Astronomers discover a Neptune-mass alien world with Earth-like density

May 17, 2016 by Tomasz Nowakowski, report
This artist's concept depicts a planetary system. Credit: NASA/JPL-Caltech

(—An international team of astronomers reports the discovery of a Neptune-mass exoplanet with a density comparable to Earth's. According to the research, the newly detected alien world, designated EPIC212521166 b, is the most massive planet with a sub-Neptune radius found to date. A paper detailing the finding was published online on May 13 in a pre-print version on

The team, led by Hugh Osborn of the University of Warwick, U.K., has analyzed the data provided by NASA's Kepler spacecraft prolonged mission, known as K2, in order to look for possible orbiting the star EPIC212521166. It is an 8 billion-year-old, metal-poor dwarf star with a mass of slightly more than 0.7 solar masses, located some 380 light years away. The K2 mission identified this celestial body as a strong candidate for hosting planets.

Osborn and his colleagues performed follow-up radial velocity observations of EPIC212521166 to confirm the presence of exoplanets around it. For their observations, conducted in March 2016, they used the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, mounted on the 3.6 m telescope at the European Southern Observatory's (ESO) La Silla observatory in Chile.

"We present the detection of a sub-Neptune radius planet around the K field dwarf EPIC212521166 from K2 photometry and the confirmation and mass measurement of this planet with HARPS radial velocities," the researchers wrote in the paper.

According to the study, EPIC212521166 b orbits its parent star approximately every 14 days. The planet has a mass of 18.3 Earth masses and radius of about 2.6 Earth radii, which makes it the most massive exoplanet with a sub-Neptune radius detected so far.

With a density of 5.7 g/cm3, the planet is expected to have a large rocky core. The researchers computed that the core must be composed mostly of enstatite (70 percent) and iron (30 percent). They also argue that the planet could have significant water content and a hydrogen-helium atmosphere.

"A two-layer iron-silicate composition model is unable to explain the density of EPIC212521166 b. Instead, either low-density volatiles such as water, an H-He atmosphere, or a combination of both must be present," the paper reads.

The scientists emphasized that the unusual density of this planet suggests that mass-radius relations should be used with extreme caution in the regime between terrestrial planets and gas giants.

The research also indicates that the evaporation process was unlikely the cause of such a high observed density of EPIC212521166 b. Thus, the planet likely formed dense, potentially after the gas disc has been photoevaporated. The scientists suggest that the planet likely disc-migrated to its current position; however, the lack of a thick hydrogen atmosphere doesn't support this theory.

The team concluded that the planet's migration and the absence of significant hydrogen atmosphere could be explained by giant impacts and accretion between a former compact multiplanet system.

"This hypothesis also supports the interpretation of an old age for this system, over which time the likelihood of dynamical instability is increased," the astronomers noted.

The EPIC212521166 system could also host other exoplanets. The team has searched for potential other transiting planets in this system but found no significant signal so far.

Explore further: Earth-like planet may exist in a nearby star system

More information: EPIC212521166 b: a Neptune-mass planet with Earth-like density, arXiv:1605.04291 [astro-ph.EP]

We report the discovery of the exoplanet EPIC212521166 b from K2 photometry orbiting on a 13.8637d period around an old, metal-poor K3 dwarf star. A joint analysis of K2 photometry and high-precision RVs from HARPS reveals it to have a radius of 2.6±0.1R⊕ and a mass of 18.3±2.8M⊕, making it the most massive planet with a sub-Neptune radius (i.e. mini-Neptune) yet found. When accounting for compression, the resulting Earth-like density is best fit by a 0.2M⊕ hydrogen atmosphere over an 18M⊕ Earth-like core, although the planet could also have significant water content. At 0.1AU, even taking into account the old stellar age of 8±3 Gyr, the planet is unlikely to have been significantly affected by EUV evaporation or tides. However the planet likely disc-migrated to its current position making the lack of a thick H2 atmosphere puzzling. With a V-band magnitude of 11.9 it is particularly amenable to follow-up observations, making EPIC-1166 b a rare and extremely important planetary system.

