Kepler-80: Analysis of a Compact System

It’s been a week for unusual planetary systems, and I’ll cap it off with Kepler-80, a star about 1100 light years away that features five planets in extraordinarily tight orbits. Such systems are now being referred to as STIPs (Systems with Tightly-spaced Planets), a nod to our apparently imperishable drive to create acronyms. Whatever we call them, though, systems like these make us realize that our own Solar System’s configuration is but one possibility in a sea of other outcomes. Yesterday’s post on ‘warm Jupiters’ is yet another confirmation of the thought.

What we have in new work from Mariah MacDonald, Darin Ragozzine (Florida Institute of Technology) and colleagues is an analysis of transit timing variations (TTVs) of the planets around this star, all of which orbit inside 1/10 AU. Here the planets’ years are 1.0, 3.1, 4.6, 7.1 and 9.5 days, respectively, close enough that gravitational perturbations can create slight changes in transit times. Although the innermost planet has a very weak TTV signal, the other four show signals strong enough for the researchers to work out the masses of each.

Gravitational interactions that disturb a perfectly periodic sequence of transits are a valuable way of making mass estimates for planets small enough that radial velocity detections are difficult. Usefully, Kepler has measured hundreds of TTV signals allowing for such estimates. They’re particularly helpful in multiple-planet transiting systems because now we can use the combination of mass and planetary radius to produce density measurements.

The Kepler-80 planets are f, d, e, b, and c in order of period. The inferred masses for the four outer planets are roughly 6.75, 4.13, 6.93 and 6.74 Earth masses, but we learn that the two outermost planets are almost twice as large as the inner two. The researchers believe this is consistent with terrestrial compositions for d and e and extended, puffy atmospheres of hydrogen and helium for b and c. Here’s how the paper describes these worlds:

Although all four planets have very similar masses, planets d and e are terrestrial and planets b and c have ?2% (by mass) H/He envelopes assuming Earth-like cores. Their orbits are similar and models suggest that photo-evaporation would have removed ?1% H/He from all four planets. Though simulations suggest the system has been affected by planetary tides, we did not consider the effect of dissipation on the atmospheric history of the planets. It is unusual to have four well-measured densities in the same system and future comparative planetology may constrain the formation and evolution of their atmospheres.

Due to orbital resonances, the four outer planets are synchronized, returning to the same configuration every 27 days. The paper notes that Kepler-80’s planetary orbits are stable in the long-term as long as we assume orbital eccentricities below about 0.2 (the researchers point out that TTVs cannot reliably detect eccentricities for this system). Although the available Kepler data are not enough to reveal the evolution of the atmospheres on these planets, the researchers’ simulations show that the outer two planets could have migrated inward from original positions in the disk where accretion of hydrogen and helium would be more likely to occur.


Image: This animation shows the position of the five planets of Kepler-80 whenever the outer two planets (green and red) pass by one another, about every 27 days over the course of four years of observations by NASA’s Kepler Space Telescope. Due to the rare synchronized nature of the system, the middle two planets (blue and purple) also return to almost exactly the same location. The innermost planet (yellow) is not synchronized and hence is found at a random location every 27 days. MacDonald et al. 2016 were able to show that this pattern indicates formation by “migration,” where the orbits shrink very slightly over time. The orbits are to scale with each other, but the planets are shown 50 times larger. The outer four planets are all about 4-6 times the mass of the Earth. The inner three planets (blue, purple, and yellow) appear rocky and the outer two planets (green and red) are likely rocky with a very puffy Hydrogen/Helium atmosphere. Credit: MacDonald/Ragozzine/FIT.

Improved mass and eccentricity estimates will fall to future space-based observatories. With its complex resonances and intriguing dynamical history, Kepler-80 should be a useful laboratory for studying planet formation. The Kepler mission has given us a wealth of information about how planetary systems can be built, and it’s clear that their formation and evolution will be the subject of study for decades. The systems we’ve looked at this week hint at what is possible as exoplanetary architectures continue to surprise us.

The paper is MacDonald et al., “A Dynamical Analysis of the Kepler-80 System of Five Transiting Planets,” accepted at The Astronomical Journal. A Florida Institute of Technology news release is available.


