How’d a nice young ice giant end up in such a hot orbit? Scientists investigate the mysterious exoplanet AU Microscopii b.

AU Microscopii is a baby red dwarf star about 32 light-years away in the southern constellation Microscopium, the Microscope. It’s only 22 million years old and surrounded by a planetary debris field, first observed in 2004. Just within the last year, independent teams have discovered two exoplanets (AU Mic b and c) orbiting the star.

A series of follow-up studies focused on AU Mic b, a young, Neptune-mass planet that whips around its star every 8½ days. This giant couldn’t have formed where it now orbits. To help determine how it got there, astronomers have sought to measure the alignment between the planet’s orbit and its host star’s spin.

There are many things that might cause a planet’s orbit to change, such as a large body passing near the system or interactions with the planet-forming disk around the star. Over the past year, multiple measurements made with various telescopes and methods have shown that AU Mic b’s orbit is still aligned with its star’s spin. While the individual measurements are more uncertain, the evidence is mounting that a more peaceful transition occurred, like disk interactions, rather than gravitational ping-pong.

Pinning Down Spin

The first of the studies was led by Teruyuki Hirano (Tokyo Institute of Technology) and published in Astrophysical Journal letters in August 2020. His team used the Subaru telescope to obtain the first tentative proof that AU Mic b’s orbit is aligned with its star’s spin.

Then, one month later, Eder Martioli (Institut d’Astrophysique de Paris) published the same good spin-orbit alignment using the Canada-France-Hawaii Telescope and the NASA Infrared Telescope Facility, reporting their results in the September 2020 Astronomy and Astrophysics.

In a third study published October 2020 in Astronomy and Astrophysics, Enric Pallé (Institute of Astrophysics of the Canary Islands, Spain) and colleagues took spectroscopy measurements with the Very Large Telescope in Chile. Using a couple different techniques to check Mic b’s spin-orbit angle, they again found good alignment.

The latest of these studies appears in the October 1st Astronomical Journal. Brett Addison (University of Southern Queensland, Australia) led the project, using radial velocity measurements from the Minerva-Australis telescope array to compare the angle of the planet’s orbit with the spin-axis of its host star.

Exo-Neptune AU Mic b (art)
An artist's concept shows one interpretation of planet AU Mic b.
NASA's Goddard Space Flight Center / Chris Smith (USRA)

AU Microscopii is so young, it hasn’t even begun fusing hydrogen into helium in its core, and a massive planetary debris field surrounds it. But it already has two fully formed gas giants, both of which probably made a long trek from beyond the “ice line,” where they must have formed, into very close orbits around the host star. The temperatures close to a star are too hot for gases like water and methane to condense during planet formation, and most of the hydrogen and helium gets blown away by solar winds. But on the outer edges of the star system, all this material is free to accrete on a truly massive scale.

It’s impossible to watch planets form and migrate in real time. But if we can observe many different, comparable systems at various stages of development, then we have the next best thing: snapshots of planets’ development over time. The age and current arrangement of the AU Microscopii system thus contributes to a working knowledge of migration and timescales in the formation process. In this case, a star near the beginning of its lifespan has already had enough time to spin out two planets, both of which have apparently taken a stroll into completely different orbits.

Scott Gaudi (Ohio State University), who was not involved with these studies, says that making these kinds of observations is very difficult because the spin-orbit alignment has to be observed during a transit. And in the case of Addison’s study, the telescopes used  were relatively small (an array for four 0.7-meter telescopes), which affected the quality of the data.

“The other AU Mic b studies provided a more definitive answer because they were taken with larger telescopes,” Gaudi says. “The bigger the view, the more photons you can collect, the better your data.”

Right now, the easiest kind of planet to see is a gas giant very close to a small star. But with the next generation of dedicated exoplanet telescopes coming online in the next decade, it should be possible to detect worlds beyond the "ice line", closer to their birth sites. Gaudi looks forward to using the Nancy Grace Roman Space Telescope, which begins operations in 2025, to find more planetary systems which look like our own.

“It’s one of the big open questions in planetary science,” Gaudi says. “At the moment we see lots of big planets that appear to have migrated, especially those called hot Jupiters. But the solar system looks different and no one knows why. The Roman telescope should give us a better idea of how we fit into the big picture.”


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