March 29 – Where the Home Fire Burns

Today’s factismal: Heinrich Olbers discovered Vesta , the eleventh planet in the Solar System, 209 years ago today.

If there is one thing that is sure to set planetologists and astronomers fighting, it is the question of how to define a planet. Astronomers claim that they get to define what a planet is because they are in the sky; planetologists claim that right because planets are what they study. But no matter how you define a planet, both sides will agree that the definition has changed several times.

For example, when Galileo discovered the four largest moons of Jupiter he called them planets. (Well, first he called them stars before realizing his mistake.)  And astronomers agreed with him until similar planets were found orbiting Saturn and the number of planets around Jupiter reached embarrassing levels – how could a mere planet have more planets than the Sun did? So astronomers decreed that any planet orbiting another planet was actually just a moon.  When Uranus was discovered in 1781, it fit neatly into the system as a new planet. When Titania and Oberon were seen circling Uranus and Enceladus and Mimas were seen orbiting Saturn, those were moons. Problem solved.

But then came Ceres. Astronomers had been searching for a “missing” planet between Mars and Jupiter based on the assumption that planetary orbits followed a spacing pattern that they called the Titus-Bode law. Like Kepler’s laws of orbital mechanics, Titus-Bode was an empirical rule based on observation and not theory. Unlike Kepler’s laws, Titus-Bode wouldn’t work out, though we wouldn’t discover that for more than a century. In the meantime, astronomers used it to tell them where to look for new planets. And, for a while, it seemed to deliver.

In 1801, Guiseppe Piazzi discovered a planet exactly where Titus-Bode predicted and named it Ceres. Less than a year later, Heinrich Olbers discovered another planet in the same area and named it Pallas. Then Karl Harding found Juno in 1804 and Olbers spotted Vesta in 1807. All in all, there were four planets where astronomers had expected to see but one and there were a total of eleven planets in the Solar System. But the new planets were tiny little things, just barely visible in the best telescopes of the day. Because they were so small, they could hardly be discerned from the stars behind them, and so Herschel proposed calling them “asteroids” or “star shaped”.

Despite the new nomenclature, astronomers still considered the asteroids to be planets. And that’s how they were spoken of in the press and in scientific papers for nearly forty years. They were given astronomical symbols to make it easier for astronomers to look them up in their texts and the origin of these planets was hotly debated. One popular suggestion was that they were the remains of a single, larger planet that had somehow broken apart. And other asteroids were eagerly sought to help fill in the gaps. But, despite many efforts, no new planets were found for nearly four decades.

The Universe, as matters stood in 1853. The four planets of Ceres, Pallas, Juno, and Vesta all fit into a gap that had been predicted by Titus and Bode.

But when they did start finding new asteroids, the floodgates opened. By 1860, 62 minor planets had been discovered. By 1890, that number had risen to 300. And in 1891, Max Wolf perfected a means of identifying asteroids using photographic plates that allowed them to be discovered almost automatically; indeed, a variant of that method is now in use and has identified more than 700,000 different asteroids!

Asteroids closer than Mars (Image courtesy JPL)

Asteroids closer than Mars (Image courtesy JPL)

Now here’s the crazy part: even though we’ve found more than 700,000 asteroids there are probably at least another 700,000 out there. And the ones still in hiding are too small or too lumpy or too weird to be found by an automatic program. What they need to find the remaining asteroids is someone who knows how to play hide-and-seek with a lump of rock a million  miles away. They need a human.

And that’s where you come in. Asteroid Zoo needs people to look at images and mark where they think an asteroid is hiding. With enough folks like you, we can find out where the remaining “vermin of the skies” (as the astronomers call them) are so we can know things how the Solar System formed, where the hazardous asteroids are, and if any of them are worth visiting. To learn more, zip on over to:

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