The first moon outside our solar system may have been discovered and is massive

The first moon outside our solar system, a gas giant the size of Neptune, may have been discovered, say astronomers.

The potential moon would be considerably larger than the Earth – about the size of Neptune or Uranus. The planet it orbits is as big as the mammoth Jupiter. This super-size pairing of a moon and a gaseous planet is 8,000 light-years away.

Researchers Alex Teachey and David Kipping evaluated 284 planets outside our solar system that had already been discovered by NASA's Kepler Space Telescope. Only one planet had promised to host a moon, another around the star known as Kepler-1625, which was about the size of our sun but was older.

Last October, the two men led the Hubble Space Telescope to the star to try to verify – or eliminate – the possibility of a moon turning around the planet Kepler-1625b. They were on the lookout for a second temporary dimming of starlight. The main hollow in the stellar luminosity would be the planet itself crossing in front of its star. Another dive might be a moon, known as the exomoon outside our solar system.

The Hubble Telescope, more powerful and more accurate, has detected a second smaller decrease in starlight 3 1/2 hours after passing the planet in front of the star – "like a dog following his owner on a leash" as Kipping says. However, the observation period ended before the moon could complete its transit. That's why astronomers need a new look at Hubble, hopefully next spring.

Despite the evidence, Teachey stressed "we call for caution here."

"The first exomoon is obviously an extraordinary claim and requires extraordinary evidence," said Teachey. "In addition, the size we calculated for this moon, about the size of Neptune, was hardly anticipated, which also warrants caution."

He added, "We are not cracking open bottles of Champagne for the moment on this one."

If it was a moon, it would be about 2 million kilometers from its planet and would appear twice as big in the sky as the moon in our region. Astronomers do not know how this potential moon could have formed, given its size.

"If confirmed, this discovery could completely upset our understanding of moon formation and composition," said Thomas Zurbuchen, NASA's Science Mission Manager.

According to the researchers, another compelling evidence in favor of a moon is that the planet has passed its star more than an hour earlier than expected. A moon could cause this kind of uncertain and wobbly way, they noted.

Kipping stated that this is how the Earth and the Moon appear from afar. This particular planet – or exoplanet – is about the same distance from its star as the Earth is in the sun.

The researchers noted that another planet could provoke the same gravitational boost, although Kepler's observations remained blank in this regard. Kepler-1625b is the only planet found to date around this star.

For Teachey and Kipping, the best and easiest explanation is that Kepler-1625b has a moon.

"We did our best to exclude other possibilities," Kipping told reporters. "But we could not find any other hypothesis that could explain all the data we have."

Their findings were published in the journal Science Advances. The deputy editor of the journal, Kip Hodges, congratulated the researchers for their cautious tone, given the difficult and complicated process of identifying an exomoon.

"If this finding withstands further scrutiny, it represents a major step in the field of astronomy," said Hodges.

Columbia astronomers have stated that they may be able to solve this problem as early as next year, with more views from Hubble. At the same time, they encourage other scientists to join them and submit to the surveillance that will be offered to them.

Whether it is confirmed or not, the subject shows how much our own solar system can be rare or common.

Moons are abundant in our own solar system, with nearly 200. Of the eight planets in our solar system, only Mercury and Venus have none.

Since both the planet and its potential moon are gaseous giants, no one suggests conditions that support life.

"But in the future, I think we are opening the door to discover such worlds," Teachey said.