An even smaller super-earth
Published on Jun 6, 2008 at 2:33 pm.
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Filed under extrasolar planets.
A few days ago, I wrote about a possible record setting small super-earth. Now, I have read about an even smaller super-earth discovered by a team of astronomers led by David Bennett of Notre Dame.
This newly announced planet is designated MOA-2007-BLG-192L b. Actually, everything in that string of letters and numbers refers to the star except for the letter “b” at the end. That is the planet. This planet may be only about 3 times Earth’s mass (the one that I talked about a few days ago was 4 times Earth’s mass). Doing the sort of back-of-the-envelope calculations that I did before, if this planet has similar composition to terrestrial planets in the Solar System, that puts its surface gravity at less than 50% greater than Earth’s gravity. Once again, that is a figure that raises interesting questions. Humans could move about under that sort of gravity. In fact, over time, I would imagine that Earth species could adapt to such gravity.
Interestingly, this planet was discovered using a microlensing technique. As the planet passes in front of a background star, its gravity slightly bends the light from the star, acting rather like a lens. Carefully monitoring of the light intensity from the star can detect this effect.
Another interesting bit of information from this system is that the host star is nothing like our own Sun. It is tiny. When extrasolar planets were first discovered, most were found around stars more or less like our own. This led to speculation that perhaps very massive stars form too quickly to permit planets to form, and that stars that are too small take so long to form that the planets simply spiral into the star like the rest of the material in its accretion disk. There is some indication that some stars do have planetary debris in their atmospheres. But, in recent years, several low mass stars have been found to have planets. This is the lowest mass star to date to be found with a planetary system. In fact, this star’s mass is so low that it is very near the cutoff point where a body can sustain nuclear fusion. The initial estimates for the mass are indeed that it is about 0.06 times the mass of the Sun. The cutoff is 0.08 solar masses. Objects with less mass than this cannot sustain nuclear fusion, and so they are not proper stars. We call them brown dwarfs. But, the mass estimate for this new object is very uncertain, so it may or may not be a brown dwarf. Though the initial estimate is 0.06 solar masses, it could well be just barely big enough to sustain fusion, and thus be a minimal star. At any rate, it is tiny, and it is cool.
If it is a minimal star, it will stay cool for just about forever. In fact, it would be able to sustain itself at that level of temperature and brightness for many times the current age of the universe. If it is a brown dwarf, though, then it can only produce energy through the gravitational compression of its gases. That doesn’t last as long, and it will eventually cool off. But, even in the former case, where the object is a star, the planet is still nothing that we would call habitable. The host star (or brown dwarf) is so cold that the planet itself is probably as cold as the objects in the Sun’s Kuiper belt. It may be a bit larger than Earth, but if it has any atmosphere, it is probably largely frozen solid on the surface. The surface might be warmed as much from heat working its way out from the interior of the planet than from light from the star.
What makes these findings of small super-earths exciting is that it suggests that planets like Earth may not be all that uncommon. The technology has not existed until recently to find planets this small. Now the technology is here, and we are finding planets. But, it also raises a question as the just how small something can be and still be called a super-earth. The term, itself, is not official. It is just a term that has come about from common use among astronomers. It does not have an official definition. But, I am thinking that a body only that only two or three times Earth’s mass is probably a bit on the small side to be calling a super-earth. Of course, we don’t really know anything about these bodies other than their possible masses. They may be nothing at all like Earth in composition. In fact, if this new body formed where it did around this small star, then a major component of it may be water, as is the case in planets and other bodies in our outer Solar System. So, that might make it more of a super-duper-pluto rather than a super-earth.
-Astroprof
Image courtesy NASA
