But, like snail mail, it would also take a lot longer to get there.

"Basically, if you can wait, you can pack information tightly and send it cheaply," said Christopher Rose, a physicist at the Wireless Information Network Laboratory of Rutgers University in New Jersey. "And in interstellar distances, maybe waiting is not too terrible a thing if it's just a matter of promulgating your knowledge, not waiting for a reply."

Rose's conclusions appeared in the journal Nature on the heels of a NASA announcement that astronomers have found a new class of planets with some Earthlike qualities. The recent discovery only underscores the notion that there are plenty of stars out there with plenty of planets around them. So, Rose asks, why shouldn't life be out there?

Barking Up the Wrong Tree?

This was what physicist Frank Drake concluded in the 1960s. The so-called Drake Equation showed that, given the probable number of stars and planets in faraway solar systems, the chances are good that intelligent life exists somewhere out there.

Drake was the first to tune a radio telescope to the stars and listen for a message from afar. Today, astronomers with the SETI (Search for Extraterrestrial Intelligence) Institute listen to space using a much more powerful tool — the world's largest radio telescope, Arecibo, whose dishes extend for as long as 26 football fields.

But Rose's calculations suggest we may have been tuning in the wrong way. If intelligent aliens are concerned with efficiency, he found, they would certainly write, not call.

Through a series of equations, Rose worked out that the intensity of radiation fades with the square of the distance from the source (think of the fading beam of a flashlight). That means, to get the same message to its target, the energy cost of sending the message multiplies as the distance it travels increases.

Matter, on the other hand, does not diminish as it travels. The main cost of sending matter long distances is in kinetic (active) energy, Rose explains, and that goes down with the square of how fast you throw it.

"Work the tradeoff and it usually favors mass by a lot," said Rose, who worked with physicist Gregory Wright of New Jersey's Antiope Associates to produce the results.

What's more, he adds, an intergalactic "letter" would stick around for much longer than a fleeting radio wave. Miss a radio wave signal and it's more or less gone forever. But an inscribed message could sit around for centuries, maybe millennia or longer, waiting to be found.

Galactic Express

Of course, aliens would need to know where to send such a letter and then Earthlings would need to find it. For Seth Shostak, senior astronomer at the SETI institute, that problem outweighs any benefits in efficiency.

"It's hard to figure a SETI strategy — if it arrives, where does it arrive?" he said. "How do you know where your intergalactic mailbox is? By the time the message finally reaches its destination, no one may be home."

Rose says he's still working on these issues, but, in the meantime, he and Wright have worked out a few possibilities for what aliens might do to reach out to Earth:

  Inscribe a message or entire library of information on a natural or alien-made asteroid and slam it into Earth. The message could be visible on the asteroid or in the crater it forms. The message would be pressed into the Earth's surface by the asteroid.

  Etch a library of microscopic information onto a chip or similar device, place it into a capsule and shoot it into an area of space, known as a Lagrange point, where equal-pulling forces keep the object in a stable orbit. Other possible locations include the moon and comets in stable orbits. But they'd have to hope that Earthlings would know to look there.

  Enclose a spore containing all the elements needed to reproduce your life form and send it to Earth on a comet or other type of large, solid object. Draw attention to the package by ensuring the comet or other vessel leaves a bright tail above Earth.

As for our likelihood of finding these packaged messages, Rose argues one way to narrow down the search would be to look for objects in space or on Earth that appear suspiciously smooth.

"If you believe the theories about how the solar system was formed, there was a lot of banging around out there," Rose said. "If a message arrived later, it would be less banged up."

Genetic Message?

In fact, Rose is hardly the first to suggest that there may be a bevy of alien information just waiting to be discovered. Paul Davies, an astrobiologist at Macquarie University in Australia, recently proposed that the entire contents of an "encyclopedia galactica" may be coded in the junk DNA of our own genes.

"Scientists in the United States have discovered whole chunks of human and mouse junk DNA that seem to have remained virtually unchanged for tens of millions of years," Davies wrote in the Aug. 7 issue of New Scientist. "That would be a good place to store a message."

Shostak concedes that nobody and everybody could be right. In the meantime, he says SETI will keep its focus on listening to space. Soon it will have even better tools to do so with the recent $13.5 million donation from Microsoft co-founder Paul Allen. The windfall will go toward building 350 more telescopes that will work in tandem to scan the heavens for an alien "hello."

As for other signs from afar, Rose argues, why not look for them?

"If you don't look, you're being kind of goofy because they could be there," he said. "You don't necessarily want to sell the farm, but it can't hurt to look."