Kepler-78b: The Earth-Like Planet That Shouldn't Exist 110
astroengine writes "Kepler-78b may be an exoplanet notable for being approximately Earth-sized and likely possessing a rocky surface plus iron core, but that's where any similarity to our planet ends. It has an extremely tight orbit around sun-like star Kepler-78, completing one 'year' in only 8.5 hours. It orbits so close in fact that the alien world's surface temperature soars to 2,000 degrees hotter than Earth's. Referring to Kepler-78b as a 'rocky' world is therefore a misnomer — it's a hellish lava world. But this is just a side-show to the real conundrum behind Kepler-78b: It shouldn't exist at all. 'This planet is a complete mystery,' said astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA) in a press release. 'We don't know how it formed or how it got to where it is today. What we do know is that it's not going to last forever.'"
Re:It's a Big Universe (Score:5, Informative)
That, and the results of both of our effective planet detecting schemes - transit and doppler - skew proportionately towards these hot worlds, as for both methods a shorter period will give a stronger signal and therefore be more likely to be detected. So just like with the hot jupiters detected by the doppler method, they are probably actually a minuscule fraction of the planets out there but happen to be the easiest to detect. So even though they are rare, we are guaranteed to see them, and then muse about their rarity.
Re:It's a Big Universe (Score:5, Informative)
It's not so much that it's an outlier or unlikely, it's that given our current understanding of planets/orbits/forces, it shouldn't be there at all. ie: There should be 0 planets like it in the universe. It would be like finding a neptune-like planet orbiting a sun-like star at 0.5 AUs, due to the solar wind at that distance, it should only be a 'rocky' planet, not a gas planet. The 'problem' with this planet is that it is too close to the star for it to have formed there, and there is no stable orbital migration pattern which would allow it to have formed farther out and drifted inward as close as it has w/o almost immediately falling into the star itself.
Selection effects (Score:4, Informative)
A great many of the known exoplanets are large, close to their star or both. It should be noted that this does not directly represent how common large close in planets actually are.
We find exoplanets in two ways - by Doppler shift of the star, or by transits.
When a planet orbits a star, the star also orbits their common center of mass, so it wobbles slightly. By looking for subtle Doppler shift in its spectral lines, we can try to detect this wobble. The larger (mass) the planet, the further the star wobbles, and the larger the Doppler shift. Similarly, the closer the planet, the faster (and so more detectable) the wobble. (Even though it has less distance to travel, this is more than compensated for by how much shorter the orbital period is.)
When a planet transits its star (moves between the star and us) we can detect a decrease in the received light, as some is blocked by the planet. The larger (radius) the planet, the greater the decrease, and so more likely we'll be able to detect it. The closer the planet, the more likely that chance alignment will allow us to observe a transit. Also, the closer the planet, the more frequent the transits, and so the more chance one will happen when we're observing the star.
So this weird planet was quite possibly thousands of times easier to detect than an Earth-like planet in an Earth-like orbit. (In this case, discovery was by transit, targeted observations measured the Doppler shift. The combination allowed an estimate of its density.)
Re:It's a Big Universe (Score:4, Informative)
I think Mythosaz might be remembering, the same as I do, when they said that the necessary Doppler shift measurements were so subtle that astronomers would NEVER be able to detect them. Now they're detecting Earth-sized planets, and it's incredibly cool.