# Is it coincident that all planets with tight orbit are tidally locked to their parent body?

I know mercury is tidal locked to our Sun like our Moon is to Earth, is it merely coincident or there is some sciences behind it? Some man-made communication satellites are also tidal locked to Earth! I think it is intentional so that there is minimum interruption for signal transmission, they used thrusters and gyro to help them to always point toward Earth.

• Mercury isn't tidally locked like the Moon; it's differently tidally locked. It's year:day ratio is 3:2, not 1:1. – Mike Scott Aug 28 '17 at 14:33

This is a physical result:

The change in rotation rate necessary to tidally lock a body B to a larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.

Stellar bodies get tidally locked with time. You even get graphs like there.

On how long it generally takes, from Wikipedia

$$t_{\text{lock}} \approx \frac{\omega a^6 I Q}{3 G m_p^2 k_2 R^5}$$

Where $Q$ and $k_2$ depend on the constituents and age of the satellite, which themselves evolve with time. Difficult to estimate. Moreover, you rarely know the age of your system.

But it explains that for long-time established system of relatively close bodies, tidal lock is generally the case. The more tide there is, the quicker it gets.

Tidal forces (that is, the difference in gravitational pull from one side on an object compared to the other) can slow the body's rotation. For example, the near side of the Moon feels Earth's gravity stronger than the far side, because it's closer. This tidal friction robs the orbiting body of angular momentum, slowing its rotation. The long-term effect of this, is that the orbiting body will slow enough that one side will always face the planet. What's more, the tidal force will also dampen out any wobble in the orbiting body. The Moon does wobble a little as it orbits the Earth -- this is called libration -- so that in total we see a little more than 50% of the Moon's surface throughout the month.