Kepler-78b: The Earth-Like Planet That Shouldn't Exist

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.'"

Comments

It's a Big Universe

[Greyfox]

In an inifinitely-ish sized universe, I'd be surprised NOT to find a lot of outliers. Even if it's 99.99999% unlikely ever to happen, there are still an infinite number of them out there! We might even be able to see a couple!

Re:It's a Big Universe

[harperska]

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

[IndustrialComplex]

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.

Re:It's a Big Universe

[mcgrew]

I'm not so sure about that. From TFA:

So could the planet have formed in a wider orbit and migrated inward? This is another improbability, say the researchers. âoeIt couldnâ(TM)t have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma,â Sasselov added.

That is in opposition to this:

âoeWhat Iâ(TM)m going to say is really absolutely crazy,â he said at the start of a recent seminar. âoeIf we publish this, my career might be over.â He could have made the same remark back in 2004 about what is now called the Nice modelâ"the hypothesis that he and his colleagues, including Alessandro Morbidelli of the CÃte dâ(TM)Azur Observatory in Nice, developed on the basis of dozens of computer simulations.

In essence Levisonâ(TM)s team proposed that our solar systemâ(TM)s four giant planetsâ"Jupiter, Saturn, Uranus, and Neptuneâ"had started out much more closely packed together, on nearly circular orbits, with the latter three closer to the sun than they are now. Early on they were embedded in the disk-shaped solar nebula, which was still full of icy and rocky debris. As the planets absorbed those planetesimals or flung them away after close encounters, they cleared gaps in the disk.

Because the planets were also tugging on one another, the whole system was fragileâ"âoealmost infinitely chaotic,â Levison says. Instead of each planet being linked only to the sun by a brass arm, itâ(TM)s as if they were all linked by gravitational springs as well. The most powerful one linked the two biggest bodies, Jupiter and Saturn. A yank on that spring would jolt the whole system.

And that, the team believes, is what happened when the solar system was about 500 million to 700 million years old. As the planets interacted with planetesimals, their own orbits shifted. Jupiter moved slightly inward; Saturn moved slightly outward, as did Uranus and Neptune. Everything happened slowlyâ"until at a certain point Saturn was completing exactly one orbit for every two of Jupiterâ(TM)s.

Because the planets were also tugging on one another, the whole system was fragileâ"âoealmost infinitely chaotic,â Levison says. Instead of each planet being linked only to the sun by a brass arm, itâ(TM)s as if they were all linked by gravitational springs as well. The most powerful one linked the two biggest bodies, Jupiter and Saturn. A yank on that spring would jolt the whole system.

TFA is framing the question in a sensational way. What the scientists are saying is "this is an exciting puzzle, it shouldn't happen according to what we know.

Re:It's a Big Universe

[Teancum]

Part of the problem is that many of the planetary system models have been developed with a sample size of one. That unfortunately skews the results of any such models. Now that there are literally hundreds of planetary systems to examine where the astrophysicists who make up these models can look at actual stellar systems to see how those models compare to reality, I'm sure there are going to be some changes to those models and some new theories put forward.

As usual, the science press is making up stuff to sensationalize a situation that is admittedly still unknown simply because it takes time to digest all of this new information. I don't think this is a reporter trying to attribute this to the supernatural, but they are trying to make what is otherwise dull news sound interesting.

Re:It's a Big Universe

[mythosaz]

Am I the only one who reads these things and goes: "Holy fuck, seriously? We're detecting planetary-caused star-wobble from where? That's how we do this shit?!?"

Re:It's a Big Universe

[harperska]

Yeah, when scientists say "This shouldn't happen according to current models", they are really saying "Holy shit, this is awesome! We get to come up with new models!".

Meanwhile, the mainstream media hears that and reports it either as "Scientists say this shouldn't happen. The universe is fucked up" or "Scientists say this shouldn't happen. Science is fucked up" depending on their political bent.

Re:It's a Big Universe

[cusco]

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.

Selection effects

[Michael Woodhams]

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.)

See! See!

[Ol Olsoc]

Scientists don't know something! This only proves that Global warming isn't real, that evolution is a farce, and the world was created in 4004 b.c.