Astronomy Buff

It Will Take Half the Age of the Universe To Search 4% of our Galaxy for Aliens

by Tony on January 22nd, 2007

I don’t think people really appreciate just how big the universe is. If you consider almost any problem associated with studying the universe, one quickly becomes entangled in some pretty tricky issues of scale. Consider the search for other civilizations in just our small, humble Milky Way galaxy. In a recent paper published in the International Journal of Astrobiology, the authors performed a rather remarkable simulation that attempted to answer the basic question of how long it would take to search a reasonable area of the galaxy for extraterrestrial civilizations. The answers were quite humbling.

To get you in the mood for what follows, watch this video I made last September:

There are about 100 billion stars in our galaxy, and of those, about 10% are estimated to have planets that harbor life. Most of these planets lie in what is known as the Galactic Habitable Zone (GHZ) and is located about 3000 - 11,000 parsecs from the center of our galaxy. The center of our galaxy isn’t likely to contain many planets that can hold life, what with supermassive black holes eating everything they can and the high density of stars located there.

Using estimates from this paper, there are about 0.0615 stars per cubic parsec in the GHZ if you assume that half of those stars are binaries and count them as one. If you do the math, and want to search an area 300 parsecs above and below the galactic disk from 3000 - 11,000 parsecs, it turns out that we would need to search 10,000,000,000 (ten billion) stars.

Technorati Tags: astronomy video, cosmology, Extraterrestrial Life

Ten billion stars is a pretty f*&king big number, even for a simulation. To make the problem more palatable, the authors cut the sample size to 40,000 stars from a variety of locations in the GHZ. Then, they sent out sample probes to do the exploration.

Here’s how the exploration was outlined:

• Send a probe to a destination star to investigate 40,000 stars within the GHZ
• Once the probe arrives at the destination star, it breaks up into 8 smaller probes, each designed to go out and search for signs of a civilization
• After all 40,000 stars have been searched, they return to the host probe and move on to the next destination star to start another search of 40,000.
• Each probe travels at a set speed of 0.1c, or ten percent of the speed of light. This allows for the effects of relativity to be ignored but it’s fast enough that the galaxy can be searched in a reasonable amount of time.
• The smaller 8 probes, do flybys only, they do not slow down and accelerate again.

Searching the galaxy this way takes roughly 100,000 years to explore 40,000 stars. Using this as a baseline unit, you can expand how long it would take to explore the entire galaxy. To do that the probes would have to do the above search 260,000 times, or 26 billion years - twice the age of the universe. Clearly we’re not going to get through even our own galaxy anytime soon. This was way too much for the simulation, so the authors cut it down and looked at the length of time it would take to search each system of 40,000 stars 10,000 times, or 4% of our galaxy.

After making allowances for the densities of stars and making sure each probe always went to a different star, the results were that it would take 9.57 billion years to to perform this search. That’s roughly half the age of the universe to cover 4% of our galaxy.

Next post: What kind of probes would these need to be? They certainly aren’t your father’s Voyager anymore…

Here’s the paper: Exploring the Galaxy Using Space Probes. It is an easy read, you should check it out. It still has many typos in it, telling me it hasn’t really been edited yet. Fascinating effort though.

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POSTED IN: general astronomy