Thinking About ‘Mini-Neptunes’

by Paul Gilster on January 7, 2014

Yesterday’s look at the exoplanet KOI-314c showed us a world with a mass equal to the Earth, but sixty percent larger than the Earth in diameter. This interesting planet may be an important one when it comes to studying exoplanet atmospheres, for KOI-314c is a transiting world and we can use transmission spectroscopy to analyze the light that passes through the atmosphere as the planet moves in front of and then behind its star. A space-based observatory like the James Webb Space Telescope should be able to tease useful information out of KOI-314c.

But the American Astronomical Society meeting in Washington DC continues, and it’s clear that the technique of studying transit timing variations (TTV) is coming into its own as a tool for exoplanet investigation. David Kipping and colleagues use TTV to look for exomoons, and it was during such a search that they discovered KOI-314c. But consider the other AAS news. At Northwestern University, Yoram Lithwick has been measuring the masses of approximately sixty exoplanets larger than the Earth and smaller than Neptune.

Learn the mass and the size of a planet, and you can make a call on its density, and thus learn something about its probable composition. And guess what?

“We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas,” said Lithwick. “This indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star. By contrast, Earth is thought to have formed much later, after the gas disk disappeared.”

That resonates nicely with Kipping and company’s work on KOI-314c, and Lithwick, working with graduate student Sam Hadden, used transit timing variation to achieve his results. Among the duo’s sample, planets two to three times larger than the Earth have very low density (compare with KOI-314c, which turned out to be only thirty percent denser than water). These are worlds something like Neptune except smaller and covered in massive amounts of gas.

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Image: Chart of Kepler planet candidates as of January 2014. Credit: NASA Ames.

Transit timing variations occur when two planets orbiting the same star pull on each other gravitationally, so that the exact time of transit for each planet is affected. These are complicated interactions, to be sure, but we’re beginning to see radial velocity measurements confirming trends that have been originally uncovered with TTV. I ran this by David Kipping, asking whether TTV wasn’t coming into its own, and he agreed. “My bet is that when we measure the mass of Earth 2.0,” Kipping wrote, “it will be via TTVs.”

We can also look at the work of Ji-Wei Xie (University of Toronto), who used TTV to measure the masses of fifteen pairs of Kepler planets. These ranged in size from close to Earth to a little larger than Neptune, The results appeared in The Astrophysical Journal in December and were presented at the AAS meeting. The work complements reports from the Kepler team at AAS presenting mass measurements of worlds between Earth and Neptune in size. Here the follow-up used for the Kepler findings was based on Doppler measurements. In fact, six of the planets under investigation are non-transiting and seen only in Doppler data.

So we’re seeing both radial velocity and TTV used to study this interesting category of planets. 41 planets discovered by Kepler were validated by the program of ground-based observation, and the masses of sixteen of these were determined, allowing scientists to make the call on planetary density. In the Kepler study, ‘mini-Neptune’ planets with a rocky core show up with varying proportions of hydrogen, helium and hydrogen-rich molecules surrounding the core. The variation is dramatic, and some of these worlds show no gaseous envelope at all.

Kepler mission scientist Natalie Batalha sums up the questions all this raises:

“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. 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?”

Plenty of questions emerge from these findings, but the Kepler team’s report tells us that more than three-quarters of the planet candidates the mission has discovered have sizes between Earth and Neptune. Clearly this kind of planet, which is not found in our own Solar System, is a major player in the galactic population, and learning how such planets form and what they are made of will launch numerous further investigations. The usefulness of transit timing variations at determining mass will likely place the technique at the forefront of this ongoing work.

The paper by Ji-Wei Xie is “Transit Timing Variation of Near-Resonance Planetary Pairs: Confirmation of 12 Multiple-Planet Systems,” Astrophysical Journal Supplement Series Vol. 208, No. 2 (2013), 22 (abstract). I don’t have the citation for the Kepler report, about to be published in The Astrophysical Journal, but will run it as soon as I can. Yoram Lithwick’s presentation at AAS was based on Hadden & Lithwick, “Densities and Eccentricities of 163 Kepler Planets from Transit Time Variations,” to be published in The Astrophysical Journal and available as a preprint.

