"We're going to send you back. Start all over again and keep in touch," Judge Alfred A. Delucchi told the panel on the fifth day of deliberations.
It was not immediately clear what the woman specifically did to get kicked off the jury. But a source told The Associated Press on condition of anonymity that she had apparently disobeyed the judge's orders to consider only the evidence presented at the trial.
"You must decide all questions of fact in this case from the evidence received in this trial and not from any other resource," the judge said.
The judge removed the juror after meeting behind closed doors with lawyers in the case. A day earlier, Delucchi lectured the jury about the importance of deliberating with an open mind.
What this means, says CBS News Legal Analyst Andrew Cohen is that the jury has to start over from scratch with this new juror. That may mean we won't see a verdict for many days now or it could mean that the change in jury makeup really puts some steam into deliberations and generates a quick verdict. There is just no way to tell, says Cohen.
The juror, a retired utility company employee in her 50s or 60s, was replaced with the next alternate — a woman in her 30s who worked at a bank and has four sons. Her brother was in and out of prison for drugs, leading her mother to become a drug counselor at a methadone clinic.
During the trial, the new juror seemed particularly attentive to defense presentations, and responded positively to the many jokes of defense attorney Mark Geragos. She smiled often and made a point of greeting the bailiffs each morning; she also cried openly at the sight of the autopsy photos.
Whether that means anything when it comes to deliberating the case remains to be seen, but lawyers for both sides left the courtroom smiling. Also unclear is whether the other members of the six-man, six-woman jury will be able to follow the judge's orders and set aside any conclusions made so far in deliberating Peterson's fate.
Peterson, 32, is charged with two counts of murder in the deaths of his wife, Laci, and the fetus she carried. Prosecutors claim Peterson killed Laci around Christmas Eve 2002, then dumped her weighted body from his boat into San Francisco Bay.
The jury has two choices should they decide to convict Peterson -- first- or second-degree murder. A first-degree conviction would mean jurors believe Peterson planned the killings in advance, and it could carry the death penalty or life without parole.
Dean Johnson, a former prosecutor who has been closely watching the case, said the ousted juror appeared to be sympathetic to the defense, judging from her statements during jury selection and her demeanor throughout the trial.
Her replacement, on the other hand, seems so emotionally involved in the case that it might be difficult for her to separate her feelings from the facts, Johnson said. "She's not going to be able to take her emotions out of the equation here."
Daniel Horowitz, a defense attorney who also has watched the trial, says the replacement bodes well for the defense, calling her "a woman who will not immediately join the group. She's clearly a non-conformist." Any jurors leaning toward a conviction would likely be frustrated by her, he said.