Astronomers find a planet denser than lead

By Phil Plait | October 6, 2008 10:50 am

Planets circle the stars that dot the heavens.

Before 1995, we couldn’t have said that with any certainty. Now we know of more than 300 planets orbiting distant stars, and we have a fleet of telescopes looking for them. The ultimate goal is to find another Earth orbiting a star like the Sun, but the quest on the way to that Holy Grail has yielded some strange benchmarks.

CoRoT-exo3b, a dense planet orbiting another star
COROT-exo-3b compared to Jupiter

Meet the planet COROT-exo-3b. It orbits a star slightly larger, hotter, and brighter than the Sun. The star is not an unusual one in any way, but the planet is definitely weird: it orbits the star in just over 4 days, which is pretty close in, though not a record breaker in and of itself. What’s bizarre is that it has about the same diameter of Jupiter, but has 21.6 times Jupiter’s mass. That makes it denser than lead.

If I could stand on the surface of this planet, I’d weigh 4200 kilograms*. That’s over 9000 pounds!


This is by far the most massive planet found so close to its parent star. There is another extrasolar planet found with about that mass, but it orbits its star much farther out. The ones we’ve found that orbit their stars so close tend to have masses much smaller than this. For comparison, Jupiter takes 12 years to circle the Sun once. Mercury takes 88 days. So we’re talking big planets, really close to their stars.

This planet is challenging to models. How did it form? It most likely formed farther out from the star — gravitational influences make it hard for a large planet to form close to a star — and then gradually moved in. This can happen due to friction, of all things: when the star and planet are young, there is a disk of material leftover from the planetary formation. As the planet sweeps through this material it slows its orbit. It spirals in due to drag gravitational interaction with the disk, and eventually settles down when the disk material thins out a few million kilometers from the surface of the star itself.

The mass of this newly discovered planet is pretty freaky. Normally, anything with a mass more than about 15 or so times the mass of Jupiter would be considered a brown dwarf, a "failed star", as some people call them (I don’t). But at the lower end of the brown dwarf mass range, it gets a bit hard to tell the difference between a planet and a BD. Some people say planets and BDs form in different way (planets grow in size from smaller bodies building up over time through collisions, while BDs and stars form from the collapse of material in a nebula); but I don’t like this definition. You could have two objects that look precisely the same, yet one could be a planet and the other a BD, just because they formed in different ways. That strikes me as silly.

Either way, COROT-exo-3b is weird.

It was discovered by COROT, an orbiting European Space Agency mission designed to look for stars that dip in brightness as an orbiting planet passes in front of them. That gives the size of the planet (the amount the light dims is proportional to the size of the planet). The mass was found using ground-based telescopes, by measuring the amount the planet tugs on the star as it orbits. That’s how the incredible density of this object was found.

As we search the sky for Earth analogue planets, we’re bound to find things at the limits of our understanding. This ultra-dense ball of compressed matter certainly falls into that category. And we’re still new at this! What else lies out there at the fringe of our knowledge?

*For those of you who want to complain about my use of mass as a weight, read this, and acknowledge my superior logical skills. However, given the amount of sturm and drang this has caused among people, I’ve decided to simply strike through the part that has given everyone the vapors.

CATEGORIZED UNDER: Astronomy, Cool stuff

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