The unruly planet - and not a plain, old-fashioned fear of mortality - is why people freak out when they turn 30.

At least, that's what astrologers Stefanie Iris Weiss and Sherene Schostak, authors of "Surviving Saturn's Return: Overcoming the Most Tumultuous Time of Your Life" (McGraw Hill/Contemporary Books, $14.95), say.

When you turn 30, the authors explain, Saturn - the planet that triggers "the karma of your life lessons," according to Schostak - returns to the exact same place it was the moment you were born. Hence, "Saturn's return."It's a theory that most people who believe in astrology subscribe to; No Doubt frontwoman Gwen Stefani took it so seriously that in 1999 she wrote a record called "The Return of Saturn," which dealt with all her fears about that imminent birthday.

"We ended up calling the album 'The Return of Saturn' because on a personal level, from age 28 to now - the time frame of making the album - was really hard," she has said. "I was like, 'God, what's wrong with me, who am I?' Going through the whole evaluation of who I was in different stages of life and which one's real."

In fact, the return of Saturn is a process that, say the authors, lasts about 2 1/2 years for most people.

"The psychic upheaval makes turning 30 feel like a 2 1/2-year tooth extraction," says Weiss.

Their book, they say, is meant to be a guide for women so that they won't go through the ordeal of a Saturn return in another 30 years.

"If you avoid your issues during the first one," says the 32-year-old Weiss, "you'll have an even tougher one the next time you go through it. Although it's typically a painful time, it's also a time for growth and change and empowerment in the biggest sense."

Weiss says she speaks from experience: During her Saturn return, she broke up with her boyfriend of seven years and decided to go to graduate school.

"My most profound lessons were about transformation and fear of change," she says.

The 33-year-old Schostak says that her Saturn return was equally life-altering: she quit her "rather dull" job as a reasearch assistant and went to India to "find myself." When she came back, she says "the crisis had transformed into this attitude of taking on new ventures, doing things big."

Whether or not you think you need help, the book can help you deal with your cranky, about-to-turn 30 (or 60) friends: "It'll help you to see what particular issues they are struggling with," says Schostack, "and it'll give you more compassion when they start freaking out on you."