Even if you aren’t a Star Trek fan, you probably know the first few words of the original opening sequence, “space: the final frontier.” Mankind is in its earliest stages of exploring what lies beyond our own small blue world.
For as long as we’ve looked up at the stars in wonder, we’ve fantasized about what might be out there, waiting to be discovered. As what we are capable of doing constantly expands, we are finding that some of our dreams may be close to reality…
Stars Are Easy
As far as astronomical phenomena go, stars are the easiest things to find. They’re so large and bright that you can spot thousands of them with the naked eye just by looking up on a clear night.
Probably Like Home
Anything smaller and dimmer than a star and it becomes a lot more difficult to find. In the past, we could only assume based on our own solar system that some other stars would have planets orbiting them and that some of those planets would resemble those that we’re familiar with.
Because planets don’t give off any light of their own, the best way to detect “exoplanets,” planets that are outside of the solar system, is through a technique called the “transit method.” As a planet passes in front of a star, it blocks some of the light traveling toward earth, causing a small dip in its brightness.
So when scientists spot a regular dip in the light coming from a star, they can determine that it’s caused by some large body orbiting the star. Through more careful – and technically complicated – observations, they can tell a lot about that body, including whether or not it’s a planet, how big the planet is and what it’s composed of, and how long it takes to orbit its star.
The first confirmed discovery of an exoplanet came in 1992 when two astronomers named Aleksander Wolszczan and Dale Frail discovered a pair of planets orbiting a pulsar named Lich, finally proving that the handful of planets in our neighborhood weren’t unique in the universe.
Where Are The Moons?
But there was one aspect of our solar system that we hadn’t been able to prove existed until very recently. In all our searching the night sky with the most powerful of telescopes, we had yet to find definitive evidence of a natural satellite – a moon – orbiting an exoplanet.
The moons orbiting the planets of other stars are so difficult to see for the same reasons as exoplanets, compounded by the fact that they are by definition close to a planetary body. That proximity makes it difficult to tell if any blockage of light is caused by the planet itself.
In order to be certain about the existence of an exomoon, it would help if it were very large. In 2018, astronomers believed they finally found exactly what they were looking for: evidence of an absolutely gigantic moon, orbiting an even larger planet.
Around Kepler-1625, one of the stars in the Cygnus constellation, there is a Jupiter-like gas giant named Kepler-1625b in orbit. The gas giant is somewhere between six and eleven times the size of the Earth. NASA’s Kepler and Hubble space telescopes have both spotted evidence of a moon roughly in orbit around that planet that seems to be roughly the size of Venus – about four times the size of the Earth.
Even though the colossal satellite is about eight times farther away from its planet than our moon is from the Earth, it’s huge size would make it appear about twice as large in the sky. But because it’s so big, some people have said that the newly found astronomical object should be described as a “binary” or “double planet.”
Though the planet and its moon aren’t that far apart in size, the moon is only about 1.5 percent as massive as Kepler-1625b. “The mass ratio […] defines the difference between a planet and a moon or a binary planet,” said Alex Teachey, head of the team that made the discovery. “I would call that a moon, but to some extent I think this is […] semantics.”
In The Zone
Whatever you’d like to call it, the exomoon and the planet it orbits are both within the star’s habitable zone, sometimes referred to as the “Goldilocks zone.” That means their orbit is close enough to the star to keep the planet from being frigid, but not so close that it is blazing hot. In other words, just right for liquid water to exist.
Liquid water is a necessity for all life as we know it so its presence on an extraterrestrial body suggests the possibility of life. Even though no evidence has been found that there’s water on the exomoon yet, the idea that there might be has reminded some of the life-bearing alien moons in science fiction.
The most popular example in recent years has been Pandora, the home of the tall blue aliens from the 2009 film “Avatar.” The fictional moon, which shares a name with one of the real life moons of Saturn, orbited a gas giant and supported a lush, sylvan ecosystem teeming with strange plant and animal life.
Not My Fav
But Pandora isn’t Teachey’s favorite fictional exomoon. “I’m partial to Endor myself,” he said. “I’m more of a ‘Star Wars’ fan.” Endor was another forest moon, inhabited by the Ewoks, a race of adorable and surprisingly fierce teddy-bear people.
But in all actuality, the odds of there being life as we understand it on Kepler-1625b or its giant moon are effectively nil. Based on the observations they’ve made so far, Teachey and his co-author David Kipping believe that both the planet and moon are gas giants with a solid core capable of sustaining life.
“To be honest, we were never really worried much about habitability, just because they’re both gas giants, so it really wasn’t something we spent any effort or time worrying about,” said Kipping during a press teleconference.
But there’s another interesting wrinkle to the possibility of the moon’s potential habitability. Kepler-1625 is a sun-like star but is about 10 billion years old, roughly twice the age of our sun, and nearing the end of its life. That means the star has heated up a lot over the recent millennia, which means the moon may have also grown a lot hotter.
Not Quite As It Seems
That increase in the moon’s temperature may have caused its atmosphere to expand, making it appear to be a gas giant like Jupiter from a distance when in reality, it could be a rocky planet like Earth. That leaves the door open to the slimmest of chances that there is, or was in the past, life on the exomoon.
Whatever the case, the exomoon still needs to be confirmed by follow-up observations. If Teachey and Kipping are correct in their findings, it will be a discovery that will cement their names in the history books of astronomy. Perhaps a few thousand years from now, “the first exomoon” will be a roadside attraction of sorts along some interstellar “Route 66.”