Legal experts said the ruling is a victory for the prosecution because allowing the lesser charges could make it easier for undecided jurors to convict Peterson.
The penalty for a second-degree conviction, says CBS News Reporter Tim Ryan, would be 15 years to life for each of the two counts. Murder One means life behind bars or even the death penalty.
Peterson already faces two counts of first-degree murder in the deaths of his pregnant wife and the fetus she was carrying. Prosecutors are seeking life without parole or the death penalty under those charges.
"The jury could say, 'Well, we think that Mr. Peterson killed Laci Peterson but we're not persuaded beyond a reasonable doubt that there was premeditation,"' Judge Alfred A. Delucchi said in his ruling — part of instructions that will be given to jurors next week.
Defense attorneys vehemently objected to the inclusion of the lesser charges because most experts agree the defense has done a good job at explaining away premeditation. In order for the jury to find Peterson guilty of first-degree murder, the members must first believe he planned the killing in advance.
"There is no case I've been able to find ... where you have a situation where (prosecutors) can't tell you where, they can't tell you when, they can't tell you how ... and the jury" was given the second-degree option, defense lawyer Mark Geragos told the judge.
"Well, this is going to be the first," Delucchi replied.
Geragos then argued to include voluntary manslaughter as an option. The judge declined.
"I would consider that to be at your peril," Geragos said, alluding to inevitable appeals should there be a conviction.
Prosecutors have built their entire case on premeditation, that Peterson planned for weeks to kill his eight-months pregnant wife, Laci, and had even devised a way to dispose of the body by purchasing a boat.
Peterson is accused of killing his wife on or around Christmas Eve 2002, then dumping the weighted body into San Francisco Bay. The remains of Laci Peterson and her fetus were discovered along a rocky shoreline about four months later, a few miles (kilometers) from where Scott Peterson claims to have gone fishing alone the day his wife vanished.
The trial is in its 22nd week. Closing arguments are scheduled to begin Monday.
Most legal experts agreed the judge's ruling bodes well for the prosecution.
"For the most part, it gives jurors a real option to convince or convert any holdouts who aren't so sure about premeditation but may still think he killed his wife," said Loyola Law School professor Stan Goldman, a regular trial observer. "The most difficult thing in this case for prosecutors to prove has been premeditation."
CBS News Early Show Legal Analyst Wendy Murphy, a former prosecutor, expects prosecutors to use their closing arguments to highlight what she calls the most understated piece of evidence in the case — the injuries on Scott Peterson's hand.
"I think he reached up to strangle her and we know, those of us in this business know, when there is a strangulation under way the first thing the victim does is reach up to try to remove the hand from the neck," often scratching the attacker's hands," Murphy said. "Scott Peterson lied about how he got those injuries. He said 'I cut my hand on the tool box in my truck.' They tested that tool box, every which way. No blood, no tissue, nothing, no DNA from Scott Peterson."
But fellow CBS News Legal Consultant Mickey Sherman, a defense attorney, predicts a not guilty verdict or hung jury.
"They've got the death of a very lovely young girl and a baby who certainly didn't deserve any harm to come to it. And you've got a family there who is grieving and they want to do something nice for them. You've got Scott Peterson whose alibi puts him in the place where the bodies were found. That's certainly the most damning evidence," said Sherman. "It's a case of 'he probably did it.' Statistics will show he probably did it and who else could have done it?
"But unfortunately for the state, they still don't have 'did he really do it?'"