all of these important boxes,” said Bryson Cale, at George Mason University about the system, part of a nearby collection of stars called the Beta Pictoris Moving Group, which takes its name from a bigger, hotter A-type star that harbors two planets and is likewise surrounded by a debris disk.
The new planet, AU Mic b, is described in a paper coauthored by Cale and led by his Ph.D advisor Peter Plavchan, assistant professor of physics and astronomy at George Mason. Their report was published in the journal Nature.
AU Mic is a cool red dwarf star with an age estimated at 20 million to 30 million years, making it a stellar infant compared to our Sun, which is at least 150 times older. The star is so young that it primarily shines from the heat generated as its own gravity pulls it inward and compresses it. Less than 10% of the star’s energy comes from the fusion of hydrogen into helium in its core, the process that powers stars like our Sun.
Although the systems have the same age, their planets are markedly different. The planet AU Mic b almost hugs its star, completing an orbit every 8.5 days. It weighs less than 58 times Earth’s mass, placing it in the category of Neptune-like worlds. Beta Pictoris b and c, however, are both at least 50 times more massive than AU Mic b and take 21 and 3.3 years, respectively, to orbit their star.
“We think AU Mic b formed far from the star and migrated inward to its current orbit, something that can happen as planets interact gravitationally with a gas disk or with other planets,” said coauthor Thomas Barclay, associate project scientist for TESS at NASA’s Goddard Space Flight Center. “By contrast, Beta Pictoris b’s orbit doesn’t appear to have migrated much at all. The differences between these similarly aged systems can tell us a lot about how planets form and migrate.”
Detecting planets around stars like AU Mic poses a particular challenge. These stormy stars possess strong magnetic fields and can be covered with starspots – cooler, darker and highly magnetic regions akin to sunspots – that frequently erupt with powerful stellar flares. Both the spots and their flares contribute to the star’s brightness changes.
In July and August 2018, when TESS was observing AU Mic, the star produced numerous flares, some of which were more powerful than the strongest flares ever recorded on the Sun. The team performed a detailed analysis to remove these effects from the TESS data.
When a planet crosses in front of its star from our perspective, an event called a transit, its passage causes a distinct dip in the star’s brightness. TESS monitors large swaths of the sky, called sectors, for 27 days at a time. During this long stare, the mission’s cameras regularly capture snapshots that allow scientists to track changes in stellar brightness.
Regular dips in a star’s brightness signal the possibility of a transiting planet. Usually, it takes at least two observed transits to recognize a planet’s presence.
“As luck would have it, the second of three TESS transits occurred when the spacecraft was near its closest point to Earth. At such times, TESS is not observing because it is busy downlinking all of the stored data,” said coauthor Diana Dragomir, a research assistant professor at the University of New Mexico Exoplanet Group. “To fill the gap, our team was granted observing time on Spitzer, which caught two additional transits in 2019 and enabled us to confirm the orbital period of AU Mic b.”
Because the amount of light blocked by a transit depends on the planet’s size and orbital distance, the TESS and Spitzer transits provide a direct measure of AU Mic b’s size. Analysis of these measurements show that the planet is about 8% larger than Neptune.
Observations from instruments on ground-based telescopes provide upper limits for the planet’s mass. As a planet orbits, its gravity tugs on its host star, which moves slightly in response. Sensitive instruments on large telescopes can detect the star’s radial velocity, its motion to-and-fro along our line of sight. Combining observations from the W. M. Keck Observatory and NASA’s InfraRed Telescope Facility in Hawaii and the European Southern Observatory in Chile, the team concluded that AU Mic b has a mass smaller than 58 Earths.
This discovery shows the power of TESS to provide new insights into well-studied stars like AU Mic, where more planets may be waiting to be found.
“There is an additional candidate transit event seen in the TESS data, and TESS will hopefully revisit AU Mic later this year in its extended mission,” Plavchan said. “We are continuing to monitor the star with precise radial velocity measurements, so stay tuned.”
For decades, AU Mic has intrigued astronomers as a possible home for planets thanks to its proximity, youth and bright debris disk. Now that TESS and Spitzer have found one there, the story comes full circle. AU Mic is a touchstone system, a nearby laboratory for understanding the formation and evolution of stars and planets that will be studied for decades to come.