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{ 12 comments }

ljk January 7, 2014 at 11:44

Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds

Nicolas B. Cowan (Northwestern University), Dorian S. Abbot (University of Chicago)

(Submitted on 3 Jan 2014)

http://arxiv.org/abs/1401.0720

and…

Would continents not exist without life?

http://www.dvice.com/2014-1-6/life-itself-could-be-responsible-creation-continents

Rob Flores January 7, 2014 at 12:11

Yes, a plethra of Mini-N’s. But I shouldn’t there be a disclaimer.
They seem to orbit mostly M dwarfs. Do we have enough data
from Kepler regarding K and G stars to conclude that these larger
suns also trend toward this type of planet being common.
It could also be that by accident, Kepler was best suited to find
these type of worlds. The Kepler detection defecits haunt the various
conclusions on planet formation. Maybe papers based on Kepler observation should have a qualifier “the raw planetary data represents a subselection of all planetary bodies, with the subselection uncharaterized as to overall frequency of occurence’

Abelard Lindsey January 7, 2014 at 12:48

I expected this, that these “super-Earths” would turn out to be more Neptune than Earth-like in character.

Enzo January 7, 2014 at 15:13

Paul,

Do you know what blue and yellow indicates in the graph about Kepler’s planets above ?

Also, see this article from Nov in the Systemic blog about the huge variation in densities for super-earths :
http://oklo.org/2013/11/16/all-over-the-map/

andy January 7, 2014 at 15:16

Playing around not so seriously with some what-ifs:

I am really starting to wonder whether the key to our solar system’s apparent weirdness is the “Grand Tack”. Maybe smashing up the inner system with an in-then-out migration of the gas giants might help getting terrestrials on hundred-day orbits — could the time it takes to do the migration have given the young Sun that little bit of extra time to clear the inner solar system of most of its gas supply?

If so, this could be bad news for finding “Earth 2″. If habitable terrestrials form in “clobbered” systems which start with less mass than usual after their own versions of the “Grand Tack”, they may be less likely to be located sufficiently close to another planet to have detectable TTV for mass estimation (and such systems would have had their inclinations excited by the gas giants, making multiple transiting planets less likely). RV would also suffer from having to model out the long-period gas giants before getting to the low-amplitude signals from the terrestrial planets.

On the up side, it may be that finding analogues of the Jupiter/Saturn pair may help target the search for suitable systems. Wonder how feasible a campaign on 47 UMa would be?

Incidentally this scenario appears to play nicely with the Copernican and anthropic principles: if habitable planets are all over the place, we have ended up with an unusually massive star, plus we’ve got Jupiter (Jupiter-analogues seem to be fairly rare) and an inner system that seems to be rather widely-spaced and low in mass compared to the norm. If “Grand Tacks” are required for habitable terrestrials, then the low rates of gas giant formation around the more common M-dwarfs make them unfavourable hosts. Also the apparently typical systems with lots of low-mass planets either build mini-Neptunes or planets with too low a mass to sustain the plate tectonics necessary to maintain habitable conditions for long enough that intelligence would arise to observe itself living on such a world.

Again, not sure how much I believe this idea, but it’s fun to speculate…

Paul Gilster January 7, 2014 at 15:58

Enzo writes:

Do you know what blue and yellow indicates in the graph about Kepler’s planets above ?

Enzo, I pulled the image from this NASA news release:

http://www.nasa.gov/ames/kepler/nasa-kepler-provides-insight-about-enigmatic-but-ubiquitous-planets-five-new-rocky-planets/#.Usxb-ZBDuhM

but no legend is provided there. Let me dig around for the answer.

P January 7, 2014 at 19:05

I think that the blue represents candidates from previous data releases and the yellow candidates are from the latest data release?

Hence the slow drift to less massive candidates and longer periods as multiple transits reduce the noise?

P

Ashley Baldwin January 7, 2014 at 19:14

Without knowing the class of the parent stars ( and ages) for these planets its premature to draw conclusions . Due to lower mass in the initial interstellar gas cloud , smaller stars and particularly M class ,maintain an accretion disc for far longer than larger stars ( and indeed long before the star forms) and fire up hydrogen fusion later for the same reason to finally enter the main sequence . This is atypical of stars as a whole ( the Sun took 30 million years, O stars a few million but M class can take hundreds of millions or longer to do this) with planets atypically forming before the parent star . This must obviously alters their content and particularly that of lower density material such as volatiles , hydrogen and helium being the obvious examples. Lower metallicity in this older population of stars must also play a role. Further still before fusing hydrogen proper , M dwarfs fuse their limited deuterium ( and lithium) supply first, a reaction which is highly temperature sensitive ( 700 k kelvins) can last tens of millions of years and helps delay hydrogen fusion . All of this is critical to planet formation as apart from turning a protostar into a star , main sequence entry delivers the XUV that will ultimately, and very quickly, blow away the remaining accretion disc thereby ending planetary formation ( just see the pictures of the way proplyds get blasted in the many lovely pictures of the “big star” rich birthing grounds of the Orion nebular) . This especially for the volatile elements that could contribute to the lower density planets seen in this study .( it also makes early differentiation of lower mass M dwarfs and L/T brown dwarfs hard ). A high percentage of M dwarfs in the sample could give this picture and be one explanation of the the findings .

