Neptune, the farthest of the planets, acts like a shepherd for the outer solar system, gravitationally scattering distant asteroids known as Kuiper Belt Objects (KBOs). Understanding Neptune's history gives important clues to how the rest of the solar system evolved to its present state.
Neptune itself is unique—tilted 30 degrees on its side, it is host to a few unusual moons, including the Pluto-sized moon called Triton. Triton orbits Neptune backward, an indicator that it did not form around Neptune but was instead captured by Neptune's gravity after it formed elsewhere in the solar system. New observations coupled with simulations of Neptune's evolutionary history indicate that an oft-overlooked Neptunian moon called Nereid may reveal the planet's past.
The research was led by graduate student Matthew Belyakov and conducted as a collaboration between the laboratories of Professor of Planetary Science Konstantin Batygin (PhD '12), and Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy and Terence D. Barr Leadership Chair and director of the Center for Comparative Planetary Evolution. The work is reported in a paper appearing in Science Advances on May 20.
Jupiter, Saturn, and Uranus all have "typical" moon systems, with each planet possessing several large moons that orbit closely and along the host planet's equatorial plane, as well as many smaller moons, called irregular satellites, located farther out on tilted or "inclined" orbits. Neptune, on the other hand, has just one large moon, Triton, that contains 99.9 percent of the mass in its entire moon system. Triton's orbit is retrograde—it moves clockwise, while Neptune orbits the Sun counterclockwise. This means that Triton could not have coalesced in place, as Jupiter and Saturn's moons did, out of the disk of material orbiting counterclockwise around its planet. Instead, Triton is thought to be a Kuiper Belt Object, like Pluto, that was flung into Neptune's path and ensnared gravitationally.
Prior to Voyager 2's flyby of Neptune in August 1989, only one other moon was known around Neptune, Nereid. Discovered by Dutch astronomer Gerard Kuiper in 1949, Nereid has since presented a mystery. The moon follows an eccentric orbit, swinging around Neptune in an ellipse, and is far from its planet, but not nearly as distant as irregular satellites around the other giant planets. Interestingly, Nereid does not have a retrograde orbit like Triton, and its orbit is much less inclined than other irregular moons in the solar system. Given these details, scientists debated Nereid's origin for 70 years, unable to conclude whether the moon was captured or formed in place.
In 2024, Caltech graduate students Matthew Belyakov and M. Ryleigh Davis (MS '22) used the James Webb Space Telescope (JWST) to observe the Neptunian moon system, with Nereid as one of the targets. The team used JWST's near-infrared spectrograph, which splits light into its many wavelengths in order to obtain chemical information about astronomical targets. Nereid's spectrum appeared rather different from that of Kuiper belt objects—Nereid was instead more similar to the moons of Uranus. Informed by the observational data, which hinted toward a noncaptured origin for Nereid, Belyakov then developed simulations of the evolution of Neptune's moons.
The simulations showed that as Triton crashed into the Neptunian system and was captured, existing Neptunian moons could have been kicked out on eccentric orbits that looked identical to Nereid's. This suggests that Nereid formed in situ around Neptune, rather than being a captured foreign object.
"Understanding what transpired at Neptune is one of the ways that we can solve what happened in the early solar system, and Nereid is important for pinning down key events like Triton's capture," Belyakov says. "We're hoping this work motivates people to do creative observations of Nereid, even though it is faint and distant. It's just as important as Triton. I hope Nereid will be visited by a mission within my lifetime."
Without such a mission, much about Nereid is likely to remain a mystery. Voyager images of Nereid are only a few pixels across. In continuation of their work, the team aims to build more simulations to constrain the timing of Triton's capture and the possible configurations for the initial moon system around Neptune.
The paper is titled "Nereid as a Regular Satellite of Neptune." In addition to Belyakov, Batygin, Brown, and Davis, former Caltech graduate student Ian Wong (PhD '18), now of the Space Telescope Science Institute in Baltimore, Maryland, is a co-author. Funding was provided by NASA, the European Space Agency, and the Canadian Space Agency, which jointly operate JWST.
