First moon outside our solar system discovered, astronomers think

Cesar Mills
October 7, 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.

Hints of what could be the first known exomoon (moon outside the Solar System) first came to light in 2017, after Alex Teachey and David Kipping of Columbia University in NY found some unusual behaviour in its parent planet, Kepler-1625b.

In their dedicated search for exomoons, Columbia University astronomers David Kipping and Alex Teachey have scaled up, looking for the largest possible exomoons, around the largest known exoplanets. The data suggested that a Jupiter-sized planet, Kepler-1625b, may be orbited by an exomoon.

To look for exomoons, the team analysed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. Based on computer modeling, the surface temperature on both the alien planet and the moon would be on the order of 175 degrees Fahrenheit (80 degrees Celsius), Kipping said. 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.

"Kepler recorded just three transits of the planet in front of her star, and that's largely because the planet takes nearly a year to complete an orbit".

Kepler-1625b is an exoplanet that orbits Kepler-1625, a Sun-like star, about 8,000 light years away. They were on the lookout for a second temporary dimming of starlight. A primary goal of the Kepler mission is to identify planets that are in the habitable zones of their stars, meaning it's neither too hot nor too cold for liquid water-and potentially life-to exist.

In flickering light of a distant sun, scientists may have discovered the first moon outside our solar system. While this is an exciting discovery, it's still not confirmed - it will take more time on the Hubble Space Telescope to confirm this remarkable find.

In 2017 NASA's Kepler Space Telescope detected hints of an exomoon orbiting the planet Kepler-1625b. The researchers can't be completely sure, however, since the observation of the moon transit could not be completed. That's why the astronomers need another look with Hubble, hopefully next spring. So stellar noise is possible, but Teachey and Kipping found no evidence for it.

"Furthermore, the size we've calculated for this moon, about the size of Neptune, has hardly been anticipated, and so that, too, is reason to be careful here", Teachey said.

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He added: "We're not cracking open Champagne bottles just yet on this one". This could explain why the moon is 3 million kilometers from its planet; they were probably closer in the past. The astronomers are uncertain how this potential moon might have formed, given its size. But the additional data both makes the case for the moon better and worse (don't worry, we will explain that).

This NASA photo taken on July 20, 1969 shows astronaut Edwin E. "Buzz" Aldrin, Jr. saluting the USA flag on the surface of the Moon during the Apollo 11 lunar mission.

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation", Teachey said. A moon could cause that kind of an uncertain, wobbly path, they noted.

"That's a sizable shift, and it implies that there is something in the system wobbling that planet around", said Kipping.

Thousands of planets have been detected around faraway stars in recent years.

Previous research has suggested that big (and therefore detectable) exomoons are rare around large planets that orbit very close to their parent stars.

An artist's impression of exoplanet Kepelr-1625b and its moon. He has led the field over this time, so I am delighted that his persistence has paid off.

Their findings appear in the journal Science Advances.

So Kipping and Teachey asked for 40 hours of observing time with the Hubble Space Telescope, which is four times as precise as Kepler.

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