Reason for a correlation between Hot Jupiters and higher metallicity in Kepler data

Reason for a correlation between Hot Jupiters and higher metallicity in Kepler data

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This question is from an amateur data analysis I did in May 2015 of data from There were 80 Jupiter size planets in the Kepler "confirmed" planet table on this website (through 4/1/15) that had host stars with metallicity data. Of these, 24 stars have [Fe/H}>0.2 and 40 have -0.1<[Fe/H]<+0.2. This is a much higher percentage than the sample as a whole. Would this be strong evidence that high metallicity measurements are due to accretion? Or, do the metallicity measurements given reflect abundance deeper into the star than accretion can count for (ie. higher metallicity stars are more likely to have Jupiters in general)?

This is a well known, well researched phenomena. Yes, there certainly is a correlation between metallicity and the likelihood of observing a hot Jupiter.

There are two classes of explanation.

(1) The correlation is real and due to the fact that it is easier to form planetary embryos from metal-rich material in the core-accretion model of giant planet formation.

(2) The observed exoplanets are those that weren't swallowed. The hosts are metal rich because they did swallow others.

Finally, it was thought possible that there could be an observational bias. Planets are easier to find and measure around high metallicity stars. This applied mainly to the Doppler technique, because high metallicity stars have stronger spectral lines. Transit detection should be much less affected.

Most think that some form of (1) is going on. (2) is argued against because the correlation still appears in stars with a wide variety of convection zone depths into which planetary material would have mixed. I'm not sure it is dead though, there have been a number of claims for abundance enhancements that look like the product of planet engulfment.

I'll add some references later. Nice work though to discover the correlation if you knew nothing of it previously.

LAMOST telescope reveals that Neptunian cousins of hot Jupiters are mostly single offspring of stars that are rich in heavy elements

We discover a population of short-period, Neptune-size planets sharing key similarities with hot Jupiters: both populations are preferentially hosted by metal-rich stars, and both are preferentially found in Kepler systems with single-transiting planets. We use accurate Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 4 (DR4) stellar parameters for main-sequence stars to study the distributions of short-period [Formula: see text] Kepler planets as a function of host star metallicity. The radius distribution of planets around metal-rich stars is more "puffed up" compared with that around metal-poor hosts. In two period-radius regimes, planets preferentially reside around metal-rich stars, while there are hardly any planets around metal-poor stars. One is the well-known hot Jupiters, and the other one is a population of Neptune-size planets ([Formula: see text]), dubbed "Hoptunes." Also like hot Jupiters, Hoptunes occur more frequently in systems with single-transiting planets although the fraction of Hoptunes occurring in multiples is larger than that of hot Jupiters. About [Formula: see text] of solar-type stars host Hoptunes, and the frequencies of Hoptunes and hot Jupiters increase with consistent trends as a function of [Fe/H]. In the planet radius distribution, hot Jupiters and Hoptunes are separated by a "valley" at approximately Saturn size (in the range of [Formula: see text]), and this "hot-Saturn valley" represents approximately an order-of-magnitude decrease in planet frequency compared with hot Jupiters and Hoptunes. The empirical "kinship" between Hoptunes and hot Jupiters suggests likely common processes (migration and/or formation) responsible for their existence.

Keywords: exoplanets metallicity transit.

Conflict of interest statement

The authors declare no conflict of interest.


Period–radius distribution for short-period Kepler…

Period–radius distribution for short-period Kepler planet candidates hosted by metal-rich ( Top )…

Cumulative fractions as a function…

Cumulative fractions as a function of host [Fe/H]. Hoptunes (magenta) and hot Jupiters…

The hot-Saturn valley revealed from…

The hot-Saturn valley revealed from the radius distribution of planets in our sample.…

The dependence of planet distribution…

The dependence of planet distribution ( Left ) and intrinsic frequency ( Right…


We present evidence for a correlation between the observed properties of hot Jupiter emission spectra and the activity levels of the host stars measured using Ca II H and K emission lines. We find that planets with dayside emission spectra that are well-described by standard one-dimensional atmosphere models with water in absorption (HD 189733, TrES-1, TrES-3, WASP-4) orbit chromospherically active stars, while planets with emission spectra that are consistent with the presence of a strong high-altitude temperature inversion and water in emission orbit quieter stars. We estimate that active G and K stars have Lyman fluxes that are typically a factor of 4-7 times higher than quiet stars with analogous spectral types and propose that the increased UV flux received by planets orbiting active stars destroys the compounds responsible for the formation of the observed temperature inversions. In this paper, we also derive a model-independent method for differentiating between these two atmosphere types using the secondary eclipse depths measured in the 3.6 and 4.5 m bands on the Spitzer Space Telescope and argue that the observed correlation is independent of the inverted/non-inverted paradigm for classifying hot Jupiter atmospheres.