‘Warm Jupiters’ and Nearby Worlds

Where exactly do ‘hot Jupiters’ come from? I usually see explanations involving planetary migration for Jupiter-class objects with tight orbital periods of 10 days or less, the thinking being that such planets are too close to their host stars to have accumulated a Jovian-style gaseous envelope there. Migration explains their placement, with gas giants forming much further out in their planetary systems and then migrating disruptively inward to become hot Jupiters.

Does the scenario work? Consider the hot Jupiter WASP-47b, which has two low-mass planets nearby in its system. WASP-47b is a problem because a migrating gas giant should have produced profound gravitational issues for small worlds in the inner system, likely ejecting them entirely. A new paper from Chelsea Huang and Yanqin Wu (University of Toronto), working with Amaury Triaud (University of Cambridge), tries to explain the dilemma posed by WASP-47b.

The answer turns out to be that, according to Kepler data used by the researchers, systems in which true hot Jupiters have nearby companions are extremely rare. A sample of 45 hot Jupiters (28 of them confirmed) found none with small companions in nearby orbits either closer to the star or more distant. This tends to confirm that these planets migrated to their current orbits, with expected results for the inner system. WASP-47b remains a prominent and problematic outlier.

But here we have to be careful because Huang and company make a crucial distinction between ‘hot Jupiters’ (orbital periods of ten days or less) and ‘warm Jupiters,’ whose orbital periods range from ten days to 200. The paper describes the latter category this way:

…we refer specifically to those giant planets orbiting between 10 days and 200 days in period. Unlike the hot Jupiters (inward of 10 days), they are too far out to have experienced little if any tidal circularization and therefore may be difficult to migrate inward by mechanisms that invoke high-eccentricity excitation. On the other hand, they live inward of the sharp rise of giant planets outside ? 1AU – in fact, the period range of warm Jupiters corresponds to the so-called ’period-valley’, the observed dip in occupation in-between the hot Jupiters and cold Jupiters…


Image: An artist’s portrayal of a Warm Jupiter gas-giant planet in orbit around its parent star, along with smaller companion planets. Credit: Detlev Van Ravenswaay/Science Photo Library.

Warm Jupiters present an entirely different picture than their hot, inner system cousins. In fact, among the researchers’ warm Jupiter sample (27 planets, 12 confirmed), 11 are found to have nearby worlds ranging in size from Earth to Neptune. Most of these companions are inner planets, which is interesting in itself, because outer planets would be less likely to make an observable transit. Hence the data point to outer planets being as common as inner ones. Formation in place seems likely here, a clear distinction between the warm and hot Jupiters:

Motivated by this discovery, and by recent theoretical progress in understanding gas accretion, we propose that a significant fraction of warm Jupiters are formed in situ. The prevalence of multiple low-mass planets in close proximity to one another and to the star can, in a fraction of the cases, permit some of the planets to accrete enough envelope and to trigger run-away growth. This process can operate in the warm Jupiter locale, but appears to become increasingly difficult towards the hot Jupiter region, explaining the rarity of systems like WASP-47b.

Huang speculates that the number of warm Jupiters with small neighboring worlds may encompass half of all such planets, with formation in situ becoming increasingly difficult for closer-in worlds. In this analysis, then, WASP-47b simply becomes the ‘hottest representative of the warm Jupiter population.’ We wind up with hot Jupiters being the result of violent dynamical processes that effectively eliminate (by ejection) nearby inner planets, while those warm Jupiters that form in place are much more benign neighbors and, we can add, interesting places to look for possible moons with habitable conditions on the surface.

Where next with this research? The paper suggests close monitoring of confirmed warm Jupiter systems in hopes of discovering smaller companion worlds. The masses of such planets, inner or outer, could be an interesting clue to the critical mass above which runaway gas accretion occurs. We also need more information about the warm Jupiter population to find out whether there is a second formation process that distinguishes two classes of such worlds.

The paper is Huang, Wu and Triaud, “Warm Jupiters are less lonely than hot Jupiters: close neighbours,” Astrophysical Journal Vol. 825, No. 2 (2016). Abstract / preprint.


A Deeper Look at TRAPPIST-1

Small red stars are drawing increased attention as we continue to discover interesting planets around them. The past two days we’ve looked at the four worlds around K2-72, a red dwarf about 225 light years out in the constellation Aquarius. That two of these worlds have at least the potential for liquid water on the surface makes the system a prime target for further study. Now we return to another recently discussed system of note, TRAPPIST-1.

