5. John Arnold
John Arnold
Founder, Centaurus Energy
Age: 37
Industry: Finance

The trading star who cut his teeth at Enron turned a talent for reading the notoriously fickle gas markets into an estimated $4 billion hedge fund fortune. He's bounced back from his first-ever down year, with his flagship fund up about 4%, and invests in natural-gas storage caverns and power plants, turning Centaurus into a diversified energy firm some have likened to a mini-Enron. Billionaire props include an art-filled modernist mansion in Houston and a foundation to support overhauling America's pension system.

Spare time: Arnold is said to be a Dave Matthews and U2 fan.


Last updated October 25 2011: 5:11 PM ET
Join the Conversation
Ones to watch

In addition to Fortune's 40 Under 40 list, we're also keeping our eye on these 20 innovators, young guns from a wide range of industries and backgrounds.

Search for Jobs

Video