And it could just be a matter of time before they are discovered.
In a brief passage titled "Where Are the Real Space Aliens?" Mr. Bush's budget document says that several important scientific discoveries in the past decade indicate that "habitable worlds" in outer space may be much more prevalent than once thought.
The passage acknowledges that hard evidence of alien life has yet to emerge.
"Despite all the space aliens that appear in science fiction movies and books, we have yet to find conclusive evidence for life, even microbes, anywhere in the universe besides Earth," it reads, but then adds, "Researchers have found life in very harsh environments on Earth, which expands the possible kinds of places where life might exist."
Other recent finds include evidence of currently or previously existing large bodies of water — a key ingredient of life — on Mars and on Jupiter's moons.
Astronomers also are finding planets outside the solar system, including about 90 stars with at least one planet orbiting them.
"Perhaps the notion that 'there's something out there' is closer to reality than we have imagined," the passage concludes.
The budget justifies the funding for one space project by mentioning the chance that life exists beyond this plant.
The president calls for $279 million next year and $3 billion over five years for Project Prometheus, which includes building the Jupiter Icy Moons Orbiter.
"This mission will conduct extensive, in-depth studies of the moons of Jupiter that may harbor subsurface oceans and thus have important implications in the search for life beyond Earth," the budget reads.
Mr. Bush is not the first president to show an interest in the possibility of extraterrestrial life.
Clinton aide Webster Hubbell claims in his autobiography that after President Clinton appointed him to a Justice Department post, the president asked Hubbell to investigate whether UFOs existed, as well as determining who was responsible for the assassination of President Kennedy.
The budget is the second time in recent months that the Bush administration has addressed questions about life in space.
On Dec. 24, the White House issued a September determination by Mr. Bush in which he followed his predecessors' lead by issuing a determination exempting the Air Force facility near Groom Lake, Nevada, from environmental laws allowing the release of classified information about the area.
Groom Lake is the place that UFO buffs call Area 51.
"I find that it is in the paramount interest of the United States to exempt the United States Air Force's operating location near Groom Lake … from any applicable requirement for the disclosure to unauthorized persons of classified information concerning that operating location," the president wrote.