Title: The Metallicity Distribution and Hot Jupiter Rate of the Kepler Field: Hectochelle High-resolution Spectroscopy for 776 Kepler Target Stars

The occurrence rate of hot Jupiters from the Kepler transit survey is roughly half that of radial velocity surveys targeting solar neighborhood stars. One hypothesis to explain this difference is that the two surveys target stars with different stellar metallicity distributions. To test this hypothesis, we measure the metallicity distribution of the Kepler targets using the Hectochelle multi-fiber, high-resolution spectrograph. Limiting our spectroscopic analysis to 610 dwarf stars in our sample with logg > 3.5, we measure a metallicity distribution characterized by a mean of [M/H]=−0.045±0.009, in agreement with previous studies of the Kepler field target stars. In comparison, the metallicity distribution of the California Planet Search radial velocity sample has a mean of [M/H]=−0.005±0.006, and the samples come from different parent populations according to a Kolmogorov–Smirnov test. We refit the exponential relation between the fraction of stars hosting a close-in giant planet and the host star metallicity using a sample of dwarf stars from the California Planet Search with updated metallicities. The best-fit relation tells us that the difference in metallicity between the two samples is insufficient to explain the discrepant hot Jupiter occurrence rates the metallicity difference would need to be ≃0.2–0.3 dex for perfect agreement. We alsomore » show that (sub)giant contamination in the Kepler sample cannot reconcile the two occurrence calculations. We conclude that other factors, such as binary contamination and imperfect stellar properties, must also be at play. « less

2. Observations

We used Wide Field Camera-3 (WFC3) observations of five secondary eclipses of WASP-18b from the HST Treasury program GO-13467 (PI: J. Bean). WFC3 obtains low resolution slitless spectroscopy from 1.1 to 1.7 μm using the G141 grism (R = 130), as well as an image for wavelength calibration using the F140W filter. Grism observations were taken in the spatial scan mode (Deming et al. 2013) with a forward-reverse cadence (Kreidberg et al. 2014). The first three visits, taken between 2014 April–June, are single eclipse events. Visit 4, taken in 2014 August, contains two eclipses in an orbital phase curve, and we extract those eclipses and analyze them separately.

We also re-analyze two eclipse observations of WASP-18b taken in the 3.6 μm and 4.5 μm channels of the Spitzer Space Telescope's IRAC instrument (Program ID 60185). The 3.6 μm observation was performed on 2010 January 23, while the 4.5 μm observation was taken 2010 August 23. Both observations were taken using an exposure time of 0.36 s in the subarray mode, and were first analyzed in Maxted et al. (2013).

Reason for a correlation between Hot Jupiters and higher metallicity in Kepler data - Astronomy

Probing the connection between a star’s metallicity and the presence and properties of any associated planets offers an observational link between conditions during the epoch of planet formation and mature planetary systems. We explore this connection by analyzing the metallicities of Kepler target stars and the subset of stars found to host transiting planets. After correcting for survey incompleteness, we measure planet occurrence: the number of planets per 100 stars with a given metallicity M. Planet occurrence correlates with metallicity for some, but not all, planet sizes and orbital periods. For warm super-Earths having P = 10-100 days and P = 1.0-1.7 oplus , planet occurrence is nearly constant over metallicities spanning -0.4 to +0.4 dex. We find 20 warm super-Earths per 100 stars, regardless of metallicity. In contrast, the occurrence of warm sub-Neptunes ( P = 1.7-4.0 oplus ) doubles over that same metallicity interval, from 20 to 40 planets per 100 stars. We model the distribution of planets as propto <10>β M , where β characterizes the strength of any metallicity correlation. This correlation steepens with decreasing orbital period and increasing planet size. For warm super-Earths β = - <0.3>-0.2 +0.2 , while for hot Jupiters β = + <3.4>-0.8 +0.9 . High metallicities in protoplanetary disks may increase the mass of the largest rocky cores or the speed at which they are assembled, enhancing the production of planets larger than 1.7 oplus . The association between high metallicity and short-period planets may reflect disk density profiles that facilitate the inward migration of solids or higher rates of planet-planet scattering.

