First Exomoon Possibly Discovered Orbiting in a Distant Star System

Posted October 07, 2018

The candidate moon, with the designation Kepler-1625b-i, is unusual because of its large size - comparable to the huge, icy planet Neptune in our solar system. Star Warshad the moons of Yavin IV and Endor. It makes sense, Teachey said, that the first moon scientists spot would also be a giant.

Making the move from science fiction to science fact has been hard for astronomers, however.

But the research group also says there is still a possibility that they found a planet, not a satellite, so they say they will continue monitoring to confirm their findings.

The reason for this, simply, is due to their size. Simply based on this example, exomoons should be abundant in the galaxy, yet evidence for them has remained elusive.

The researchers requested time on the Hubble Space Telescope, and their request, which was public, generated a bit of excitement among those who follow new planet discoveries. It is so big that theoretically the moon might even have its own moons. Now, it seems, their search has paid off.

The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface.

The data suggest that the exomoon orbits Kepler-1625b at a distance of about 2 million miles (3 million kilometers), Teachey said. About 3.5 hours after Kepler-1625b's transit completed, HST recorded a second smaller decrease in the star's brightness, a dimming indicative of a moon "trailing the planet like a dog following its owner on a leash," Kipping said.

The scientists made the initial observations on the Kepler Space Telescope and used their evidence to secure 40 hours time on the Hubble. "It was definitely a shocking moment to see that light curve - my heart started beating a little faster and I just kept looking at that signature", David Kipping described his feelings. "But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us". Astronomers may have discovered the first "exomoon" - a moon outside the solar system.

Astronomers can confirm those objects outside the solar system by observing a dip in the amount of light emanating from stars when they cross in front of the stars.

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Indeed, it may take just a single additional transit observation that shows a "separate clean moonlike event", Kipping said. An when Kepler observed each of 284 transiting planets, the telescope showed a short dimming. A second slight drop in the "light curve" indicates the possible presence of a large moon. The exomoon's mass is about 1.5 per cent that of its planet.

"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit", Kipping said.

Even Saturn's largest moon, Titan, has a radius that is less than 5% that of its parent planet, and while it's true that the Earth and moon have a size ratio of about four to one, they are both rocky bodies. They found the best match to be a Neptune-sized exomoon, named Kepler-1625b i, on an inclined orbit at a distance of 35 to 45 planetary radii from Kepler-1625b.

The results were presented in the paper Evidence for a large exomoon orbiting Kepler-1625b in the journal Science Advances. The researchers can't be completely sure, however, since the observation of the moon transit could not be completed.

Again, the moon under discussion here remains a candidate, not a confirmed body.

Faced with that skepticism, Teachey and Kipping are undoubtedly applying for more Hubble time during its next transit.

"If this were confirmed, then it would be a first-of-its-kind discovery, and it's important to remember that a first-of-its kind discovery demands a much higher degree of rigor, skepticism and confidence than a run-of-the-mill detection", he said.

It's an "extraordinary" find that "defies easy explanation" said co-author Alex Teachey - nothing like it exists in our own solar system. Did they form together, at the same time?

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