Actor Sean Penn filed for legal separation from his wife of 13 years, actress Robin Wright Penn, last Friday. Penn originally sought a divorce back in December of 2007. Robin filed her own paperwork that month as well but the case was later dismissed and the couple tried to work things out.

The couple has been married since 1996 and have two kids: Dylan, 18 and Hopper, 15. It appears that they will likely seek joint custody of Hopper, the only minor child.

It is unknown whether there was a prenuptial agreement. Penn’s fame and certainly a portion of his fortune pre-dates the marriage. Fast Times at Ridgemont High, Carlito’s Way and Dead Man Walking were all premarital films. Similarly, his wife had an already successful career when they married. The Princess Bride and Forrest Gump were both filmed before 1996.

Both have seen their profiles and paychecks increase in the last 13 years – Sean’s more so than Robin’s after this year’s Academy Award. A prenup could be very useful in this case, especially to identify the premarital assets which might have existed.

Another unknown is exactly why Penn is seeking a legal separation instead of a divorce now. Legal separation is not a required precursor to divorce and requires as much negotiation or litigation as a divorce to determine ownership of marital assets, responsibility for marital debt, child custody, child support and spousal support issues. There may be legal reasons or personal reasons for the choice – and we may never know. But we’ll keep an eye out and update you if we hear!

UPDATE: Looks like Sean Penn is represented by San Francisco divorce attorney Mindy Ross. The Penns live in the Bay Area. Ross’s firm also has offices in Beverly Hills.