Deadly tides mean early exit for hot Jupiters

Bad news for planet hunters: most of the "hot Jupiters" that astronomers have been searching for in star clusters were likely destroyed long ago by their stars. In a paper accepted for publication by the Astrophysical Journal, John Debes and Brian Jackson of NASA's Goddard Space Flight Center in Greenbelt, Md., offer this new explanation for why no transiting planets (planets that pass in front of their stars and temporarily block some of the light) have been found yet in star clusters. The researchers also predict that the planet hunting being done by the Kepler mission is more likely to succeed in younger star clusters than older ones.

"Planets are elusive creatures," says Jackson, a NASA Postdoctoral Program fellow at Goddard, "and we found another reason that they're elusive."

When astronomers began to search for planets in star-packed globular clusters about 10 years ago, they hoped to find many new worlds. One survey of the cluster called 47 Tucanae (47 Tuc), for example, was expected to find at least a dozen planets among the roughly 34,000 candidate stars. "They looked at so many stars, people thought for sure they would find some planets," says Debes, a NASA Postdoctoral Program fellow at Goddard. "But they didn't."

More than 450 exoplanets (short for "extrasolar planets," or planets outside our solar system) have been found, but "most of them have been detected around single stars," Debes notes.

"Globular clusters turn out to be rough neighborhoods for planets," explains Jackson, "because there are lots of stars around to beat up on them and not much for them to eat." The high density of stars in these clusters means that planets can be kicked out of their solar systems by nearby stars. In addition, the globular clusters surveyed so far have been rather poor in metals (elements heavier than hydrogen and helium), which are the raw materials for making planets this is known as low metallicity.

Debes and Jackson propose that hot Jupiters -- large planets that are at least 3 to 4 times closer to their host stars than Mercury is to our sun -- are quickly destroyed. In these cramped orbits, the gravitational pull of the planet on the star can create a tide -- that is, a bulge -- on the star. As the planet orbits, the bulge on the star points a little bit behind the planet and essentially pulls against it this drag reduces the energy of the planet's orbit, and the planet moves a little closer to the star. Then the bulge on the star gets bigger and saps even more energy from the planet's orbit. This continues for billions of years until the planet crashes into the star or is torn apart by the star's gravity, according to Jackson's model of tidal orbital decay.

"The last moments for these planets can be pretty dramatic, as their atmospheres are ripped away by their stars' gravity," says Jackson. "It has even been suggested recently the hot Jupiter called WASP-12B is close enough to its star that it is currently being destroyed."

Debes and Jackson modeled what would have happened in 47 Tuc if the tidal effect were unleashed on hot Jupiters. They recreated the range of masses and sizes of the stars in that cluster and simulated a likely arrangement of planets. Then they let the stars' tides go to work on the close-in planets. The model predicted that so many of these planets would be destroyed, the survey would come up empty-handed. "Our model shows that you don't need to consider metallicity to explain the survey results," says Debes, "though this and other effects will also reduce the number of planets."

Ron Gilliland, who is at the Space Telescope Science Institute in Baltimore and participated in the 47 Tuc survey, says, "This analysis of tidal interactions of planets and their host stars provides another potentially good explanation -- in addition to the strong correlation between metallicity and the presence of planets -- of why we failed to detect exoplanets in 47 Tuc."

In general, Debes and Jackson's model predicts that one-third of the hot Jupiters will be destroyed by the time a cluster is a billion years old, which is still juvenile compared to our solar system (about 4-1/2 billion years old). 47 Tuc has recently been estimated to be more than 11 billion years old. At that age, the researchers expect more than 96% of the hot Jupiters to be gone.