Designated 2MASS J23062928-0502285, this ultracool dwarf is also in Aquarius, though at forty light years, much the closer target. As with K2-72, we have multiple planets here (three), and also like the K2 discovery, TRAPPIST-1 orbits a star small and dim enough to make planet detection easier — a transiting world presents a clear signature and the study of planetary atmospheres is possible through what is known as transmission spectroscopy, wherein light from the star that has passed through the planet’s atmosphere is analyzed.

Today we have a paper in Nature from an international team including Michaël Gillon (University of Liège) and Julien de Wit (MIT), who have been tightly focused on TRAPPIST-1 for some time. TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm robotic instrument operated out of Liège, Belgium but sited at the European Southern Observatory’s La Silla Observatory in Chile. The instrument has been studying 70 nearby dwarfs at infrared wavelengths, uncovering the TRAPPIST-1 planets with orbital periods of 1.5 and 2.4 days and an outer world with period not yet well determined.

It was Gillon and de Wit who announced the discovery of the planetary system around TRAPPIST-1 on May 2. The work received a bit of buzz because although the two inner planets are too close to the star to be in the habitable zone, a tidally locked world in these orbits could have regions near the terminator where liquid water could exist. To probe further, the researchers studied data from the Spitzer Space Telescope, allowing them to refine the planetary orbits. At this point, they realized a double transit was in the offing.

Moreover, the event was in a scant two weeks, making for frenzied work, as de Wit explains:

“We thought, maybe we could see if people at Hubble would give us time to do this observation, so we wrote the proposal in less than 24 hours, sent it out, and it was reviewed immediately. Now for the first time we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time.”

The result: A combined transmission spectrum of TRAPPIST-1b and c, meaning the team could analyze the atmospheres of both worlds as the transit occurred. The transmission spectrum was featureless, the data sufficient to show that both transiting planets have relatively compact atmospheres rather than large, gaseous envelopes like Jupiter and Saturn. That would imply rocky planets like the terrestrial worlds — Mars, Earth, Venus — in our own Solar System.


Image: Comparison between the Sun and the ultracool dwarf star TRAPPIST-1. Credit: ESO.

That’s a useful insight because we have no other information about the nature of these planets. Their masses have not been measured, and we have no other data about the kind of planets that can exist around ultracool dwarf stars (TRAPPIST-1 is an M8 dwarf) because the TRAPPIST-1 worlds are our first transiting example.

The excerpt below shows the team’s reasoning, building on the fact that the lack of features in the combined spectrum rules out certain kinds of atmospheres:

…the first observations of TRAPPIST-1’s planets with HST allow us to rule out a cloud-free hydrogen-dominated atmosphere for either planet. If the planets’ atmospheres are hydrogen-dominated, then they must contain clouds or hazes that are grey absorbers between 1.1 μm and 1.7 μm at pressures less than around 10 mbar. However, theoretical investigations for hydrogen-dominated atmospheres predict that the efficiencies of haze and cloud formation at the irradiation levels of TRAPPIST-1b and TRAPPIST-1c should be dramatically reduced compared with, for example, the efficiencies for GJ 1214b… In short, hydrogen-dominated atmospheres can be considered as unlikely for TRAPPIST-1b and TRAPPIST-1c.


Image: The binary transit visualized. Credit: NASA/ESA/STScl.

With an extended gas envelope ruled out, we wind up with a range of possible atmospheres, ranging from the CO2-dominated Venus to an Earth-like atmosphere with heavy clouds or a depleted atmosphere like what we see on Mars. To push further into the possibilities, the team has formed a consortium called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), the good news being that they are building larger versions of the TRAPPIST instrument in Chile that will focus on the brightest ultracool dwarf stars in the southern hemisphere. Consider the effort an attempt to build the kind of pre-screening tools that our future space telescopes like the James Webb instrument will need for their target list.

The paper is de Wit et al., “A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c,” Nature 20 July 2016 (preprint). The discovery paper is Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star,” published online in Nature 2 May 2016 (abstract). An MIT news release is available.


