Last summer, scientists announced that they had found what could be the first moon to be spotted outside of the solar system. But new research on the supposed moon's evolution calls its existence into question.
If it does exist, the moon is most likely a large, Neptune-size object orbiting an even larger gas-giant planet. But the unwieldy system strains understanding of how it may have formed, researchers have said.
In July 2017, scientists reluctantly announced the possible discovery of an exomoon. A candidate planet identified by NASA's Kepler telescope revealed lopsided dips in the light streaming from the planet's star, suggesting the possibility of a moon. After exomoon hunter David Kipping, of Columbia University in New York, requested time on the Hubble Space Telescope to follow up on the unusual activity, various media outlets probed the research. This led Kipping and Columbia's Alex Teachey, the lead scientist on the potential discovery, to announce the possibility of the first sighting of an exomoon.
René Heller, an astrophysicist at the Max Planck Institute in Germany, took the opportunity to independently analyze the Kepler data. In addition to teasing out a size range for the potential moon, Kepler 1625 b-i, he also explored its possible formation methods. [The Most Intriguing Alien Planet Discoveries of 2017]
"It turns out that Kepler 1625 b-i is, in fact, not a good candidate for an exomoon," Heller told Space.com by email, pointing out that the original research team said that the Kepler data alone was ambiguous. (That's why they planned to follow up using the Hubble Space Telescope.) A large part of the problem stems from the fact that the parent star is so far from Earth that it appears dim, resulting in poor data quality, Heller said.
"The bottom line is that Kepler 1625 b-i is one of the best exomoon candidates so far, but it's still not a good candidate," Heller said.
"A tiny solar system"
In Earth's solar system, moons are fairly common; only Mercury and Venus have no rocky or icy satellites. While most of our solar system's moons are inhospitable to life as we know it, three are potentially habitable. Jupiter's Europa contains a liquid ocean beneath the moon's icy crust. Around Saturn, the icy moon Enceladus also hosts an ocean, while smoggy Titan has lakes of methane and ethane that could have allowed a type of life different from that on Earth to form. So, the solar system's only habitable planet (Earth) is outnumbered by the system's potentially habitable moons.
That could mean good news for those looking for life on moons around other stars. Even if few planets are capable of hosting life as we know it, their moons could turn out to be habitable, Heller said.
"On the challenging side, moons are expected to be significantly smaller and lighter than their planets," Heller said. "That's simply what we learn from observations of the solar system's moons."
Because objects with a larger mass or radius are easier to find from afar, be they planets or moons, that makes natural satellites harder to spot, Heller said.
When Kepler hunts planets, it does so by watching the light streaming from a star in what scientists call a light curve. (Kepler didn't study one star at a time but instead examined thousands of stars at once.) When a planet moves between its star and Earth, the star's light dims, allowing researchers to determine the planet's size. Researchers observe multiple passes to determine how long it takes the planet to orbit its star.
What the original researchers noticed about one object, Kepler 1625 b, was that it contained a strange secondary dip. Heller used the publicly available data set from Kepler to study three transits of a Jupiter-size object moving across the star, along with some wiggles that could have been caused by a moon orbiting the object.
"If, and only if, these additional wiggles really stem from the moon, then it is possible to derive the mass and radius of both the planet and the moon from the dynamics of the planet-moon system that can be derived from the light curve," Heller said.
Heller determined that the massive object could be anything from a planet slightly more massive than Saturn up to a brown dwarf, an almost-star not quite massive enough to ignite fusion in its core, or even a very low-mass star (VLMS) that's a tenth the mass of the sun. The proposed moon could range from an Earth-mass gas satellite to a rock-and-water companion with no atmosphere.
Heller concluded that a Neptune-mass exomoon around a giant planet or low-mass brown dwarf would not match up with the mass-scaling relationship found in our solar system's moons. While Earth and Pluto both have large moons compared to the planets' sizes, the solar system's gas giants have moons closer to 0.01 to 0.03 percent of the planets' sizes, according to the Planetary Habitability Laboratory at the University of Puerto Rico.
Previous theories predicted that this relationship should extend to larger worlds, seeming to rule out the existence of the potential exomoon. On the other hand, a mini-Neptune around a high-mass brown dwarf or a VLMS would be more in line with that ratio, Heller said. [What Is the Moon Made Of?]
"If the primary transiting object is a very low-mass star and if its Neptune-sized companion turns out to actually exist, then we would see a tiny solar system in orbit around a sun-like star at about the Earth's distance to the sun. This would be something on its own!" Heller said.
Even without the potential for a habitable exomoon, the tiny solar system could help scientists understand how worlds form, he said.
"If the primary [object] were either a [brown dwarf] or a VLMS with a large companion, then this would represent a fascinating bridge between planet formation around stars and moon formation around giant planets," Heller said.
Heller posted his research on the arXiv preprint server.
The birth of moons
With estimates of the moon and planet — or star — in hand, Heller decided to look at how the moon could have formed.
"The moons in the solar system serve as tracers of their host planets' formation and evolution," he said in the new paper. "It can thus be expected that the discovery of moons around extrasolar planets could give fundamentally new insights into the formation and evolution of exoplanets that cannot be obtained by exoplanet observations alone."
With this in mind, Heller applied the three different models of moon formation in the solar system to the new potential exomoon.
First up was the impact model, which describes how scientists think the Earth's moon formed. When a large body slammed into Earth billions of years ago, the debris carved from the planet created a new companion. According to Heller, one peculiar characteristic of this model is the high size ratio of satellites to the planets. While the large size of the proposed moon compared to its host would be consistent with an impact, he expressed concern that the mass of the host planet or star was far higher than that of any planet in Earth's solar system.
In the second model of moon formation, they develop from the gas and dust left over after the planet is born, and this is how most of the gas giants' moons are thought to have formed. The mass-scaling ratio that keeps the moons so much smaller than their planets is a natural outcome of moon formation occurring in the gas-starved environment around a completed planet, Heller wrote in the paper. That same relationship makes this formation method unlikely, he said.
"If the companion around Kepler 1625 b can be confirmed and both objects can be validated as gas-giant objects, then it would be hard to understand how these two gas planets could possibly have formed through either a giant impact or in-situ accretion at their current orbits around the star," Heller wrote.
The remaining possibility is that the distant world captured a Neptune-size object. Neptune's moon, Triton, and both Martian moons are thought to have formed this way. The exomoon could have originally formed with an Earth-size companion, before being pulled away from it by the gravity of the larger object, Heller said. He determined that the capture of a Neptune-mass object by Kepler 1625 b is possible at the planet's current location.
Still, while such a capture is possible in principle, Heller told Space.com he thinks the scenario is "very unlikely."
And though scientists currently hold to those three different moon-formation scenarios for planets around Earth's sun, that doesn't mean that natural satellites couldn't form another way, Heller said.
"It is possible that this system actually formed through a mechanism that we haven't seen in the solar system," Heller said.
He suggested an alternate theory, similar to that of giant-planet formation, in which the two objects started out as a binary system of rocky planets. The pair could have drawn gas from the disk of leftover material, like the process by which giant planets form, with the future planet consuming more gas than its would-be moon. He cautioned that this was speculation and that the two objects might not be stable over long timescales.
Still, if the Neptune-size exomoon around Kepler 1625 b is real, the new system could provide an intriguing glimpse at moon formation outside the solar system, Heller said.
The Kepler data isn't the only available research. In October, Teachey and Kipping looked at the system using Hubble. Results from those observations should be announced soon.
Until then, however, things don't look good for the potential exomoon.
"The extraordinary claim of an exomoon is not supported by extraordinary evidence for it," Heller said.