Discovery of Neptune-sized planet orbiting young, nearby star may help scientists understand how planets form
An exoplanet about the size of Neptune has been discovered circling around an especially young, nearby star. The planet, named AU Mic b, is orbiting AU Microscopii, which is relatively close to the Milky Way at 31.9 light-years away. AU Microscopii, known as AU Mic for short, is estimated to be about 20 or 30 million years old -- at least 150 times younger than our Sun, making it a stellar infant compared to the Sun. There are only two or three known stars that are both nearby and young. For over a decade, astronomers have searched for planets orbiting AU Microscopii, a star still surrounded by a disk of debris leftover from its formation. The current breakthrough was made possible in part by using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope.
According to scientists, the new finding creates a major opportunity for breakthrough research into solar system formation dynamics. Since AU Mic b is so young, studying this planet, and hopefully others like it, can give scientists insight into how our own solar system formed. “One of the things we want to understand is, 'When do planets form, and what do they do in their early days?'" says study author Tom Barclay in the analysis published in Nature. He is an associate research scientist with the University of Maryland Baltimore County’s Center for Space Sciences and Technology, a partnership with the NASA Goddard Space Flight Center in Greenbelt, Maryland. AU Mic b is likely primarily comprised of gases. “This star probably hasn't had time to form small, rocky planets yet. It gives us a chance to get a picture of what might have happened before our terrestrial planets like Earth and Venus formed,” says Barclay.
The research team now plans to learn more about the atmosphere of the new planet. In addition to the rate of atmosphere loss, observations can also help determine what the planet's atmosphere is made of. Determining the atmosphere's components could help the team figure out where the planet formed because certain substances can only exist at a known distance from the star. Further, knowing where the planet formed would provide clues about how it had moved since it first came into being, the authors explain. This information would get scientists closer to understanding more generally how planets form and migrate in a new solar system. But the work is not easy. “Understanding the migration of planets is a really difficult problem. One of the fun things and one of the most frustrating things about studying stars is that we can never go to them. So this discovery is just one more puzzle piece in trying to understand what's going on,” he writes.
Space telescopes to the rescue
Detecting planets around stars like AU Mic b 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 powerful stellar flares. Both the spots and their flares contribute to the star’s brightness changes. 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. The TESS mission observes the same section of sky for weeks at a time, collecting data about the brightness of stars in its field of view every two minutes. TESS can help detect planets by recording when a star's brightness temporarily dims. That can sometimes signal a planet crossing in front of the star, or transiting. Dips in brightness tell scientists about the size of the planet and measuring how regularly spaced the transits shows how long it takes the planet to go around the star. Usually, it takes at least two observed transits to recognize a planet’s presence.
TESS detected two transits of AU Mic b, but the research team needed a third to be confident of their observations. So they called on additional data collected by NASA's Spitzer satellite and ground-based instruments in Hawaii and Chile, which confirmed the transits of AU Mic b. “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. 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,” says co-author Diana Dragomir, a research assistant professor at the University of New Mexico in Albuquerque, in a statement. Since the amount of light blocked by a transit depends on the size of the exoplanet and its distance from its star, these observations allowed scientists to determine that AU Mic b is about as large as Neptune. The analysis shows that the planet is about 8% larger than Neptune, it has a mass of no more than 58 Earths and completes an orbit of AU Microscopii every 8.5 days. An orbit that short indicates that the planet is extremely close to the star, say researchers.
A unique laboratory
AU Mic system provides a unique laboratory for studying how planets and their atmospheres form, evolve and interact with their stars. Very few systems like AU Mic are known. Not only is the detection of exoplanets difficult in these systems, but they are also very rare because a system's period of planetary formation is relatively short compared to the life of a star.
AU Mic is a cool red dwarf star. The star is so young that it primarily shines from the heat generated as its 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. The system is located in the southern constellation Microscopium. It is 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. While the systems have the same age, their planets are markedly different. Beta Pictoris b and c are both at least 50 times more massive than AU Mic b and take 21 and 3.3 years, respectively, to orbit their star.
The AU Mic system is close to Earth and therefore appears brighter, allowing astrophysicists to observe it with a range of instruments, such as the SPIRou spectrograph. “This instrument, with its polarimetric capabilities, will allow us to better distinguish the effects of stellar activity, which are often confused with the signal from the planets. This will allow us to determine the mass of AU Mic b accurately and to know if this exoplanet is more like a large Earth or a Neptune twin,” writes Étienne Artigau, a project scientist at Université de Montréal, in the study. The researchers believe that 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. “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,” says Barclay.