A Sunny Day on a ‘Super-Earth’?

by Paul Gilster on June 13, 2013

When a distant planet moves in front of its star as seen from Earth, the slight drop in starlight is often enough to allow sensitive instruments to make a detection. We call the degree to which the star’s light is diminished the ‘transit depth,’ and even with transiting gas giants, the figure is usually on the order of one percent. What we’re getting at is the ratio of the area of the planet to the area of the star behind it. The transit depth of the ‘hot Jupiter’ HD 189733b is unusually large at three percent. Obviously both a planet’s size and the the size of the star come into play.

In the case of the super-Earth GJ3470b, the primary star is relatively nearby and is also an M-dwarf, allowing greater transit depth and propelling a series of investigations from the ground. GJ3470b orbits its star at 0.036 AU, completing its orbit in a mere 3.3 days. The new work, led by Akihiko Fukui and Norio Narita (NAOJ), along with Kenji Kuroda (University of Tokyo), looks at the atmosphere of a planet with some fourteen times the mass of the Earth.

The team calculates the radius of GJ3470b at 4.3 times larger than that of the Earth, a figure about 10 percent smaller than previously reported. Its calculations also indicate that the planet possesses a hydrogen-rich envelope of considerable mass. Says Fukui:

“Suppose the atmosphere consists of hydrogen and helium, the mass of the atmosphere would be 5 to 20% of the total mass of the planet. Comparing that to the fact that the mass of Earth’s atmosphere is about one ten-thousandth of a percent (0.0001%) of the total mass of the Earth, this planet has a considerably thick atmosphere.”

Using the Near-Infrared Imager/Spectrograph (ISLE) mounted on the Multicolor Imaging Telescopes for Survey and Monstrous Explosions (MITSuME) instrument in Okayama, the team looked at the lightcurve of the transit in four colors from visible to near-infrared. This news release from the National Astronomical Observatory of Japan provides the image below, showing estimates of planetary radius by each of the colors observed. The radius derived from near infrared turns out to be 6 percent less than that derived from visible light.

20130612-oao-fig2

Image: Radius of each color measured (observation wavelength) of GJ3470b (shown as Planet-to-Star radius ratio). Credit:NAOJ.

From the paper:

A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed Rp / Rs [the planet-to-star radius ratio] variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H2O, CH4, and CO, without being prevented by clouds.

In other words, the lack of a substantial cloud cover should make it easier to find traces of water or methane in the atmosphere that could give us clues as to how the planet formed — thick clouds would have masked the differences in radii by color that the researchers detected. The team hopes to conduct further observations of the planet using the 8.2-meter optical-infrared Subaru telescope on Mauna Kea in Hawaii, looking for further ways to characterize the world’s atmosphere and get a clue as to whether it formed in its present position or further out in the system, later migrating inward.

The paper is Fukui et al., “Optical-to-Near-Infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint of a Cloud-Free Atmosphere,” in The Astrophysical Journal, Vol. 770 (2013), p. 95 ff. (abstract).

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{ 5 comments }

Jim Early June 13, 2013 at 10:08

As implied in the title of this paper, this planet has almost the same radius, mass and density as Uranus. The title described the planet as a “hot Uranus”. Did the paper give an estimate of the temperature or a calculation of the radiation flux for the planet?
If the planet shows a Raleigh scattering pattern in its spectra, that could also indicate a low level of absorbing carbon compounds in the atmosphere.

Klaus Lang June 13, 2013 at 12:50

Hello,
can you please explain, why this planet should have a thick atmoshere but no thick cloud layer? Is there a planet like this in the solar system?
Klaus Lang

andy June 13, 2013 at 12:51

The full paper is available on arXiv here.

I do find it odd that GJ 3470b is described as a “super-Earth” in the press release. It has roughly the same mass as Uranus and is slightly larger in diameter, the paper’s use of the terms “hot Uranus” and “hot Neptune” seem to me to better convey the nature of the planet.

