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Artist's illustration of the exoplanet Kepler-1625b with its supposed moon, which would have the size of Neptune. The researchers say that this candidate Moon, and others, could host their own satellites.
Credit: Dan Durda
Extrasolar moons, or exomoons, are moons gravitating around a planet outside our solar system, called an exoplanet. Although nearly 4,000 exoplanets have been discovered by NASA's Kepler and TESS space telescopes since 2009, only one exomoon has been described – and it is still unclear whether it is about the same. ;a moon.
Astronomer David Kipping and his graduate student, Alex Teachey, of Columbia University in New York, reported for the first time on a possible exomoon in October 2018 in the journal Science Advances .
Using the NASA Hubble Space Telescope, the two men saw Kepler-1625b – an exoplanet the size of Jupiter – pass in front of his star, causing a slight drop in the amount of visible light from the Earth.
This "transit method" is the way thousands of exoplanets have been discovered so far. But watching Kepler-1625b in transit in front of his star brought two surprises. First, the exoplanet completed its transit about 1.25 hours earlier than expected, suggesting that something was pulling it by gravity. Then there was a slight dip in the light shortly after the planet finished passing in front of the star, probably indicating a satellite trailing behind Kepler-1625b.
"We have done our best to exclude other possibilities such as spacecraft anomalies, other planets in the system or a stellar activity, but we are unable to find another single hypothesis. that can explain all the data we have, "Kipping said. during a teleconference with reporters.
The alleged exomoon has the size of Neptune – about a third of that of Kepler-1625b. It's huge for a satellite, which is usually much smaller than the planet on which it orbits. These models should be quite rare, if not impossible, according to the current models of formation of the planet-moon system.
"We have not opened the champagne bottles yet," Teachey told Nature. But "things seem exciting, tempting, even convincing".
Exomoons in search of life?
According to some scientists, exomoons have the potential to be "super-habitable" worlds, which means that they are particularly conducive to the evolution of life. Indeed, moons do not depend exclusively on the light energy of the star of their solar system. Instead, they can get energy elsewhere, such as:
- Reflected light – Light reflected or emitted by the heat of the neighboring planet of an exoon can provide long periods of stable temperatures that would encourage life to reproduce.
- Radioactive elements – The radioactive elements, such as uranium and radon, which are found in the rocks inside an exomoon of the Earth's size, slowly decompose over time and release of the heat on the surface.
- Tidal forces– The gravity of a large exoplanet – say the size of Jupiter or Saturn – can shoot an exomoon to create tidal forces similar to how our moon pulls the Earth's oceans to create tides. When the gravity of an exoplanet kills the surface of a rocky exomoon, it can generate heat that could spread to the surface.
There are about 175 moons in our solar system. Many of them have the above characteristics, and two of these moons have become powerful opponents of life as we know it: the Europa moon of Jupiter and Enceladus, the moon of Saturn.
Both satellites have icy surfaces covered with stretch marks from the tidal forces of the parent planets and never exceed 200 degrees Fahrenheit (minus 128 degrees Celsius). But a few kilometers under each of their surfaces is an ocean that contains more water than on Earth. And where there is water, there could be life.
Where are the other exomoons?
According to some estimates, there should be 100 to 1,000 times more icy and oceanic moons in the Milky Way than there are Earth-like planets, but they are hard to find.
The sun-like star of Kepler-1625b is about 8000 light-years from Earth. This distance makes the star a tiny hint of light, but advanced technology has allowed scientists to detect the meticulous silhouette created by crossing a planet in front of its star. And for the so-called giant moon of Kepler-1625b, detection is easier because of its size.
But for any potentially European-sized exomoon, which would only represent a quarter of the Earth's surface, the dip of the light when it moves in front of a star may still be too small to be clearly deciphered with current technology.
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