Massive object could be an interstellar moon, a rare find


It’s only the second space object discovered that may be an exomoon, or a moon outside of our solar system. The giant moon was found orbiting a Jupiter-size planet called Kepler 1708b, located 5,500 light-years from Earth.

The newly detected celestial body is 2.6 times larger than Earth. There’s no analog for such a large moon in our own system. For reference, our own moon is 3.7 times smaller than Earth.

It’s the second time that David Kipping, assistant professor of astronomy and leader of the Cool Worlds Lab at Columbia University, and his team have found an exomoon candidate. They discovered the first one, a Neptune-size moon orbiting a giant exoplanet called Kepler-1625b, in 2018.

“Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are far more challenging,” Kipping said in a statement. “They are terra incognita.”

Understanding more about moons, such as how they form, whether they could support life, and if they play a role in the potential habitability of planets, could lead to a greater understanding of how planetary systems form and evolve.

Hard-to-find objects

Kipping and his team are still working on confirming that the first candidate they found is actually an exomoon, and this latest discovery will likely face the same uphill battle.

Moons are common in our solar system, which has more than 200 natural satellites, but the long search for interstellar moons has largely been unfruitful. Astronomers have had success locating exoplanets around stars outside our solar system, but exomoons are harder to pinpoint because of their smaller size.

Scientists may have found a fiery exomoon that's like a volcanic planet from 'Star Wars'

More than 4,000 confirmed exoplanets have been discovered across the galaxy, but that doesn’t mean finding them was easy. Many of them have been detected using the transit method, or looking for dips in starlight when a planet passes in front of its star. Spotting moons, which are smaller and cause even more diminutive dips in starlight, is extra difficult.

To find this second potential moon, Kipping and his team used data from NASA’s retired planet-hunting Kepler mission to survey some of the coldest gas giant exoplanets the telescope found. The researchers used this criteria in their search because in our solar system, the gas giants Jupiter and Saturn have the most moons orbiting them.

Out of the 70 planets they studied, only one revealed a companion signal that appeared to be a moon, with only a 1% probability of it being something else.

“It’s a stubborn signal,” Kipping said. “We threw the kitchen sink at this thing but it just won’t go away.”

3 ways a moon could form

The newly discovered candidate shares similarities with the first potential exomoon discovery. Both are likely gaseous, which accounts for their massive size, and they are far from their host stars.

This potential moon outside of the solar system isn't anything like ours

There are three primary theories about how moons form. One is when large space objects collide and the blasted-off material becomes a moon. Another is capture, when objects are captured and pulled into orbit around a large planet — such as Neptune’s moon Triton, which is believed to be a captured Kuiper Belt object. And the third is moons forming from the materials, like gas and dust swirling around stars,…



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