Kepler provides insight about enigmatic but ubiquitous planets, five new rocky planets

Jan 06, 2014

(Phys.org) —More than three-quarters of the planet candidates discovered by NASA's Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth. Such planets dominate the galactic census but are not represented in our own solar system. Astronomers don't know how they form or if they are made of rock, water or gas.

The Kepler team today reports on four years of ground-based follow-up observations targeting Kepler's exoplanet systems at the American Astronomical Society meeting in Washington. These observations confirm the numerous Kepler discoveries are indeed planets and yield mass measurements of these enigmatic worlds that vary between Earth and Neptune in size.

Included in the findings are five new ranging in size from ten to eighty percent larger than Earth. Two of the new rocky worlds, dubbed Kepler-99b and Kepler-406b, are both forty percent larger in size than Earth and have a density similar to lead. The planets orbit their host stars in less than five and three days respectively, making these worlds too hot for life as we know it.

A major component of these follow-up observations were Doppler measurements of the planets' host stars. The team measured the reflex wobble of the host star, caused by the gravitational tug on the star exerted by the orbiting planet. That measured wobble reveals the mass of the planet: the higher the mass of the planet, the greater the gravitational tug on the star and hence the greater the wobble.

"This marvelous avalanche of information about the mini-Neptune planets is telling us about their core-envelope structure, not unlike a peach with its pit and fruit," said Geoff Marcy, professor of astronomy at University of California, Berkeley who led the summary analysis of the high-precision Doppler study. "We now face daunting questions about how these enigmas formed and why our solar system is devoid of the most populous residents in the galaxy."

Using one of the world's largest ground-based telescopes at the W. M. Keck Observatory in Hawaii, scientists confirmed 41 of the exoplanets discovered by Kepler and determined the masses of 16. With the mass and diameter in-hand, scientists could immediately determine the density of the planets, characterizing them as rocky or gaseous, or mixtures of the two.

These density measurements dictate the possible chemical composition of these strange, but ubiquitous planets. The density measurements suggest that the planets smaller than Neptune—or mini-Neptunes—have a rocky core but the proportions of hydrogen, helium and hydrogen-rich molecules in the envelope surrounding that core vary dramatically, with some having no envelope at all.

The ground-based observation research validates 38 , six of which are non-transiting planets only seen in the Doppler data. The paper detailing the research is published in the Astrophysical Journal today.

A complementary technique used to determine mass, and in turn density of a planet, is by measuring the transit timing variations (TTV). Much like the gravitational force of a planet on its star, neighboring planets can tug on one another causing one planet to accelerate and another planet to decelerate along its orbit.

Ji-Wei Xie of the University of Toronto, used TTV to validate 15 pairs of Kepler planets ranging from Earth-sized to a little larger than Neptune. Xie measured masses of the 30 planets thereby adding to the compendium of planetary characteristics for this new class of planets. The result also was published in the Astrophysical Journal in Dec. 2013.

"Kepler's primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone," said Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center in Moffett Field, Calif. "But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky, terrestrial globe?"

The dynamical mass measurements produced by Doppler and TTV analyzes will help to answer these questions. The results hint that a large fraction of planets smaller than 1.5 times the radius of Earth may be comprised of the silicates, iron, nickel and magnesium that are found in the here in the solar system.

Armed with this type of information, scientists will be able to turn the fraction of stars harboring Earth-sizes planets into the fraction of stars harboring bona-fide rocky . And that's a step closer to finding a habitable environment beyond the .

Explore further: Newfound planet is Earth-mass but gassy

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vlaaing peerd
not rated yet Jan 07, 2014
"But the question in the back of our minds is: are all planets the size of Earth rocky?"

I'm not an expert at all, just wondering. Wouldn't that be something that is possible to calculate rather than observe? I expect a water or gas planet would need a certain size and mass to keep it's water or gas gravitationally bound and therefore have minimum size or at least a likeliness for such to be possible.
LariAnn
3 / 5 (1) Jan 07, 2014
While extremely fascinating, these announcements seem to indicate that most extra-solar systems have hot Jupiters orbiting close to their stars, something totally unlike what we have here. I know the detection methods would miss small planets like Mercury but so many of the planets found are orbiting really close to their stars. Are our detection methods selective for this type of system or are we missing or miscalculating something somewhere? I wonder what would change if a rigorous critique of the assumptions involved in evaluating the data revealed that things in these extra-solar systems are not what they have seemed to be.
Requiem
4 / 5 (1) Jan 07, 2014
Are our detection methods selective for this type of system


Yes. Low real-world sensitivity versus the ideal needs to take a proper survey does strongly select for hot jupiters. The disparage is still huge with the instrumentation that Kepler had and how long it ran, considering that the mission didn't even run long enough to declare a detection on Earth itself had it been sensitive enough to observe a star with an Earth analogue. It's also worth noting that it would have had to have been within ~1000ly from Earth, if Kepler had actually ran long enough to observe 3 transits.

Also note that many of the Earth-mass(ish) exoplanets/moons that have been discovered were not actually by Kepler, but by follow-up observations to confirm other planets in the same system that Kepler selected for.
Requiem
not rated yet Jan 07, 2014
I'd also like to point out that the jury is still out, at least as far as I'm concerned, on a hot jupiter precluding the existence of an Earth-like body within a solar system. It's not difficult to imagine a system with a very radiant star, or perhaps multiple stars(which is actually the norm, not the exception), supporting an earth-like biosphere at many AU, beyond the perturbative effects of a hot jupiter.

Models are great and everything but all you have to do is cite DM as one low-hanging example of how unreliable even simple Newtonian calculations are when compared with the real universe. It's simple fact that we can't accurately model a system like that over billions of years.
Torbjorn_Larsson_OM
not rated yet Jan 08, 2014
After checking the CMB finding our universe is essentially flat, DM follows out of newtonian gravity (really the CMB alone, but NG as approximation for the background space). Locally decidedly so, out of galaxy rotation curves.

I think you are thinking of weak lensing measurements, where general relativity comes in?
Requiem
not rated yet Jan 08, 2014
I'm not sure what you got from what I typed but to clarify, I was stating that I don't really trust any present-day model that predicts that hot jupiters will expel earth-like planets in their host star's habitable zone. Citing the difference between what we observe and what we can model, which we call "dark matter", seems, as I said, like a very low hanging example of why any solar-system scale model would never be accurate when it has to simulate billions of years of evolution to become "stable".

I could elaborate on that point, for example by noting that we don't even know the distribution or shape of DM within and around celestial entities to any high degree of certainty, or what it's relationship is with the matter we can see there. It seems ridiculous to suggest that without that knowledge we could even come close when modelling billions of years.

Or I could move on to a more speculative one, like how exactly are we supposed to model planets that get captured during potentially any point in the lifetime of a solar system? With what interval do we introduce them? What are their properties? How often does a planet that will become habitable once captured come by?

It's not hard to find reasons why it seems likely(at least to me) that models would come up short when determining the makeup of a solar system at this time.

The second post was just an aside though, meant as nothing more than a possibly interesting thought. I have no bone to pick here.