An illustration of an artist from the extrasolar planet Kepler-1625b with a hypothesized moon, which is considered the size of Neptune. These moon candidates, and others like that, can host their own satellites, the researchers said.
Credit: And Durda
Extrasolar moons, or exomoons, are moons orbiting planets outside our solar system, called exoplanets. Although nearly 4,000 exoplanets have been discovered by space telescopes such as Kepler and NASA TESS since 2009, only one exomoon has been described – and it is still unclear if it really is the moon.
Astronomer David Kipping and his graduate student Alex Teachey at Columbia University in New York first reported the possibility of eksomoon in October 2018 in the journal Science Advances.
Using NASA's Hubble space telescope, the pair observed Kepler-1625b – an exoplanet of Jupiter size – which passes in front of its star, causing a slight decrease in the amount of light seen from Earth.
This "transit method" is how thousands of exoplanets have been found so far. But watching the Kepler-1625b transit in front of the star came with two surprises. First, the exoplanet completes transit of about 1.25 hours earlier than expected, indicating that something is gravitationally interesting. Then there is shallow light in the light as soon as the planet has finished passing in front of the star, perhaps indicating the satellite is lagging behind Kepler-1625b.
"We have tried our best to rule out other possibilities such as spacecraft anomalies, other planets in systems or star activity, but we cannot find another single hypothesis that can explain all the data we have," Kipping said. during a teleconference with reporters.
The supposed exomoon is the size of Neptune – about one third the size of Kepler-1625b. That's huge for satellites, which are usually much smaller than planets in orbit. Such a moon is predicted to be quite rare, if not impossible, by the current planetary-moon system models.
"We have not opened an open champagne bottle," Teachey said in Nature. But "things look attractive, tempting, maybe interesting."
Expressions in the search for life?
Exomoons have the potential to become a "super-inhabited" world, according to some scientists, which means that they are especially good places for developing lives. That's because the moon does not exclusively depend on the light energy of their solar system stars. Instead, they can get energy elsewhere, such as:
- Reflected light – Light reflected or emitted as heat from the closest planet near the planet can provide a long stable period of temperature which will encourage life to lay eggs.
- Radioactive element – Radioactive elements, such as uranium and radon, which are found in rocks deep within Exomoon the size of Earth, are slowly damaged over time and release heat to the surface.
- Tidal forces– The gravity of a large exoplanet – say the size of Jupiter or Saturn – might attract an exomoon to create a tidal force, similar to the way our moon draws in the Earth's ocean to create waves. When the rocky exomoon surface is supported by the gravity of the exoplanet, it can produce heat that can move to the surface.
About 175 months are in our solar system. Many of them have the above characteristics, and two of those months have been strong contestants to save life as we know it: Jupiter Europa Moon, and the moon Saturn Enceladus.
Both satellites have an ice surface covered by stretch marks from the tidal forces of their mother planet and never exceed minus 200 degrees Fahrenheit (minus 128 degrees Celsius). But a few miles below each surface they sit oceans that hold more water than those found on Earth. And where there is water, there may be life.
Where are the other exomons?
In some estimates, there must be 100 to 1,000 times more than the watery oceanic moons in this Milky Way than Earth-like planets, but they are hard to find.
Stars like the Kepler-1625b sun sit around 8,000 light years from Earth. The distance makes the star just a little light, but advanced technology has enabled scientists to detect minute silhouettes made when a planet crosses in front of its star. And for the alleged giant moon Kepler-1625b, detection is easier because of its size.
But for every potential Europa size, which is only a quarter the size of Earth, the slope of light when moving in front of a star may be too small to be clearly understood with current technology.