TR: Astronomers Define New Class of Planet: The Super-Earth

[bystander]

Astronomers Define New Class of Planet: The Super-Earth
Technology Review | The Physics arXiv Blog | kfc | 2011 Aug 03

Rocky planets that are almost as big as Uranus seem far more common than anyone suspected.

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In our Solar System, planets fall into two types. First, there are the rocky planets like Earth, Mars, and Venus, which are similar in size and support gaseous atmospheres. Then there are the gas giants, like Jupiter, Saturn, and Uranus. These huge puff balls are two or more orders of magnitude bigger than their rocky cousins.

Perhaps strangest of all, there are no planets in between; nothing that sits on the borderline between rocky minnow and gas giant.

This sharp distinction has driven much of astronomers' thinking about planet formation. One of the main challenges they have faced is to come up with a theory that explains the formation of two entirely different types of planet, but no hybrids that share characteristics of both.

That thinking will have to change. It now looks as if we've been fooled by our own Solar System. When astronomers look elsewhere, this two-tiered planetary division disappears.

Astrophysicists have now spotted more than 500 planets orbiting other stars and all of these systems seem entirely different to our Solar System. They've seen entirely new class of planets such as the Super-Jupiters that are many times larger than our biggest planet with orbits closer than Mercury.

But the one we're interested in here has a mass that spans the range from Earth to Uranus, exactly the range that is missing from our Solar System.

Astronomers are calling these new types of planet Super-Earths, and so far they have found more than 30 of them.

Today, Nader Haghighipour at the University of Hawaii in Honolulu reviews what we know about Super-Earths and shows they are changing the way astronomers think about planet formation. Their mere existence, for example, should allow astrophysicists to reject a large portion of current theories about planet formation.

Of course, the question about Super-Earths that generates the most interest is whether they can support life. To that end, Haghighipour discusses the possibility that these planets may be rocky with relatively thin atmospheres, that they have dynamic cores that generate a magnetic field and that they may support plate tectonics. Above all, there is the question of whether they can support liquid water.

It makes for fascinating reading. But when all this new information has been absorbed by the community, astronomers will still be left with an important puzzle. That is why our Solar System is so different from all the others we can see, why it has this sharp distinction in planet type and what relevance this has to the question of habitability.

This is a mystery that astronomers are only just getting their teeth into.

Coming To A Solar System Near You… Super-Earth!
Universe Today | Tammy Plotner | 2011 Aug 06

Planetary system of HR 8799 imaged by Marois et al (2010) [i](Figure with the permission of Nature Publishing Group)[/i]

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It is our general understanding of solar system composition that planets fall into two categories: gas giants like Jupiter, Saturn, Neptune and Uranus… and rocky bodies that support some type of atmosphere like Earth, Mars and Venus. However, as we reach further into space we’re beginning to realize the Solar System is pretty unique because it doesn’t have a planetary structure which meets in the middle. But just because we don’t have one doesn’t mean they don’t exist. As a matter of fact, astronomers have found more than 30 of them and they call this new class of planet a “Super-Earth”.

“Super-Earths, a class of planetary bodies with masses ranging from a few Earth-masses to slightly smaller than Uranus, have recently found a special place in the exoplanetary science.” says Nader Haghighipour of the Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii. “Being slightly larger than a typical terrestrial planet, super-Earths may have physical and dynamical characteristics similar to those of Earth whereas unlike terrestrial planets, they are relatively easier to detect.”

Having a super-Earth in the neighborhood opens the avenue towards habitability. Chances are planets of this type have a dynamic core and are able to maintain a type of atmosphere. When combined with being within the habitable zone of a host star, this raises the bar towards possible life on other planets.

“It is important to note that the notion of habitability is defined based on the life as we know it. Since Earth is the only habitable planet known to humankind, the orbital and physical characteristics of Earth are used to define a habitable planet.” says Haghighipour. “In other words, habitability is the characteristic of an environment which has similar properties as those of Earth, and the capability of developing and sustaining Earthly life.”

But being a super-Earth means that there is a lot more going on than just being in the zone. To qualify it must meet three requirements: its composition, the manifestation of plate tectonics, and the presence of a magnetic field. For the first, the presence of liquid water is a high priority. In order to determine this possibility the values of its mass and radius have to be known. To date, two super-Earth planets for which these values have been determined – CoRoT-7b and GJ 1214b – have given us fascinating numerical modeling to help us better understand their composition. Plate tectonics also plays a role through geophysical evolution – just as the presence of a magnetic field has been considered essential for habitability.

“Whether and how magnetic fields are developed around super-Earths is an active topic of research.” notes Haghighipour. “In general, in order for a magnetic field to be in place around an Earth-like planet, a dynamo action has to exist in the planet’s core.”

