Hubble boosts the case for Exomoon

The Kepler-1625b exoplanet could harbor a moon the size of Neptune – potentially the first confirmed exomoon – but researchers recommend caution in interpreting the data.

Last year, astronomers announced that they had discovered the first known moon outside our solar system. Now, these same researchers think their claims are a bit stronger.

New observations from the Hubble Space Telescope suggest that a moon of Neptune's size and mass orbited approximately 3 million kilometers from the Jupiter-sized exoplanet, Kepler-1625b, at about 7,800 years ago. Earth light.

For some, according to René Heller (Max Planck Institute for Solar System Research, Germany), a moon is about 10 times more massive than all the moons and rock planets in our solar system combined.

But the discoverers themselves call for caution. "The first exomoon is obviously an extraordinary claim," said Alex Teachey (Columbia University) at a press conference on Oct. 1. "We are not ready to open champagne bottles yet."

Teachey and his colleague David Kipping (Columbia University) had their first glimpse of an exomoon after examining data on 284 planetary systems from the Kepler Space Telescope. The telescope identifies the exoplanets with the help of the transit method, which is based on the immersions of starlight created by a planet passing in front of its sun. The team discovered a system that looked promising – a gas giant that was transiting every 287 days, associated with a hint of a second, a smaller transition from a potential moon – but they needed more data for be certain. So they turned to the Hubble Space Telescope, which spent 40 hours searching for a confirmation on October 28 and 29, 2017, when the planet had to move again.

They were not disappointed. The second hoped transit took place a few hours after the exoplanet. In addition, the planet began transit about 78 minutes earlier than planned. While this may be evidence that other planets in the system are shooting at it, it could also be caused by a planet and moon in orbit.

"We have done our best to exclude other possibilities such as spacecraft anomalies, other planets in the system or stellar activity," Kipping said. "But we are unable to find another single hypothesis that can explain all the data we have."

The research is published on October 3 in Progress of science.

Extraordinary requests require extraordinary evidence

"I think we have to be very, very cautious," says Stephen Kane (University of California, Riverside). "But it's very encouraging." Hubble's data is essential, he says, because these observations were specifically intended for this system, as opposed to Kepler's observations, which cast a wide net to maximize the chances of finding planets.

"Alex and David have done a great job looking at the data from a variety of angles and being very transparent about the methods they have used," says Heller. "As for the confidence I have in the very existence of this exomoon, I remain skeptical."

It's not that the exomoons are improbable. And it's not that the signal is too small to be reliable. As a world the size of Neptune, the moon blocks a quantity of stellar light comparable to many planets already discovered, says Heller. The problem is the noise in the data. Heller emphasizes that it is essential to understand all the potential sources of noise – the telescope, the instruments and the star itself.

The authors try to solve these problems. One of the tests was to look for transit in several wavelengths. If the star twinkled, the red and blue stars would dive differently. But these transits are similar in all colors.

The reliability of the data aside, there is also the question of the unusual size of the moon. While the mass ratio between the Moon and the planet is about the same as that of our Moon and Earth – about 1% – the entire system is about 1,000 times larger than ours.

"The type of system they found is extremely different from what we are used to," says Kane. "This raises a lot of questions about how this happened."

There are three supposed ways to create a moon. It can form as a result of something of a major impact on a planet (like the Earth's Moon). It can form in a disk of gas and dust surrounding a planet (like the Galilean satellites of Jupiter). You can also capture it by moving too close to a planet (like Neptune's largest moon, Triton).

The three methods are difficult to reconcile with this situation. The orbit of the moon is stable, but the question remains open. Nobody has really examined the impact that an impact on a giant gas planet could produce on the moon, Kipping said. A disc around a mass world Jupiter sometimes gives birth to such a moon, according to recent simulations. "It's a bit like the tip of the iceberg, but sometimes you make such big moons," Kipping said.

However, Heller argued in an article published earlier this year that the masses in the Kepler-1625b system are not on the charts with respect to the calculations of how things are formed in a circumplanetary disk. And capturing, he finds, requires that the moon once had a mini-Neptune mate who was ejected during a close encounter with the planet – a plausible but unlikely scenario that poses two large, gravitationally bound worlds one to the other.

"This moon, if confirmed, must have been formed by an as yet unknown astrophysical mechanism," he says.

The next step is to get more data. To this end, Teachey and Kipping filed an application to participate in Hubble in May 2019, at which time the next transition of the planet is planned. Another passage with the moon in the right position depending on its orbit planned could solve the case. "If we see that," Kipping said, "then I think we're done."

The references:

A. Teachey and D. M. Kipping. "Proof of a great Exomoon orbiting Kepler-1625b." Progress of science. October 3, 2018.

R. Heller. "The nature of the giant candidate at the Exomoon Kepler-1625 b-i." Astronomy and astrophysics. February 26, 2018.

M. Cilibrasi et al. "The satellites are formed quickly and late: population synthesis for the moons of Galilee." Monthly Notices from the Royal Astronomical Society. August 9, 2018.