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May 17, 2016
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5 / 5 (6) May 17, 2016

Not really, sontar has surface gravity of 6 G's, this world is around 2.7 (due to the larger radius) D~
1 / 5 (6) May 17, 2016
I do not understand that without the basic information about the star draw conclusions about the planet?
There is only "..metal-poor dwarf star with a mass of slightly more than 0.7 solar masses ..).
Perform other values:
- Orbit of 14 days means that the star has a fast rotation on its axis, This results that is comes to white, hot dwarf fast rotation and a small radius (0.7 mass / say, ~ 0.7 radius) and high surface gravity (above 4).
It follows that the measurements of the planet is incorrect, it is about hot body with a temperature above 1000 K.
http://www.svemir...e-causal "The causal relation between a star and its temperature, gravity, radius and color"
1 / 5 (3) May 17, 2016
"The causal relation between a star and its temperature, gravity, radius and color"

With Sirius A for example, I have found no convincing science that proves it is a star at all. The way the instruments work means it is likely a planet whos ionosphere is emitting radiation at UV/EUV wavelengths, and only when that radiation interacts with Earths atmosphere is the visible light created. We have to trust NASA that Hubble is seeing stars, as without independently verifiable confirmation (nobody else has a visible light telescope outside of Earths atmosphere) it is most likely that all of the objects they claim to be stars are not even visible from space, and are not stars.
3.5 / 5 (4) May 17, 2016
With more research it'll be interesting to see whether our solar system is exceptional or common. So far it seems we're just finding bizarro systems totally different from ours.
5 / 5 (4) May 17, 2016
The way the instruments work means it is likely a planet whos ionosphere is emitting radiation at UV/EUV wavelengths, and only when that radiation interacts with Earth's atmosphere is the visible light created.

No, because 1) it has a clear blackbody radiation spectre with present absorption lines and 2) such interaction of light with atmosphere (or just any matter) would disperse the light as well (Compton scattering).

Also, the important thing about the absorption lines is that you can use them to 1) determine lower bound for rotational velocity because of their Doppler broadening (one side of the star is moving towards the Earth, the other away) and 2) determine mass from gravitational redshift.
1 / 5 (5) May 18, 2016

To be trusted only in the evidence, it does not matter who they collected evidence. Article builds a roof without a foundation. One might say deliberately hiding information that tell the suspect story about the possible existence of life. For article Science is not important. Sory NASA.
5 / 5 (2) May 20, 2016
@wduckss, Solon: When people don't read the paper *evidence* but refers to the press release shorthand info and to other stars, and come up with conspiracy theories there is no reason to trust *you* at all - no evidence.

wduckss, planet orbit period is not solar rotation. When a protoplanetary disk formed, there is some coupling in between, but it disappears as the disk does.

Do you really think the Sun rotates once every 400 year or so, bound to Neptune orbit period? How do you decide which planet is bound to Sun rotation, since our planets differ in orbit period?

Solon, this planet was observed by HARPS, which is Earth based. If you travel outside your city, you will see ~ 3.000 stars with your naked eye. Everyone knows, or should know, about stars and how they are detected. (Spectroscopically confirming the H/He elements at 6.000 K as on the Sun's surface, say - that is not a planet.)
not rated yet May 20, 2016
@tblakely: It is often claimed, but with more evidence the question is still open and seems tilted the other way.

First, our methods are biased to see 'bizarro' systems, either planets close to (radial velocity, transit observations) or far from (direct imaging) the star.

Second, there is a long discussion where the limits between different planet categories goes on average. This work adds one outlier to the discussion.

However, with massive statistics and unbiased methods it may be that the border between terrestrial planets and gassy mini-Neptunes goes at 1.2 Earth radii. [ http://www.planet...ets.html ]

That chart, based on precision data and an unbiased data filter agrees with a bayesian method that tried to remove bias. It would make Earth (and perhaps Venus) the superEarths we are looking for.

not rated yet May 20, 2016

And since Saturn is classified as likely a giant mini-Neptune rather than a dwarf Jupiter, with or without Planet Nine our mini-Neptunes are as frequent as we should expect, as is our Jupiters.

That classification, which seems viable so far, makes our system dead on normal.

in sum, we will have to wait and see how the chips fall. So far it seems like the Copernican expectation of being average, or at least common, is tentatively looking good.

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