A New Presence Outside the Solar System

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Astronomers Alex Teachey and David Kipping, from Columbia University, USA, may have found an exomoon orbiting a planet in a solar system thousands of light years away. Kepler is best equipped to find planets that orbit close to their stars, and analyses of orbital dynamics - not to mention the evidence in our own solar system - suggest that moons will be more common further out. We won't know for sure until researchers can do more tests, of course, but even so, what we do have is very promising.

The two researchers, from New York's Columbia University, examined data on 284 planets outside our Solar System and only one of them, Kepler-1625b, showed evidence it may host a moon.

The moon candidate is estimated to be only 1.5 percent the mass of its companion planet, and the planet is estimated to be several times the mass of Jupiter. And excitingly, some of the moons around the giant planets in our solar system - including Europa, Enceladus and Titan - are now our best bet for finding life outside our own planet (or possibly on Mars). They will use the Hubble Space Telescope for more observations in May 2019 to confirm their finding. Jupiter's moon Ganymede has a diameter of about 3,270 miles (5,260 km). The researchers monitored a 19-hour event when the planet known as Kepler 1625b passed in front of its parent star blocking out some of the light coming from the star, which lies at a distance of 8,000 light-years from Earth and matched with their expectations. However, scientists know that Earth's moon is gradually moving away from our planet at a rate of about 1.5 inches (4 centimeters) per year. 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.

So last October, the pair directed the Hubble Space Telescope at the star in an attempt to verify - or rule out - the possibility of a moon orbiting the planet Kepler-1625b. Kepler-1625b stood out. Unfortunately, the scheduled Hubble observations ended before the complete transit of the candidate moon could be measured and its existence confirmed. Exomoons, or moons outside of our solar system, are hard to spot due to their expected smaller size compared to planets.

Now the researchers say that what Hubble saw seems to confirm the idea that this planet has a moon. In addition, the transit occurred about one-and-a-quarter hours earlier than predicted. The second was a delay in the planet passing in front of its star.

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In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.

The evidence was published in Science, where the authors said: 'We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon.

"It's raising new questions about sort of the dynamical processes that go on to create the planets and moons", Teachey said.

Another planet could cause the same gravitational nudge, the researchers noted, although Kepler observations have come up empty in that regard.

However, in the case of the Earth-Moon system and the Pluto-Charon system - the largest of the five known natural satellites of the dwarf planet Pluto - an early collision with a larger body is hypothesised to have blasted off material that later coalesced into a moon. They chose to look at exoplanets with the widest orbits, or those that take about 30 days to circle their stars.