Expert: Courtney Seligman - 5/23/2009

Question
Can planets become tidally locked to stars in the same way that moons are tidally locked to their planets?

Answer
Yes, they can. In fact, until the 1960's, it was thought that Mercury was tidally locked to the Sun. Only a coincidence involving its eccentricity and rotation rate (discussed on my website, at http://cseligman.com/text/planets/mercuryrot.htm) has allowed it to avoid this fate.

In addition, many of the known exoplanets are so-called 'hot Jupiters', which are extremely close to their stars (far closer than Mercury and the Sun). (The fact that so many exoplanets are like this is a result of the usual way in which they are found -- very large planets very close to their stars are far easier to 'observe' than smaller planets, further away.) For such very close planets, tidal forces would be far greater than normal, and they should be tidally locked, more often than not.

So yes, planets can become tidally locked to their stars, and such a situation is probably fairly common among currently known exoplanets.