Last, but not least, comes an atmosphere – the “presence of which has profound effects on its capability in developing and maintaining life.” From its chemical properties we can derive the “planet’s possible biosignatures” as well as the chemicals which formed it. Atmosphere means environment and all of this leads back to being within a habitable zone and of sufficient gravity to keep atmospheric molecules from escaping. Says Haghighipour, “It would not be unrealistic to assume that super-Earths carry gaseous envelopes. Around low-mass stars, some of such atmosphere-bearing super-Earths may even have stable orbits in the habitable zones of their host stars.”

Has a super-Earth been detected? You betcha’… and studied right down to its spectral signature. “The recently detected super-Earth GL 581 g with its possible atmospheric circulation in the habitable zone of its host star may in fact be one of such planets.” says Haghighipour. “More advanced telescopes are needed to identify the biosignatures of these bodies and the physical and compositional characteristics of their atmospheres.”

Super-Earths: A New Class of Planetary Bodies - Nader Haghighipour

• arXiv.org > astro-ph > arXiv:1108.0031 > 29 Jul 2011

[ExplorerAtHeart]

Wonder what the core of one of these big guys must be like. Woofh! O~O I would imagine something with high gravity would attact more volatiles to its surface. Image an ocean thousands of kilometers deep.

[bystander]

Super-Earths may come in two flavors
Science News | Nadia Drake | 2011 Sept 08

Some of the exoplanets appear to be denser than others

[i]Planets with masses that fall between Earth’s and Neptune’s might be common around distant stars. (Credit: Aldaron/Wikimedia Commons)[/i]

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Now that known exoplanets have become almost as numerous as fireflies on a midsummer’s eve, two top planet-finding missions are starting to disagree over the abundance of low-mass planets that are heavier than Earth but smaller than Neptune.

The Swiss-led HARPS mission suggests that between 30 and 50 percent of sunlike stars in the solar neighborhood host super-Earths and sub-Neptunes. Meanwhile, NASA’s Kepler mission is finding that these planets circle roughly 15 percent of the stars in its far-flung field of view.

That discrepancy is of great interest to astronomers, because the number of planets in the weight class just above Earth hints at how many bodies of terrestrial proportions are likely to be discovered.

But there may not be a discrepancy at all.

“We know the Geneva team does a good job observing, and they have a good technique. And we know the Kepler telescope is working beautifully. So we wanted to see if there was a plausible, believable way in which you could have the difference between those two surveys,” says Greg Laughlin, an astronomer at the University of California, Santa Cruz. In a paper posted online August 30 at arXiv.org, he and U.C. Santa Cruz graduate student Angie Wolfgang propose that there are two kinds of low-mass planets out there, one of which is more amenable to discovery by HARPS.

Laughlin and Wolfgang created a simulation based on the HARPS data. In it, they created a population of planets between one and 17 Earth-masses around the more than 100,000 stars being monitored by Kepler. Giving those virtual planets varying characteristics and orbital periods between one and 50 days, the researchers then asked whether simulated planet populations could reproduce the Kepler observations. The answer was “yes.”

When peppered with two distinct kinds of planets in the size range — one rocky and dense, the other gaseous and fluffy — the simulation mimics Kepler’s data. “You don’t really have to force things into some strange configuration. You can explain the results of those surveys if you have these two distinct populations,” Laughlin says.

HARPS searches for distant planets by looking for the telltale signs of an orbiting planet tugging on its host star in what’s called a radial velocity survey. Surveys using the transit method, like Kepler, monitor a star field for blips in brightness caused by a planet briefly blocking some of the star’s light. These different methods can bias the types of planets detected — denser planets tug more on their stars, and bloated planets with bigger radii block more light.

“Nature can be quite a bit more clever than our simple models are, especially in the realm of exoplanets. We always find that things are much more interesting and complicated than we imagine,” says Caltech planetary astronomer John Johnson.

Ideally, densities for detected planets could be found by peering at the exo-worlds using both methods — but in practice that’s often tricky.

Two super-Earths that have been detected using both methods — CoRoT-7b and GJ 1214b — have similar masses but different densities. Both are examples that fit well with Wolfgang and Laughlin’s ideas, says Andrew Youdin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

There are other explanations for the different results, though they’re less probable. Johnson points to the possibility that the surveys are peering at different stellar populations, but says that’s far-fetched.

Either way, there are lots and lots of planets out there, and some might be quite familiar, Johnson says. “Kepler’s been telling us that the local universe is teeming with Earths. They’re just all over the place. We need to go out and find them.”

Combining Kepler and HARPS Occurrence Rates to Infer the Period-Mass-Radius Distribution of Super-Earths/Sub-Neptunes - A. Wolfgang, G. Laughlin

• arXiv.org > astro-ph > arXiv:1108.5842 > 30 Aug 2011