The Kepler mission, which is searching for hot Jupiters and smaller, Earth-like planets, gives Debes and Jackson a good chance to test their model. Kepler will survey four open clusters -- groups of stars that are not as dense as globular clusters -- ranging from less than half a billion to nearly 8 billion years old, and all of the clusters have enough raw materials to form significant numbers of planets, Debes notes. If tidal orbital decay is occurring, Debes and Jackson predict, Kepler could find up to three times more Jupiter-sized planets in the youngest cluster than in the oldest one. (An exact number depends on the brightness of the stars, the planets' distance from the stars, and other conditions.)

"If we do find planets in those clusters with Kepler," says Gilliland, a Kepler co-investigator, "looking at the correlations with age and metallicity will be interesting for shaping our understanding of the formation of planets, as well as their continued existence after they are formed."

If the tidal orbital decay model proves right, Debes adds, planet hunting in clusters may become even harder. "The big, obvious planets may be gone, so we'll have to look for smaller, more distant planets," he explains. "That means we will have to look for a much longer time at large numbers of stars and use instruments that are sensitive enough to detect these fainter planets."

The Kepler mission is managed by NASA's Ames Research Center, Moffett Field, Calif., for the Science Mission Directorate at NASA Headquarters in Washington.

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Nuclear fusion in the deuterated cores of inflated hot Jupiters

Ouyed et al. (Astrophys. J. 501:367, 1998) proposed Deuterium (DD) fusion at the core-mantle interface of giant planets as a mechanism to explain their observed heat excess. But rather high interior temperatures ( (sim10^

mbox) ) and a stratified D layer are needed, making such a scenario unlikely. In this paper, we re-examine DD fusion, with the addition of screening effects pertinent to a deuterated core containing ice and some heavy elements. This alleviates the extreme temperature constraint and removes the requirement of a stratified D layer. As an application, we propose that, if their core temperatures are a few times (10^

mbox) and core composition is chemically inhomogeneous, the observed inflated size of some giant exoplanets (“hot Jupiters”) may be linked to screened DD fusion occurring deep in the interior. Application of an analytic evolution model suggests that the amount of inflation from this effect can be important if there is sufficient rock-ice in the core, making DD fusion an effective extra internal energy source for radius inflation. The mechanism of screened DD fusion, operating in the above temperature range, is generally consistent with the trend in radius anomaly with planetary equilibrium temperature (T_>) , and also depends on planetary mass. Although we do not consider the effect of incident stellar flux, we expect that a minimum level of irradiation is necessary to trigger core erosion and subsequent DD fusion inside the planet. Since DD fusion is quite sensitive to the screening potential inferred from laboratory experiments, observations of inflated hot Jupiters may help constrain screening effects in the cores of giant planets.


This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 336792. D.K.S., F.P. and N.N. acknowledge support from STFC consolidated grant ST/J0016/1. Support for this work was provided by NASA through grants under the HST-GO-12473 programme from the STScI. A.L.E., P.A.W. and A.V.M. acknowledge support from CNES and the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 ‘Exo-Atmos’. P.A.W. and H.W. acknowledge support from the UK Science and Technology Facilities Council (STFC). G.W.H. and M.H.W. acknowledge support from NASA, NSF, Tennessee State University and the State of Tennessee through its Centers of Excellence programme.

Extrasolar Planets: A Matter of Metallicity

Astronomers have discovered more than 130 planets orbiting nearby stars in our galaxy. Although the solar systems they have found are very different from ours, by studying the planets that have been found – their masses, their orbits and their stars – they are uncovering intriguing hints that our galaxy may be brimming with solar systems like our own.

According to Greg Laughlin, an assistant professor of astronomy and astrophysics at UC Santa Cruz, planet hunters can expect, over time, to find hundreds of nearby stars with Neptune-like planets circling them at about 5 AU. (One AU, or astronomical unit, is the distance between the sun and Earth. Jupiter orbits our sun at about 5 AU.) A solar system with a large planet at 5 AU, astronomers believe, is one in which a habitable terrestrial-sized planet could also safely exist.

Laughlin’s prediction comes from studying a characteristic of stars that, until a few years ago, few paid much attention to: metallicity. New stars form when vast clouds of interstellar dust and gas collapse. This dust and gas is mostly primordial hydrogen and helium, but it also contains a smattering of heavier elements, which astronomers call "metals" (even though non-astronomers don’t normally think of all of these elements as metals). The metallicity of a star tells you what portion of its material is made of metals.

