Astronomers Find Densest Planet In The Universe (So Far)

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Astronomers have found what may be the biggest rocky planet in the neighborhood.

The planet is called 55 Cancri e. It is 60 percent larger than the Earth, eight times as massive and orbits its star so fast that an entire year passes in less than a day.

The findings are based on data from the Microvariability & Oscillations of STars telescope (MOST). MOST is a mission from the Canadian Space Agency, an orbiting telescope that measures tiny changes in a star's light. That allows it to see planets and measure stellar variability.

At about 41 light years from Earth, 55 Cancri is much closer than the stars and planets observed by the Kepler Space Telescope, which are hundreds or even thousands of light years distant. Also called Rho Cancri, the star is in the constellation Cancer and visible to the naked eye, if faint.

The planet found was originally thought to be a Neptune-sized world that orbited the star in 2.8 days. But new calculations based on the MOST data found that 55 Cancri's light dipped every 17 hours and 41 minutes. That meant that the planet was passing in front of the star, and that its orbit was much faster (and closer in) than anyone thought. The planet dimmed the star's light by about 0.02 percent, enough to tell the astronomers that its diameter was about 21,000 kilometers. Earth's diameter is about 12,740 kilometers.

The orbital period gives a good idea of the planet's mass. The team got a figure of about 8.57 times the mass of Earth, plus or minus about 0.64 Earth masses. That gives a density of about 10.9 grams per cubic centimeter, twice that of Earth, which averages about 5.5 grams. The surface gravity would be about 2.7 times that of Earth, comparable to what one would feel at the cloud tops of Jupiter.

An orbital period so short also means the planet orbits 55 Cancri at a distance of only three times the star's radius, or about 2 million kilometers. It speeds along at 738,000 kilometers per hour.

Dimitar Sasselov, a professor of Astronomy at Harvard University and one of the team of researchers on the project, said the planet is probably tidally locked to its star and keeps the same face towards it all the time. The equilibrium temperature at the substellar point - the equatorial part of the planet where the sun is directly overhead - approaches 2,900 degrees Kelvin, or about 2,627 degrees Celsius. That is hot enough to melt most metals.

But that doesn't tell the whole story, Sasselov said. As soon as you start moving away from the substellar point the temperature drops, he said. Also, many materials would never be liquids unless there is some kind of atmosphere around the planet. Without an atmosphere you don't get liquids, as anything that heats enough to melt turns right into a gas.

That means that if there is any atmosphere at all, it might well be made of silicates or even some metals. Sasselov said it would not likely be very thick or heavy, despite the high gravity.

Sasselov noted that the density of the planet was rather like that of Mercury in our solar system, and that implies its internal structure is likely to be similar. Mercury has an iron core that takes up about 70 percent of its volume, making the planet a lot denser than the Earth is. (Earth has an iron core as well, but it only takes up about 30 percent of the volume of the planet).

The team of astronomers involved hailed from the Massachusetts Institute of Technology, the University of British Columbia and the University of California at Santa Cruz, and will publish its results in the Astrophysical Journal.

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