Paul Gilster January 7, 2014 at 21:11

Enzo, this just in from NASA’s Michele Johnson:

“The blue data points are the planet candidates through Q8 which were observations made May 2009 through March 2011. The yellow data points are the new candidates through Q12 which were observations made through March 2012.”

Ronald January 10, 2014 at 7:04

Rob Flores and Ashley Baldwin, with regard to M dwarfs in the sample: I am not sure about this, but I think that Kepler is specifically intended to observe a relatively large number (and fraction) of roughly solar type (F, G, K) stars.

Andy, again I tend to agree with you. As I commented under the previous and the following posts, the question is whether and to what extent these ‘secondary’ terrestrials (gas envelope eroded Neptunes) are terrestrial and habitable.

What we see in this very important graph is that, as time goes by, more and more small planets are discovered, the population is moving to the bottom right.
However, what worries me is that the very outskirts at the bottom right, earthsized planets in earthlike orbits, remains conspicuously empty.
Now, I do understand, that this is to a large degree observational bias.
However, I would expect that by now a greater number in that corner would be within detection limits. For instance, there are the two lonely blue dots around 200 days and just below Re. And yet those outskirts remain almost empty.
Is this still observational bias, or is there a real drop-off in earthlike planets beyond a certain orbital distance, as suggested by a few recent studies?

ljk January 15, 2014 at 11:53

First Planet Found Around Solar Twin in Star Cluster

Six-year search with HARPS finds three new planets in Messier 67

15 January 2014

Planets orbiting stars outside the Solar System are now known to be very common. These exoplanets have been found orbiting stars of widely varied ages and chemical compositions and are scattered across the sky. But, up to now, very few planets have been found inside star clusters [1]. This is particularly odd as it is known that most stars are born in such clusters. Astronomers have wondered if there might be something different about planet formation in star clusters to explain this strange paucity.

Anna Brucalassi (Max Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the new study, and her team wanted to find out more. “In the Messier 67 star cluster the stars are all about the same age and composition as the Sun. This makes it a perfect laboratory to study how many planets form in such a crowded environment, and whether they form mostly around more massive or less massive stars.”

The team used the HARPS planet-finding instrument on ESO’s 3.6-metre telescope at the La Silla Observatory. These results were supplemented with observations from several other observatories around the world [2]. They carefully monitored 88 selected stars in Messier 67 [3] over a period of six years to look for the tiny telltale motions of the stars towards and away from Earth that reveal the presence of orbiting planets.

This cluster lies about 2500 light-years away in the constellation of Cancer (The Crab) and contains about 500 stars. Many of the cluster stars are fainter than those normally targeted for exoplanet searches and trying to detect the weak signal from possible planets pushed HARPS to the limit.

Three planets were discovered, two orbiting stars similar to the Sun and one orbiting a more massive and evolved red giant star. The first two planets both have about one third the mass of Jupiter and orbit their host stars in seven and five days respectively. The third planet takes 122 days to orbit its host and is more massive than Jupiter [4].

The first of these planets proved to be orbiting a remarkable star — it is one of the most similar solar twins identified so far and is almost identical to the Sun (eso1337) [5]. It is the first solar twin in a cluster that has been found to have a planet.

Two of the three planets are “hot Jupiters” — planets comparable to Jupiter in size, but much closer to their parent stars and hence much hotter. All three are closer to their host stars than the habitable zone where liquid water could exist.

“These new results show that planets in open star clusters are about as common as they are around isolated stars — but they are not easy to detect,” adds Luca Pasquini (ESO, Garching, Germany), co-author of the new paper [6]. “The new results are in contrast to earlier work that failed to find cluster planets, but agrees with some other more recent observations. We are continuing to observe this cluster to find how stars with and without planets differ in mass and chemical makeup.”

Full article here:

http://www.eso.org/public/news/eso1402/

andy January 16, 2014 at 18:29

This is probably relevant to the discussion of mini-Neptunes:

Lammer et al., arXiv 2014 “ Origin and Loss of nebula-captured hydrogen envelopes from “sub”- to “super-Earths” in the habitable zone of Sun-like stars

As a result of considerations of accretion and loss of hydrogen-atmospheres, they suggest that habitable planets are in the range 1±0.5 Earth masses and radii 0.8–1.15 times that of Earth. They are rather scathing of the claims that the Kepler-62 planets (1.6 and 1.4 Earth radii for planets e and f respectively) are plausible candidates for being habitable.

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