And, says Laughlin, "the one true indicator of whether a star is likely to have a detectable giant planet is its metallicity." These hot Jupiters and eccentric Jupiters, as they are known, are the easiest types of planets to detect almost all the planets discovered to date are of these two types. And "the vast majority of extrasolar planets that are known so far are around metal-rich stars."

Scene from a moon orbiting the extra-solar planet in orbit around the star HD70642.
Credit:David A. Hardy, (c)

Here’s why. When a metal-rich interstellar cloud collapses, it forms a metal-rich star. According to the core-accretion theory, the dominant theory of planetary formation, this abundance of heavy material also enables large rocky planetary cores to form relatively quickly, within a few million years. Once these cores reach 10 Earth masses or more, they begin attracting hydrogen and helium gas from the collapsing cloud they become gas giants. How big these giants get depends on how much gas they attract.

But the hydrogen and helium don’t stick around forever. So timing is critical: only large rocky cores that form before the gas disappears become gas giants. Cores that grow too slowly – the lower the metallicity of the collapsing cloud, the more slowly the cores grow – can’t grab any gas. "If the disk lifetime is 4 million years and it takes you 5 million years to build a core, then you’re out of luck," says Laughlin. "But if you can get that core buildup time down to 2.5 million years, say, then there’s still plenty of gas available."

Both of these types of planets can be seen in our solar system. "The sun is a metal-rich star, but not dramatically so," Laughlin says. When our solar system was forming, there was enough heavy material around for Jupiter and Saturn to form their cores quickly. They got gas. Neptune and Uranus, however, didn’t make it to the starting gate.

There is a strong correlation between high solar metallicity and hot Jupiters. The picture is fuzzier, though, for eccentric Jupiters, planets with elongated elliptical orbits that have been found out to an average distance of about 3 AU from their stars. And it is fuzzier still for planets with orbits like Jupiter’s. Planets out at 5 AU take more than a decade to complete their trips around their stars astronomers have only begun to confirm their presence.

HD 28185 b is the first exoplanet discovered with a circular orbit within its star’s habitable zone. Credit: STScI Digitized Sky Survey

But Laughlin thinks he knows what to expect once all the data are in: lots of Neptune-mass planets, with some as massive as Saturn, in Jupiter-like orbits. Why Neptunes? Metallicity. The majority of the stars that U.S.-based planet hunters are studying have a bit more than half the metallicity of the sun. That’s enough to form a large rocky planet like Neptune. There’s no time limit on Neptunes. But it’s not enough to form a core quickly it’s not enough to become a gas giant.

So what are the prospects of finding solar systems that contain Earth-like planets? Pretty good, according to Laughlin. The solar systems that have been found so far, the ones that contain hot Jupiters or eccentric Jupiters, probably don’t contain habitable Earth-like planets. The motions of these closer-in giants prevent terrestrial planets from forming stable orbits in the habitable zone. But a solar system with a large planet in a circular orbit at 5 AU – even a Neptune-sized planet – is a solar system in which a habitable Earth-like planet could exist quite comfortably.

Indeed, Laughlin believes that, when all the data are in, we’ll have discovered hundreds of nearby stars with solar systems much like our own, although the majority of them will have a Neptune or a Saturn at 5 AU rather than a Jupiter. True, planet hunters haven’t found any such planets yet. But that doesn’t mean they’re not there. Astronomers just haven’t been looking long enough to confirm their presence. With current planet-hunting techniques, Laughlin says, "it’s not like you discover a planet – boom!" – in a single observation. "The planets emerge gradually," as a result of many, many observations over time.

So just how long will it take to find such worlds? Well, that’s the unfortunate part of the story. Although astronomers have already begun to detect large planets in Jupiter-like orbits, it will take another 10 to 20 years to complete the census of planets orbiting at 5 AU around nearby stars. "The amount of patience that you have to exercise to get a true Jupiter analog is really enormously more than the amount of patience that you need to find and detect a hot Jupiter or an eccentric giant," Laughlin says. But considering that 10 years ago no-one knew for sure whether there was even a single planet around a star other than our sun, perhaps another 10 or 20 years isn’t such a long time to wait.