They find an exoplanet shaped like a football 0:39
Astronomers may have found a moon that is completely unlike anything in our solar system.
It is only the second space object discovered that may be an exomoon or a moon outside our solar system.
The giant moon was found orbiting a Jupiter-sized planet called Kepler 1708b, located 5,500 light-years from Earth.
A study detailing these findings was published Thursday in the journal Nature Astronomy.
The newly detected celestial body is 2.6 times larger than Earth.
There is no analogy for such a large moon in our own system.
For reference, our own moon is 3.7 times smaller than Earth.
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This is the second time that David Kipping, assistant professor of astronomy and leader of Columbia University's Cool Worlds Lab, and his team have found an exomoon candidate.
They discovered the first, a Neptune-sized moon orbiting a giant exoplanet called Kepler-1625b, in 2018.
"Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are much more challenging," Kipping said in a statement.
"They are terra incognita" (unknown land)."
Understanding more about moons—for example, how they form, whether they could support life, and whether 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 items
Kipping and his team are still working to confirm 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 been largely unsuccessful.
Astronomers have been successful in locating exoplanets around stars outside our solar system, but exomoons are more difficult to identify due to their smaller size.
More than 4,000 confirmed exoplanets have been discovered across the galaxy, but that doesn't mean finding them has been easy.
Many of them were detected using the transit method, or by looking for dips in starlight when a planet passes in front of its star.
Detecting moons, which are smaller and cause even tinier dips in starlight, is very difficult.
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To find this second potential moon, Kipping and his team used data from NASA's retired planet-hunting Kepler mission to study some of the cooler gas giant exoplanets the telescope found.
The researchers used this criterion in their search because in our solar system, the gas giants Jupiter and Saturn have the largest number of moons orbiting them.
Of the 70 planets they studied, only one revealed a companion signal that appeared to be a moon, with only a 1% chance that it was something else.
"It's a stubborn sign," 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 probably gaseous, which explains their enormous size, and they are far from their host stars.
There are three main theories about how moons form.
One is when large space objects collide and the detached material becomes a moon.
Another is capture, when objects are captured and put 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 the moons that form from the materials, such as gas and dust swirling around stars, that created the planets in the early days of the solar system.
It is possible that both exomoon candidates started out as planets that were eventually pulled into orbit around larger planets like Kepler 1625b and Kepler 1708b.
Giant moons are probably an anomaly
Kipping thinks it's unlikely that all the moons outside our solar system are as big as these two candidates, which may make them the oddballs, rather than the standard.
"The first detections in any survey will generally be the weirdos," he said.
"The big ones that are just easier to detect with our limited sensitivity."
Confirming that the two candidates are exomoons will require follow-up observations by the Hubble Space Telescope and the James Webb Space Telescope in 2023. Meanwhile, Kipping and his team continue to gather evidence in support of exomoons.
The fact that each associated planet takes more than an Earth year to complete one orbit around its star slows down the discovery process.
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"Confirmation requires lunar transits to be repeated multiple times," Kipping said.
"The long-period nature of our target planets means we only have two transits available here, but not enough to see a series of lunar transits needed to claim a confirmed detection."
If confirmed, it could be the start of a new acceptance that exomoons are just as common as exoplanets outside our solar system.
The first exoplanet wasn't discovered until the 1990s, and most of the exoplanets known today weren't revealed until the Kepler launch in 2009.
"Those planets are alien compared to our home system," Kipping said.
"But they have revolutionized our understanding of how planetary systems form."