Ravi Kopparapu: Looking at K2-72

Is the K2-72 system, discussed yesterday as part of a recent exoplanet announcement from Ian Crossfield and colleagues, as intriguing as it looks? Ravi Kopparapu has some thoughts on the matter. Dr. Kopparapu’s work on exoplanet habitability is well known to Centauri Dreams readers — he offered an overview in these pages called How Common Are Potential Habitable Worlds in Our Galaxy?, which ran in 2014. An assistant research scientist at NASA GSFC and the University of Maryland, Dr. Kopparapu began his exoplanet career with James Kasting at Penn State following work on the LIGO collaboration enroute to his PhD from Louisiana State. Analyzing habitable zone possibilities around different kind of stars, as well as modeling and characterizing exoplanet atmospheres, plays a major role in his research interests. I was pleased to receive the following note on the recently announced K2-72 system and want to run his thoughts today given the interest this unusual system has already begun to generate.

By Ravi Kumar Kopparapu


Having read your article on new K2 planet discoveries on Centauri Dreams (see Intriguing System in New Exoplanet Haul), I was interested to go back to Crossfield’s paper to look more carefully at the data tables given at the end of their paper. I found couple of interesting things, which I am sure the team must have noticed too.

1. The paper, and your article, mentions a system with four potentially rocky worlds (I am using ‘potentially’ here because I am hoping there may be more refined measurements of the stellar radius eventually), and “The irradiation levels for several planets are also quite consistent with Earth’s insolation.” This system is listed as K2-72 in Ian’s data tables.

I looked at this table, and found that out of the four, two of them (K2-72c and K2-72e) can be considered to be in the Habitable Zone (HZ) of the host star. The habitable zone limits are from my climate model calculations. [See citations at the end of this post].

Particularly, these two planets are very close in size with each other….just like Earth and Venus. I think K2-72e is most definitely in the HZ (incident flux = 0.76 Earth flux), while K2-72c is receiving about twice (~1.41 Earth flux) the stellar flux as K2-72e. So, I think the ‘e’ and ‘c’ planets are like Earth and Venus, respectively, in our Solar system. (Venus receives about twice the Earth flux).

Now, there are some 3-D climate model results, including some from our group, that keep the planet ‘c’ comfortably within the HZ, if that planet is covered with oceans. In that case, the K2-72 system would have two potential habitable planets (please note my stress on ‘potential’). We do not know the water content of planet ‘c’, so we can not make any definitive statements. So, to be on the safe side, let’s assume there is an Earth-Venus twin in the K2-72 system.

Screenshot from 2016-07-19 08:48:31

Image: Photometry of K2-72 (EPIC 206209135), which hosts four transiting planets. Top: Full time series with colored tick marks indicating each individual transit time. Bottom: Phase-folded photometry with the color-coded, best-fit transit model overplotted for each planet. Credit: Crossfield et al.

2. There is another interesting system that also got my attention: K2-3d and K2-3c. These planets are nearly Earth-size, and as with the K2-72 system, they are also very similar in size with each other….as are Earth & Venus in our Solar system. What’s more, the stellar flux incident on these planets also varies by a factor of two between each other (0.8 Earth flux for K2-3d, and 1.77 Earth flux for K2-3c)…just as Earth & Venus!

The similarities of these systems with Earth and Venus based only on size and incident flux (which is the only thing we can measure now with transit photometry) are astonishing. These two systems would be excellent candidates for follow-up characterization campaigns depending upon how bright are the host stars. It is amazing that within the bounty of planets from this data, there are already two systems VERY close to Earth-Venus similarities.

For more on Dr. Kopparapu’s habitable zone calculations, see Kopparapu et al., “Habitable Zones Around Main-Sequence Stars: New Estimates,” Astrophysical Journal, 765 (2013), 131 (abstract). See also Kopparapu et al, “Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass” Astrophysical Journal Letters, 787 (2014), L29 (abstract).


Intriguing System in New Exoplanet Haul

Today’s announcement of the confirmation of over 100 planets using K2 data reminds me of how much has gone into making K2 a success. You’ll recall that K2 emerged when the Kepler spacecraft lost function in two of its four reaction wheels. Three of these were needed for pointing accuracy, but ingenious pointing techniques and software updates have made K2 into a potent project of its own. The latest announcements demonstrate that certain benefits emerged from the changed mission parameters, especially in the ability of K2 to move away from the original field of view (toward Cygnus and Lyra) and focus on targets in the ecliptic plane.

