Hell on & off Earth


Venus may be the hottest terrestrial world in our solar system, super-volcanic Io might most resemble Dante’s Inferno, but we have a new candidate for most hellish earth-like world and it’s not even a close call:

Red Orbit — According to the Harvard scientists involved with the discovery, Kepler 78b is the first known Earth-sized planet that also shares an Earth-like density. While the exoplanet is only about 20 percent larger than Earth in terms of size and volume, it weighs almost twice as much – suggesting an Earth-like composition of iron and rock.

“It’s Earth-like in the sense that it’s about the same size and mass, but of course it’s extremely unlike the Earth in that it’s at least 2,000 degrees hotter,” said team member Josh Winn, an associate professor of physics at MIT. “It’s a step along the way of studying truly Earth-like planets.”

The determination of the planet’s orbit and size was made by analyzing the light given off by its host star as the planet passes in front of it in a phenomenon known as a transit. The researchers recorded a transit each time the star’s light dipped, and calculated the amount of dimming to determine the planet’s size.

It may seem strange that the very first rocky-iron earth-sized exoplanet we find is so close to its star. Until you stop and think about how it was detected. NASA’s Kepler is basically a horrendously accurate photometer connected to special telescope. It’s so accurate it can detect minute changes in brightness as a tiny planet passes in front of its sun. If a planet is only a million miles away from a star a million miles wide, the odds of it passing in front of that star’s disk as seen from our perspective many light-years away is much higher than it would be for a planet orbiting at a more familiar distance.

How in the hell 78b ended up a million miles away, with an eight-and-a-half hour year, is a much bigger mystery. We have only the most rudimentary understanding of early solar nebula dynamics. We’re only just now starting to piece together clues from important events like Late Heavy Bombardment or the Giant Impact theory that shaped our own solar system and planets, and those clues are right next door by comparison to exo-systems. Odds are it has to do with chaotic proto-planetary formation in the early accretion disk around a new star. Some objects gang up on others and kick them into higher orbits or clear out of the system, to roam the galaxy as rogue planets until the last stars wink out. Others get kicked in, Kepler 78b is probably one of those.

But it won’t last long, the planet is on a death spiral. It will become part of it’s star in a relatively short time by astronomical standards, leaving virtually no direct trace behind. If something similar happened in the early history of our own solar system, it would be extremely difficult and perhaps impossible to reliably infer such a planet ever existed.




  1. Pierce R. Butler says

    From the BBC story:

    The results suggest Kepler 78b is about 1.2 times the size of Earth, and 1.7 times as massive. From the same measurements, the astronomers calculated the planet’s density as 5.3 grams per cubic centimetre, close to the 5.5 grams per cubic centimetre value for Earth.

    I’m down with the flu (misery loves company, huh?) and never got high grades in math, and recognize that the term “size” is ambiguous (diameter? volume?), but I read the first quoted sentence as implying K78b has higher density than our home planet, and the second sentence as saying the opposite.

    In any case, I eagerly look forward to the movie about invading lava monsters!

  2. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    it won’t last long, the planet is on a death spiral. It will become part of it’s star in a relatively short time by astronomical standards.

    Well, three billion years or so according to this space-dot-com article :


    Which is rather a while still.

    Remarkable discovery and very hot news indeed.

    Astonishing that we can learn so much about something this small and distant.

  3. sawells says

    @1: if the diameter is 1.2 times that of earth than the volume scales as 1.2 cubed, which is 1.728. Dividing 1.7 earth masses by 1.728 earth volumes gives 0.98 times earth density which is about 5.4.

    The problem was the ambiguous term “size” in the first sentence.

  4. Orakio says

    Pierce – The 1.2 is probably radius / diameter, and mass goes up by the cube of radius. 1.2*1.2*1.2 = 1.73 times the volume. And… amusingly, it’ll pull just under 1.2 g on the surface.

  5. lpetrich says

    Here’s how such planets may have originated: Stars Can Strip Gas Giants Naked

    The most remarkable exoplanet discovery is likely of “hot Jupiters”, gas giants that orbit much closer than what one would expect from their composition. They are usually thought to have spiraled in from where they formed, and some have spiraled in remarkably close to their stars.

    Their stars can then strip off most of their material, leaving behind a rocky core.

  6. Amphiox says

    I think that whenever the word “size” is used to refer to a Kepler planet, it means radius/diameter, because that is what Kepler actually observes – the cross-sectional shadow the planet casts against the light of its star.

    If the word “size” is used to refer to a radial velocity/doppler shift planet, then it probably refers to mass, because that’s what the radial velocity method actually measures/determines – the mass of the planet from the effect its gravity has on the motion of its parent star.

  7. StevoR : Free West Papua, free Tibet, let the Chagossians return! says

    @ ^ Amphiox : yep. That makes sense.

    @8. lpetrich : Also agreed. The Bad Astronomer in his blog article on this exoplanet suggested something similar . Be interesting to see if anyone can determine if this world is shrinking with gas and /or even molten rock being blown off to form a comet like tail and perhaps even calculate back over how much mass /gas it likely started with before it migrated too close to its sun.

    @3. Raging Bee : “So basicaly you’re saying I shouldn’t buy land on this exoplanet?

    Well, if it isn’t tidally locked with one side molten and the other frozen* then its seems likely the whole planet is one huge magma ocean – not much land for you to buy!

    * As depicted in a youtube clip about a similar exoplanet here :


    namely Kepler-10b. Imagine Kepler78 b is very similar, probably also tidally locked – although maybe an atmosphere evens out the searing temperature making it lava all over – and a reasonable number of these Mustafar class lava worlds exist.

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