A moon could have been found orbiting an exoplanet

Over the past three decades, the field of extrasolar planet studies has advanced by leaps and bounds. Nowadays, 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 Aquatic Worlds, has raised many questions about the nature of planet formation and evolution. A rather important question is the role and commonalities of natural satellites, aka. “exomones”.

Given the number of moons in the solar system, it’s entirely reasonable to assume that moons are ubiquitous in our galaxy. Unfortunately, despite thousands of known exoplanets, there are still no confirmed exomoons available for study. But thanks to Columbia University Professor David Kipping and an international team of astronomers, who may have changed. In a recent Study supported by NASA, Kipping and colleagues report the possible discovery of an exomoon they found while examining data from the Kepler Space Telescope.

The research team included members of the NASA Exoplanet Science Institute (NExScI) at Caltech, NASA’s Ames Research Center, the Kavli Institute of Astrophysics and Space Research at MIT, the Mani Bhaumik Institute for Theoretical Physics at UCLA, the Institute of Particle Physics and Astrophysics at the Swiss Federal Institute of Technology Zurich (ETHZ) and Institute of Astronomy and Astrophysics at the Academia Sinica in Taipei. The article describing their research and findings recently appeared in the journal natural astronomy.

Artist’s rendering of NASA’s Kepler spacecraft. Credit: NASA/Kepler

Professor Kipping is well known for his pioneering work in exoplanet studies. As head of the Cool Worlds Laboratory 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 Exolune hunting with Kepler (HEK), a campaign affiliated with the Harvard-Smithsonian Center for Astrophysics (CfA) which is dedicated to finding evidence of exomoons in Kepler mission data. As Kipping told Universe Today via email:

“Astronomers tend to fall into two categories, those who want to understand how the universe works and those who want to know if we are alone or not. In both themes, the exomoons are very promising. Regarding the former, they will provide further 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 (probably by a giant impact) a 1 in 1 trillion chance, or are we looking at the inevitable result of planet formation?

“And on the latter, moons can be frequent abodes for life, a common trope in science fiction of course. Since a primary goal of NASA is to understand how common Earth-like worlds are, the search for moons is a necessary part of that – as far as we know they can, in fact, dominate habitable real estate in the cosmos.

The formation and evolution of Earth’s only natural satellite, the Moon, is closely linked to that of the Earth itself. According to the giant impact hypothesis, the two formed after a Mars-sized object (Theia) collided with a primordial Earth around 4.5 billion years ago. Moreover, some scientists believe that this giant impact could be the reason the Earth is habitable today. Another theory is that the Moon helps keep the dynamo going inside the Earth, which generates the magnetic field that protects us from radiation.

For these reasons, Kipping and his colleagues studied exoplanet systems and worked to create ways to detect exomoons. One of the methods that Kipping and his colleagues have devised to search for them is the Transit Time Variations (TTV), where the gravitational oscillations of an exoplanet are interpreted as the influence of exomoons (similar to the radial velocity method). Another method is to look for the transits of the exomoons themselves, which is consistent 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 present new evidence obtained by the The Hubble Space Telescope which reinforced their previous conclusions. Kepler-1625b-i remained the only candidate exomoon because they are very difficult to detect. kipping said:

“Exomoons are difficult to detect because they are expected to be smaller than your typical planet, making them hard to find, and furthermore, their signals mix with planetary transit, making them difficult to disentangle. There are many methods to search for exomoons. But for the sake of brevity, we believe that transits are the most effective approach, so they are clearly very effective for planet discovery and offer repeatable events from which to build a falsifiable hypothesis.

As noted, natural satellites are extremely common around the gas/ice giants of the Solar System, all of which orbit beyond the frost line and are “cold” (as opposed to hot Jupiters and Neptunes). Therefore, it seems logical that the same is true for cold gas/ice giant exoplanets. This led Kipping and his associates at HEK to examine Kepler data for possible indications of exomoons transiting their parent exoplanets.

An artist’s conception of a habitable exomoon. Credit: NASA

“We assume that hot Jupiters are unlikely, for example, because they are thought to migrate inwards, which would be dangerous to the moon’s survival,” Kipping said. “In our work, we have bet [theorized] that the cool giants were the best place to look, but that was a punt. This was driven by the outer giant planets which have an abundance of moons and the decrease in size of Hill spheres which occurs for nearby planets.

To further test this hypothesis, Kipping and his team examined archival data obtained by Kepler for transits of cold gas giants about twice the size of Jupiter and orbital periods of more than 400 days. After eliminating any object with less than two transits (and likely false positives), this resulted in a sample of 73 exoplanets. They then analyzed the sample based on a planet+moon pattern to a planet-only pattern 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,” Kipping said. “The best way to describe it is that it is a transit signal for which the best-fitting astrophysical model is a planet + moon model, and we can find no reason to reject that hypothesis after a battery of verification tests.

Of course, this research is still in its infancy, and Kipping and his colleagues recognize that it takes time to develop their methods and refine their techniques. “I’m optimistic that we can build on these successes to eventually find even smaller moons where we’re likely to have less discomfort with their nature as they converge more and more towards the moons we find in our solar system,” summarizes Kipping.

Artist’s impression of the view of a hypothetical moon around an exoplanet orbiting a triple star system. Credit: NASA

In addition, research on exoplanets and exomoons will greatly benefit in the near future, as next-generation observatories like the James Webb and the Nancy GraceRoman space telescopes become available. Now that the James Webb has finally launched and deployed its mirrors and heat shield, astronomers expect it to 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 limit the search for exomoons.

Using their advanced suites of giant primary mirrors, spectrometers, coronagraphs and adaptive optics, these observatories will perform direct imaging studies of exoplanets. Particularly smaller rocky planets that orbit closer to their stars where one would expect to find Earth-like planets. These advanced capabilities can also spot faint light signatures caused by orbiting exomoons.

Further reading: natural astronomy

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