In the past three decades, the field of extrasolar planet studies has advanced by leaps and bounds. To date, 4,903 extrasolar planets have been confirmed in 3,677 planetary systems, with another 8,414 candidates awaiting confirmation. The diverse nature of these planets, ranging from Super-Jupiters and Super-Earths to Mini-Neptunes and Water Worlds, has raised many questions about the nature of planet formation and evolution. A rather important question is the role and commonality of natural satellites, aka. “exomoons.”
Given the number of moons in the Solar System, it is entirely reasonable to assume that moons are ubiquitous in our galaxy. Unfortunately, despite thousands of know exoplanets, there are still no confirmed exomoons available for study. But thanks to Columbia University’s Professor David Kipping and an international team of astronomers, that may have changed. In a recent NASA-supported study, Kipping and his colleagues report on the possible discovery of an exomoon they found while examining data from the Kepler Space Telescope.
The research team included members from the NASA Exoplanet Science Institute (NExScI) at Caltech, the NASA Ames Research Center, the Kavli Institute for Astrophysics and Space Research at MIT, the Mani Bhaumik Institute for Theoretical Physics at UCLA, the Institute for Particle Physics & Astrophysics at the Swiss Federal Institute of Technology (ETH) Zurich, and the Institute of Astronomy and Astrophysics at the Academia Sinica in Taipei. The paper describing their research and findings recently appeared in the journal Nature Astronomy.
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Professor Kipping is well-known for his pioneering work in exoplanet studies. As the Cool Worlds Laboratory leader at Columbia University, he and his colleagues have spent years developing methods for the study and characterization of exoplanets. Kipping is also the principal investigator of the Hunt for Exomoons with Kepler (HEK), a campaign affiliated with the Harvard-Smithsonian Center of Astrophysics (CfA) that is dedicated to finding evidence of exomoons in Kepler mission data. As Kipping told Universe Today via email:
“Astronomers tend to come into two flavors, those who want to understand how the universe works and those who want to know if we’re alone or not. In both themes, exomoons hold much promise. Concerning the former, they will provide other examples of how moons manifest in the Universe beyond our cosmic shore. When we look at the Moon, for example, we wonder – was its formation (likely through a giant impact) a 1 in a trillion fluke, or are we looking at the inevitable outcome of planet formation?
“And on the latter, moons may be frequent abodes for life, a common trope in sci-fi of course. Since an overarching goal of NASA is to understand how common are Earth-like worlds, looking for moons is a necessary part of that – for all we know that they may, in fact, dominate the habitable real estate in the cosmos.”
The formation and evolution of Earth’s only natural satellite, the Moon, is closely linked to that of Earth itself. According to the Giant Impact Hypothesis, both formed after a Mars-sized object (Theia) collided with a primordial Earth roughly 4.5 billion years ago. Furthermore, some scientists speculate that this giant impact may be the reason why Earth is habitable today. Another theory has it that the Moon helps maintain the dynamo in Earth’s interior, which generates the magnetic field that shields us from radiation.
For these reasons, Kipping and his colleagues have studied exoplanet systems and worked towards creating means for detecting exomoons. One of the methods Kipping and his colleagues have devised to look for them is the Transit Timing Variations (TTV), where an exoplanet’s gravitational wobbles are interpreted as the influence of exomoons (similar to the Radial Velocity Method). Another method is to look for the transits of exomoons themselves, which is in keeping with Transit Photometry (aka. The Transit Method).
In 2017, Kipping and the HEK campaign identified the strongest exomoon candidate to date: Kepler-1625b-i. Using Transit Photometry (aka. the Transit Method) from Kepler, the team found evidence of a possible Neptune-sized exomoon (or double planet) orbiting a Sun-like star 8,000 light-years from Earth. A year later, they presented new evidence obtained by the Hubble Space Telescope that reinforced their previous findings. Kepler-1625b-i has remained the only candidate exomoon since they are very difficult to detect. Said Kipping:
“Exomoons are challenging to detect because they are expected to be smaller than your typical planet, thus making them hard to find, and further their signals get mixed up with the planetary transit making it hard to disentangle. There are many, many methods to look for exomoons. But for the sake of brevity we believe that transits are the most effective approach, so they are clearly highly successful for planet discovery and offer repeatable events enabling a falsifiable hypothesis to be constructed.”
As noted, natural satellites are extremely common around gas/ice giants in the Solar System, all of which orbit beyond the Frost Line and are “cool” (as opposed to Hot Jupiters and Neptunes). Therefore, it seems logical the same is true of cool gas/ice giant exoplanets. This led Kipping and his associates at HEK to examine through the Kepler data for possible indications of exomoons making transits along with their parent exoplanets.
“We speculate that hot-Jupiters are improbable, for example, since they are thought to migrate inwards which would be hazardous to moon survival,” said Kipping. “In our work, we punted [theorized] that cool giants were the best place to look, but that was a punt. It was motivated by the outer giant planets that have an abundance of moons and the decreased Hill sphere sizes that occur for close-in planets.”
To test this hypothesis further, Kipping and his team examined archival data obtained by Kepler for transits by cool gas giants about two times the size of Jupiter and orbital periods of more than 400 days. After eliminating any object with fewer than two transits (and likely false positives), this yielded a sample of 73 exoplanets. They then analyzed the sample based on a planet+moon to a planet-only model to see where a planet+moon signal was strongly favored. In the end, Kepler-1708b was the strongest candidate.
“It was the only object that passed every test we could think of,” said Kipping. “The best way to describe it is that it’s a transit signal for which the best fitting astrophysical model is a planet+moon model, and we can find no reasons to discard that hypothesis after a battery of vetting tests.”
Of course, this research is still in its infancy, and Kipping and his colleagues acknowledge that time is needed to develop their methods and refine their techniques. “I’m optimistic that we can build upon these successes to eventually find even smaller moons for which we will likely have less discomfort with their nature as they increasingly converge upon the moons we find in our Solar System,” Kipping summarizes.
In addition, exoplanet and exomoon research will benefit considerably in the near future as next-generation observatories like the James Webb and the Nancy Grace Roman space telescopes become available. Now that the James Webb has finally launched and has deployed its mirrors and heat shield, astronomers anticipate that it will take its first images in just six months. Meanwhile, ground-based telescopes like the Extremely Large Telescope (ELT) and Giant Magellan Telescope (GMT) will also narrow the search for exomoons.
Using their advanced suites of giant primary mirrors, spectrometers, coronographs, and adaptive optics, these observatories will carry out Direct Imaging studies of exoplanets. Particularly smaller, rocky planets that orbit closer to their stars where Earth-like planets are expected to be found. These advanced capabilities are may also spot the faint light signatures caused by orbiting exomoons.
Further Reading: Nature Astronomy