A New Way to Search for Exomoons

We’d love to find another planet like Earth. Not exactly like Earth; that’s kind of ridiculous and probably a little more science fiction than science. But what if we could find one similar enough to Earth to make us wonder?

How could we find it? We progress from one planet-finding mission to the next, compiling a list of planets that may be “Earth-like” or “potentially habitable.” Soon, we’ll have the James Webb Space Telescope and its ability to study exoplanet atmospheres for signs of life and habitability.

But one new study is focusing on exomoons and the role they play in a planet’s habitability. If we find a Moon-like exomoon in a stable orbit around its planet, could it be evidence that the planet itself is more Earth-like? Maybe, but we’re not there yet.

Scientists believe that the stable relationship between the Earth and the Moon is part of what makes Earth habitable. For one thing, the Moon keeps Earth’s axial tilt stable, which nurtures a stable climate. Researchers also know that many factors can disrupt long-term planet-moon stability. In the paper “Exomoons in Systems with a Strong Perturber: Applications to ? Cen AB” the authors explore the orbital relationships between moons, their planets and stars. The lead author is Billy Quarles, an astrophysicist and planetary dynamicist from Georgia Tech. The paper is published in The Astrophysical Journal.

In our own Solar System, there are many more moons than there are planets. There’s an average of 20 moons per planet, thanks largely to Jupiter and Saturn and their combined 160+ moons. Mercury and Venus have none, Earth has only one, and Mars has only two, which are likely captured asteroids.

But our Solar System isn’t representative of other systems. Ours has only a single star. When we look out into the night sky, more than 80% of the points of light that we see are actually a pair of stars—or more—grouped together. Binary star systems can host planets, and the planets can orbit these systems in two ways. Circumstellar means a planet orbits only one of the stars, and circumbinary means a planet orbits both stars.

Previous research, including some by the lead author of this study, Billy Quarles, has shown that giant circumbinary planets can host exomoons. But the same hasn’t been found for circumstellar planets in multiple star systems. That’s part of the purpose of this study.

The researchers focused on our nearest stellar neighbour, the Centauri system. Centauri is “only” 4.37 light-years away, which is close in astronomical terms. But even at that distance, the planets are hard to detect. And moons? Almost impossible.

“We know they are there,” said study co-author Siegfried Eggl in a press release. “We just need to look harder. But because it is so difficult to see them, we identified a way to detect them through the effect they have on a planet using transit timing variations.”

“We know the planets, stars, and moons in our solar system interact gravitationally like a giant board game,” Eggl said. “The moon is tidally interacting with the Earth and slowing its own rotation, but the Sun is also there, tugging on both. A second star would act as another external perturber to the system.”

The researchers used Transit-Timing Variation (TTV) to search for moons in the Alpha Centauri system. Alpha Centauri is a triple star system, and at least two planets, which both orbit the star Proxima Centauri. Proxima Centauri is a red dwarf and the two planets are Proxima Centauri b and Proxima Centauri c. PC b is a terrestrial planet about 1.17 Earth masses, and PC c is a super-Earth, or maybe a mini-Neptune.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer to its star than Mercury. As a result, it lies well within the habitable zone, where liquid water can exist on the planet’s surface. Image Credit: Palereddot.org

The TTV method measures the tiny tugs that bodies exert on one another as they go about their orbital business. It’s most often used to find exoplanets. When a planet passes in front of its star, the starlight dims a little from our perspective. If another object is exerting a force on the planet, then the timing of the dimming starlight will be variable.

If the planet has a moon, the moon will exert a small force on the planet, making the planet wobble a little. That wobble can be enough to change the timing of the starlight blocked by the planet. Measuring those small changes is at the heart of TTV.

In a system like Alpha Centauri, with multiple planets and stars, there’s a lot going on, and many tugs to sort out. The planets in AC are in a circumbinary orbit, which means the orbits are more elliptical than Earth’s for example. So some of the timing variations are caused by the planet and its elliptical orbit. Other of the timing variations can be due to exomoons.

If observers can figure out which of the timing variabilities are due to exomoons, then they can infer some of the properties of both the planet and its moon. “This is an indirect proof of a moon because there’s nothing else that could tug on the planet in that kind of fashion,” Eggl said.

It’s difficult to tease out all the variables in a multiple-star system like Alpha Centauri. The elliptical orbits make it more difficult because the planet and its moon(s) can move at different speeds. “When moons and planets have slightly elliptical orbits, they don’t always move at the same speed. The more eccentric an orbit, the more frequencies can be excited, and we see these resonances become more and more important,” Eggl said.

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser

At some point, the resonances can overlap one another and make the whole system kind of chaotic. But even in the chaos, there should be some periods of stability. “At some point, there will be overlapping resonances that can lead to chaos in the system. In our study, we have shown, however, that there is enough stable ‘real estate’ to merit a thorough search for moons around planets in double-star systems,” said co-author Eggl.

This study is aimed at the stable real estate in a planet’s orbit in a three-body system, where there are two stars and a planet. Where the planet is stable, there’s like another nested three-body hierarchy between the binary star, the planet, and its moon. There’s more: “In addition, a secondary star orbits the center of mass of three-body system at such a distance so that the planetary orbit remains stable to produce a hierarchical four-body configuration,” the authors write. The authors say you can look at the whole arrangement as two three-body problems that are couples together.

Lead author Quarles used the ocean’s tides here on Earth to help explain their efforts. “The major difference with binary systems is the companion star acts like the tide at the beach, where it periodically comes in and etches away the beachfront.  With a more eccentric binary orbit, a larger portion of the stable ‘real estate’ is removed. This can help out a lot in our search for moons in other star systems.”

One of the central issues behind this work is not only identifying exomoons but determining their longevity. A moon’s longevity can aid a planet’s long-term habitability. That’s what’s happened here on Earth. But there are many reasons a moon can be ejected, or escape, from its planet’s orbital grip. Eccentric orbits in a binary star system are one of the primary reasons. One of the stars in a binary system acts as a perturber, driving the moon away from its planet.

The Moon plays a big role in Earth’s habitability. The same is likely true in other solar systems. Image Credit: NASA SVS/Ernie Wright

Scientific models show that the dissipation of tidal forces between a planet and its moon can free a satellite from its host planet. When the team applied these models to the Alpha Centauri system, they found that the star Alpha Centauri A acts as a pertuber and leads to more eccentricity in the orbit of the Earth-like planet Alpha Centauri B. That can lead to any moons orbiting the planet becoming unstable on 10 Gyr timescales. But not always. They also found that exomoons can withstand some of the eccentric forcings and remain stable.

So what does all this mean? What does it lead to?

By studying the TTV of the Earth-analog planet in Alpha Centauri, the team developed constraints on what an Earth-Moon configuration could look like in other binary solar systems. The TTV for an Earth-Moon combo in other systems could be very small. It could be as small as some of the TTVs detected by the Kepler mission in distant solar systems. Some of those detections are likely astronomical noise, some probably are evidence of exomoons.

Finding exomoons, and understanding their longevity, could turn out to be a critical method of evaluating planets for potential habitability, right up there with a planet’s position in a star’s habitable zone.

“If we can use this method to show there are other moons out there, then there are probably other systems similar to ours,” Quarles said. “The moon is also likely critical for the evolution of life on our planet because without the moon the axis tilt of the Earth wouldn’t be as stable, the results of which would be detrimental to climate stability. Other peer-reviewed studies have shown the relationship between moons and the possibility of complex life.”

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