(Main caveat being we do not know if the heavy elements in GJ 3470b are mainly icy or mainly rocky, though apparently we don’t know particularly well whether Uranus and Neptune are icy or rocky either.)

ljk June 13, 2013 at 14:05

13 June 2013

** Contacts are listed below. **

Text & Image:

http://carnegiescience.edu/news/exoplanet_formation_surprise

EXOPLANET FORMATION SURPRISE

A team of researchers has discovered evidence that an extrasolar planet may be forming quite far from its star — about twice the distance Pluto is from our Sun. The planet lies inside a dusty, gaseous disk around a small red dwarf TW Hydrae, which is only about 55% of the mass of the Sun. The discovery adds to the ever-increasing variety of planetary systems in the Milky Way. The research will be published in the Astrophysical Journal.*

This dusty protoplanetary disk is the closest one to us, some 176 light-years away in the constellation Hydra. The astronomers made Hubble Space Telescope observations over a wide range of wavelengths from visible to near infrared and modeled the color and structure of the disk in a way that has not been done before. They found a deficit of disk material, or partial gap, at about 80 astronomical units (AU) (1 AU is the Earth/Sun distance). Their models indicate that the depression is about 20 AU wide, just slightly wider than necessary for a planet-opening gap and consistent with a planet of between 6 and 28 Earth masses. The feature is seen at all wavelengths indicating it is structural and not a local compositional difference. The team believes the evidence is strong for planet formation causing the gap.

“TW Hydrae is between 5 and 10 million years old, and should be in the final throes of planet formation before its disk dissipates,” remarked coauthor Alycia Weinberger of the Carnegie Institution and principal investigator of the observations. “It is surprising to find a planet only 5 to 10% of Jupiter’s mass forming so far out since planets should form faster closer in. In all planet formation scenarios, it’s difficult to make a low-mass planet far away from a low-mass star.”

The goal of these observations was to understand not only whether planets have formed, but also what conditions can result in planet formation and what chemical constituents are available for new planets. Models by coauthor Hannah Jang-Condell, a former Carnegie postdoctoral researcher, showed that the disk was brighter than expected, which indicates that very small dust grains are being lifted high above the midplane. This is surprising because observations with radio telescopes have previously shown that the disk contains dust that has conglomerated into pebbles.

Weinberger designed the observations to be able to detect large water ice grains in the surface layer of the disk. These grains weren’t seen, which probably means that they have grown and sunk to the midplane of the disk where they can aggregate into water-rich planets.

Planet formation far away from a small parent star is at odds with the conventional planet-making dogma. Under the most accepted scenario, planets form over tens of millions of years from the slow accretion of dust, rocks, and gas. That happens most easily close to the central star, where orbital timescales are short. Even under a disk instability scenario, in which planets can collapse quickly from the disk, it’s not clear such a low mass planet could form.

Carnegie astrophysicist Alan Boss, who works on disk instability models, said “If the mass of this suspected planet is as low as it seems to be, this presents a real puzzle. Theory would say that it cannot exist!”

Lead author of the study, John Debes of the Space Science Telescope Institute and also a former Carnegie postdoctoral researcher remarked, “Typically, you need pebbles before you can form a planet. So, if there is a planet in the gap and there is no dust larger than a grain of sand farther out, we have provided a challenge for traditional planet formation models.”

PIO Contact:

Tina McDowell
+1 202-939-1120
tmcdowell@carnegiescience.edu

Science Contact:

Alycia Weinberger
+1 202-478- 8852
aweinberger@carnegiescience.edu

*Authors on the study are John Debes, Hannah Jang-Condell, Alycia Weinberger, Aki Roberge, and Glenn Schneider. Support for this work was provided by NASA through the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under contract NAS 5-26555. Debes, Jang-Condell and Roberge are all former Carnegie postdoctoral Fellows.

d.m.falk June 14, 2013 at 12:30

Klaus: To answer your question, yes we do- Neptune, actually, has very few clouds! Its colour is due to almost entirely Raleigh light scattering, and is so thick, we’re unable to see the core.

This particular “hot Neptune” is consistent with what is predicted, insofar as to the cloudiness of gas planets, as planets like Jupiter and Saturn are at just the right temperature balance with solar activity to produce cloud layers- Any closer, such as closer than the Earth, the gas giant would lose its cloudiness, but not atmospheric activity (wind patterns). Too cold, and the same thing happens- Not enough atmospheric temperature to cause heavier particles (gases, etc.) to stir up. This can be seen in Saturn during its seasonal extremes- The upper atmosphere goes cloudless, and turns bluish, due to Raleigh scattering, though Saturns lower layers can still be cloudy, being still warmer.

A “hot Neptune” around a red dwarf, such as this one, will look a bit closer to a bluish-grey, if we could see it, since the light from its sun is skewed more toward the red and upper (near) infrared.

d.m.f.

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