What we gain from that change is that working in the ecliptic allows more chances for observation from ground-based observatories in both northern and southern hemispheres as they perform the needed exoplanet follow-up. But there are other factors that make K2 potent. With all targets being chosen by the entire scientific community (not limited to the original science team members), we’re drilling down into smaller red dwarf stars. Thus Ian Crossfield (University of Arizona), who is behind the latest tranche of exoplanet discoveries:

“Kepler’s original mission observed a small patch of sky as it was designed to conduct a demographic survey of the different types of planets. This approach effectively meant that relatively few of the brightest, closest red dwarfs were included in Kepler’s survey. The K2 mission allows us to increase the number of small, red stars by a factor of 20 for further study.”

The paper on the new announcement elaborates on the Kepler/K2 distinction:

K2 observes a qualitatively different stellar population than Kepler, namely a much larger fraction of late-type stars [i.e., K and M-class]… Stellar parameters for these late-type systems derived from photometry alone are relatively uncertain, and follow-up spectroscopy is underway to characterize these stars… In addition to the difference in median spectral type, K2 also surveys a much broader range of Galactic environments than was observed in the main Kepler mission. These two factors suggest that, once K2 ’s detection efficiency is improved and quantified, the mission’s data could address new questions about the intrinsic frequency of planets around these different stellar populations.

That factor of 20 increase in small red stars is paying off handsomely. We now have 104 newly confirmed planets, among them a planetary system containing four interesting potentially rocky worlds. Although all four of these planets orbit within the distance of Mercury’s orbit around the Sun, the star itself is an M-dwarf less than half the Sun’s size. The planetary orbital periods go from 5.58 days to 24 days, and according to the paper on this work, “The irradiation levels for several planets are also quite consistent with Earth’s insolation.”

The host of the four planets is the M-dwarf K2-72, all four of whose planets have been validated. The orbital periods here are 5.58, 7.76, 15.19, and 24.16 days, with the authors noting that planets c and d orbit near a first-order 2:1 mean motion resonance, or MMR (in a first-order resonance, the integers in the ratio differ by one), while b and c orbit near a second-order 7:5 MMR. Planetary radii are in the range of 1.2–1.5 R? for all planets.

All of this is exciting news, though we still have challenges in future observation. The star is faint enough to make Doppler or transit spectroscopy observations, needed to measure planetary mass or perform atmospheric analysis, difficult. It may be that transit timing variations will be helpful in analyzing the masses and bulk densities of these worlds.


Image: A montage showing the Mauna Kea Observatories, Kepler Space Telescope, and night sky with K2 Fields and discovered planetary systems (dots) overlaid. An international team of scientists discovered more than 100 planets based on images from Kepler operating in the K2 Mission. The team confirmed and characterized the planets using a suite of telescopes worldwide, including four on Mauna Kea (the twin telescopes of Keck Observatory, the Gemini-North Telescope, and the Infrared Telescope Facility). The planet image on the right is an artist’s impression of a representative planet. Credit: Karen Teramura/IFA; Miloslav Druckmüller/NASA.

Of the 104 planets, 64 are validated in this paper for the first time, and we still have another 63 remaining planet candidates. The paper tells us that the new discoveries include 37 planets smaller than two Earth radii (2R?), and several multi-planet systems. The complete list of these worlds is found in the research paper cited below, which points out that K2 may be able to double or triple the number of small planets detected around nearby stars. 500 ? 1000 planets are likely to be discovered in K2’s planned four-year mission.

That’s good news, of course, for future attempts to measure the composition of planetary atmospheres with the James Webb Space Telescope, to be launched in 2018, and it feeds excitement for the upcoming Transiting Exoplanet Survey Satellite (TESS) mission, due for launch next year.. We’re just getting a taste here of what TESS is likely to give us. From the paper:

The size of our validated-planet sample demonstrates yet again the power of high-precision time-series photometry to discover large numbers of new planets, even when obtained from the wobbly platform of K2. Since K2 represents a natural transition from the narrow-field, long-baseline Kepler mission to the nearly all-sky, mostly short-baseline TESS survey, the results of our K2 efforts bode well for the productivity of the upcoming TESS mission. The substantial numbers of intermediate-sized planets orbiting moderately bright stars discovered by our (and other) K2 surveys… will be of considerable interest for future follow-up characterization via radial velocity spectroscopy and JWST transit observations…

The paper is Crossfield et al., “197 Candidates and 104 Validated Planets in K2’s First Five Fields,” to be published in Astrophysical Journal Supplement Series (preprint). A Keck University news release is also available.