Published: Sat, October 06, 2018
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Astronomers Discover Elemental Proof For A Moon Outside Our Solar System

Astronomers Discover Elemental Proof For A Moon Outside Our Solar System

"I'm more of a "Star Wars" fan".

Did the Hubble Space Telescope come up with evidence of the first moon orbiting a planet outside of our solar system? Or perhaps, like Earth's moon, it is actually a product of its planet, formed in some catastrophic collision. Kepler-1625b is a Jupiter-sized planet that orbits far enough away from its star-about the same distance Earth is from ours-that a moon could be stable.

"That's been a key driver for us for a while, just trying to understand the cosmic habitats out there that we might look for, for life", says Kipping.

With the Hubble, they monitored the exoplanet Kepler-1625b as it passed between the star it orbits - Kepler-1625 - and Earth, and they looked at how the brightness of that star dimmed as the planet passed in front of it. Yet there were weird signatures in the flicker of this far-flung star system. This observation suggested transit time variations, which is a major signal that there could be an exomoon. Such a scenario is unlike anything that's seen in our own solar system, of course, and it doesn't fit any of the standard theories for moon formation.

This artist's impression depicts the exomoon candidate Kepler-1625b-i. Only one planet held promise for hosting a moon, one around the star known as Kepler-1625, which is about the size of our sun but older. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash", Kipping said.

At first, the researchers saw the dip in light caused by the planet passing across the face of the star, but not long afterward, they saw a second, smaller dip, likely caused by the moon transiting across the star. "Science can't operate by teams such as ourselves refusing to publish our results and hiding behind closed doors", Kipping says in an online outreach video posted concurrently with the paper. This is interpreted as a so-called transit timing variation, caused by an unaccounted for gravitational tug on the planet by an unseen body.


Teachey and Kipping are submitting proposals for more time on Hubble to observe this planet and its moon during another transit. These enabled astronomers to derive the masses of the planets directly from the transit light curve. Alternatively, it could be caused by an exomoon.

Scientists may have bagged the first-ever exomoon. They want some more observational data to be certain what they're looking at is indeed an exomoon.

Is it really a moon?

In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. "While most of these moons orbit Saturn and Jupiter, which are outside the Sun's habitable zone, that may not be the case in other solar systems", said Stephen Kane, an associate professor of planetary astrophysics and a member of the University of California Riverside's Alternative Earths Astrobiology Center. The star was probably cooler in the past, so this heat could be a reason for the size of the moon, inflating the gas giant as the temperature rises.

Jupiter's moons are believed to have formed from a ring of material; but again, none of Jupiter's moons are the size of Neptune.

Researchers Alex Teachey and David Kipping evaluated 284 planets outside our solar system that had already been discovered by NASA's Kepler Space Telescope. And the tenuous nature of the supporting data, combined with the weird properties of the system, give us reasons to take that skepticism seriously, even if a moon is still the best explanation for them.

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