Alex McColgan

The difference is that none of our planets are tidally locked to the Sun, as they are far enough away from the parent star, which means their moons have a stable orbit

This is one of the reasons we believe that none of the TRAPPIST system planets have moons, as they are so close to their parent star that we assume the planets are all tidally locked.

I hope this didn't disappoint you too much, so I'm going to leave you with this.

There are some other curious objects in the solar system, far away from any other object so that they aren't tidally locked to anything.

This particular asteroid, called Ida, looks like a standard 30km wide asteroid as seen by the Galileo spacecraft, but you might notice this little blob here.

This is actually Ida's moon, Dactyl.

It's only 1.5km across, and orbits only 60km away from Ida.

We don't know especially how stable this orbit is, but a very cool thing to observe nonetheless.

We are used to tectonic activity on Earth, it being a daily occurrence somewhere on the planet.

But what about our closest neighbour, the moon?

During the Apollo missions, seismometers were left on the moon to detect the presence of moonquakes.

From these instruments, we now know that the moon does have quakes, and relatively frequently too.

But unlike Earth, most can be attributed to meteors striking the moon's surface.

Although, there is also evidence to suggest that a small percentage of these quakes could be due to the moon shrinking and contracting.

Early in the solar system's life, most of the celestial objects were hot, a result of the process of how they were formed, and followed up by a period of heavy bombardment.

Since then, impacts have become much less frequent, and these objects, including the Moon, have cooled down.

As something cools, it contracts.

Photos taken by the Apollo missions imaging the moon's surface, and photos from the current lunar mission, the Lunar Reconnaissance Orbiter, show that the moon is covered with these kilometres-long ridges, or lobate scarps, which can rise up to 100 metres high.

Scientists believe they are connected to the moon cooling.

They predict that these scarps are caused by thrust faults, faults caused by the lunar crust being pushed together until it gives, causing a distinctive scarp on the surface which is steep on one side and shallow on the other.