You can’t see infinity. Somehow you know it exists, though.

This weird fact was pointed out by the Italian philosopher Giordano Bruno in 1584, and 16 years later he was burned at the stake for saying so. Or, it was one thing he said that got him in trouble. He also said the universe itself is infinite and has an infinite number of suns with infinite habitable planets circling around them.

These ideas contradicted the established wisdom of Inquisition authorities who some years later threw Galileo in prison just for saying the Earth circles around the sun.

It turns out 400 years later that we still can’t see infinity. But powerful telescopes have apparently seen to the edge of everything and collected information that implies Bruno might, for all intents and purposes, have been right. The latest evidence of planets circling stars cropped up last month when astronomers announced that a possibly habitable planet appears to be revolving around a star called Gliese 581.

Gliese 581, a “red dwarf,” is so small and far away that it was invisible to Galileo (and to Bruno too, though he never looked through a telescope), like almost all of the 178 stars where astronomers believe at least 210 planets are orbiting. The planets themselves are so small they are mostly impossible even for telescopes to spot.

To figure out whether a star has any planets, astronomers watch how the star’s light behaves. Although they seem stationary, all stars are in motion, which can be detected by “astrometry.” So can their speed, or radial velocity. A planet affects a star’s motion or speed, and so when either of these varies in certain ways, then a planet may be present. Also, a planet passing between a star and us may dim the star’s light, and this can be measured by “photometry.” Astronomers measure the variations, turn the measurements into mathematical equations, and reason logically from what the equations imply about the light. Planet Gliese 581c was detected by the radial velocity method.

In a few cases telescopes have taken pictures of actual planets, a method called “direct imaging.” But so far most extra-solar planets are too small and too close to their stars’ glare to be actually seen. They do exist, though, if the astronomers’ reasoning is right.

Some astronomers think it’s possible most stars have planets. How many might be habitable by humans is debatable because so far most of the planets detected appear to be large and gaseous like Jupiter, or so close to their stars the weather would be deadly to life as we know it. Planet Gliese 581c seems to be of an Earth-like size, though, and at a distance from its sun that may make it warm enough to have liquid water – which, logically, means living beings could inhabit it.

Now, Giordano Bruno was not a scientist. But he arrived logically at this idea: “There are innumerable suns, and an infinite number of earths revolve around those suns, just as the seven [planets] we can observe revolve around this sun which is close to us. … We discern only the largest suns, immense bodies. But we do not discern the earths because, being much smaller, they are invisible to us.” Given the billions of billions of stars that apparently exist, this idea is in principle pretty much the same one some modern astronomers have.

1584. Lucky guess. Or is infinity, despite being invisible, recognizable by some combination of perception, reason and imagination common to scientists, philosophers, and even the rest of us? I hesitate to answer, not wanting to get in hot water with the authorities.

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