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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).


Comments on this entry are closed.

  • Harry R Ray July 19, 2016, 9:47

    Dr Kopparapu: I derive from the graphs(correct me if I am wrong), a radius of 1.5 Earth radii for K2-72c and 1.35 Earth radii for K2-72e, so it appears to be a Venus-Earth in reverse. A recent paper by Kipping and Chen argues that both if these planets would be a lot more like Neptune than Earth, but the 1.5 Earth radii K2-3d appears*(more on this later) to have a mass of 8-13 Earth mass(with 11 Earth mass being the most likely value*). Radial velocity studies of the K2-72 system will me much more difficult than for K2-3 because it is at a greater distance and dimmer, but TESS may be able to detect TTV’s in it. As for K2-3d(only, NOT K2-3b and K2-3c) the radial velocity derived mass is somewhat uncertain because its orbital period is relatively close to the stars’ rotation rate, but continued RV observations of Gamma Cephi eventually disentangled the planet’s mass from the star’s rotation rate, and I assume that RV observations with ESPRESSO will do the same for K2-3d. If K2-3d DOES turn out to be a Mega-Earth instead of a Sub-Neptune, internal heat ADDED to the 0.8 solar flux may make the surface too hot for life.

  • Michael July 19, 2016, 10:35

    I am suspecting both of these sets of worlds (K2-72e, c and K2-3d, c) to sit quite comfortably in the HZ of their stars. I would think that the inner edge of the M-type stars HZ would allow a higher light flux than larger massed stars as they generally have lower UV outputs in their light flux’s. UV can break down water allowing it to escape if the world is not massive enough to hold on to it leading to a greenhouse runaway process. But M-types have long contraction phases which could have limited the amount of water delivered to planet surfaces during their formation years. If so any outer worlds may offer more chances of subsurface life due to more water been delivered early on but are now frozen over due to their distances from their stars.


  • Andrew LePage July 19, 2016, 11:24

    I had done a “Habitable Planet Reality Check” on K2-3d (also known as EPIC 201367065) two months ago based on earlier results:


    The newer properties derived by Crossfield et al. indicate that the planets in this system are smaller and have much lower effective stellar fluxes than had been thought earlier. This certainly improves the prospects of K2-3d is a potentially habitable exoplanet.

    • Harry R Ray July 19, 2016, 15:44

      I had no luck getting a DIRECT revision for the radii of K2-3c and K2-3d. I had to go to Wickipedia to get thr REVISED MASS of K2-3d(7.50+ or – 3 earth mass), but THEY gave a radius of 1.61 Re for K2-3d, which is LARGER than the ORIGIONAL radius, and therefore, OBVIOUSLY WRONG! I am GUESSING HERE, but a drop in solar flux from 1.5 Earth flux to 0.8 Earth flux would mean a drop in radius from 1.5 Re to 1.15 Re(about the same as Kepler 186f), which would also mean that K2-3c’ REAL radius would be about 1.25 Re. If you or anyone else has the ACTUAL NEW RADII, please post them here. A revised DISTANCE based on the Gaia September 24 release should FINALIZE ALL OF THE PARAMETERS! ALSO: Will TESS be observing K2-3 sometime in its observational run? If so, it should ELIMINATE A LOT OF THE UNCERTANTY about the true value of the revised radii.

      • Andrew LePage July 20, 2016, 10:22

        Personally, I am still sorting through all the various new sources of info about the planets of the K2-3 system that have come out since I did my original analysis back in January to figure out where the various parameter values came from and which should be trusted more than others. There is even an important caveat about the new parameters in the Crossfield et al. paper where they readily admit that they are underestimating key stellar properties compared to more accurate spectroscopically-derived parameters. In addition, it appears that the mass for K2-3d found by others is poorly constrained because of stellar noise issues (not too surprising) so I wouldn’t put too much faith in those numbers. I do recall that TESS should be able to observe K2-3. These observations should tamp down the transit observation uncertainties and allow TTV measurements. But some newly derived stellar parameters (e.g. a Gaia distance), which appear to dominate the uncertainties in the derived planetary parameters, would help pin this down better including possibly a TTV-derived mass.

  • andy July 19, 2016, 13:52

    Out of interest, how much carbon dioxide would be needed to maintain liquid water on these planets? Would the levels be low enough to be non-toxic for human beings, or high enough to allow (oxygenic) photosynthesis?

    • Eniac July 19, 2016, 21:38

      No carbon dioxide would be needed for liquid water, but high levels should be expected nevertheless, consistent with what we observe on places other than Earth in our solar system. Toxic for humans, most likely, and more than enough for photosynthesis, if there were organisms (unlikely).

  • Ravi Kopparapu July 19, 2016, 15:14

    Harry Ray:

    According to Ian Crossfield’s paper (https://www.lpl.arizona.edu/~ianc/docs/crossfield_K2s_new_planets.pdf), if you look at the data tables provided at the end, on page 51, I see that the radii of all the planets in K2-72 are less than 1 EArth radius. For K2-3, the planets that are closest to Earth-Venus analog are planets K2-3c and K2-3d. The radii of the K2-3c is 1.18 Earth radius, and K2-3d is 0.96 Earth radius (see page 47). So they may be rocky if the radius measurements hold.

    • Harry R Ray July 20, 2016, 9:55

      Thanks for the update. Previously, I had just skimmed the PDF, and did not pay attention at all to page 47 because it appeared SIDEWAYS on the screen and I thought that it would be in agreement with the following: “Huber et al(2016)reports a stellar radius of 0.23, but notes this is likely an understatement. The weighted mean of our four stellar density measurements is 9.0 +/- 3.69gcm3; using the mass reduction of Moldonado et al(2015)implies a stellar radius of 0.40+0.12 -0.07Rs, and planetary radii of 1.2-1.5 for all planets.”(Crossfeld et al. page 21). Which values do you believe are correct; Huber et al’s or the authors’ DERIVED(via Moldonado et al)values?

  • Malcolm Sargeant July 20, 2016, 5:10

    I’m not qualified to comment on the Kepler data, but I guess the James Webb telescope is going to be very busy once it starts observing.

  • Mike Fidler July 20, 2016, 9:59

    Potassium-40 and the Evolution of Complex Life.
    Could this effect lead to different development times for life in stellar systems with different Potassium levels?
    Take a look at the chart – K-40 has a half-life of 1.248 Gyr, right in the mid-range of geological time, with the K-clock being wound by the supernova that expelled what became our solar nebula, and running down ever since. Consider the following table, which takes the current K-40 dose as one and extrapolates over the age of the Earth and into the future. “Fraction Remaining” arbitrarily starts at 1 at the time the Earth was formed — the actual isotopic abundance will vary from star to star depending on the properties of the medium from which it formed. “Relative Radiation” compares the radiation received by an organism from K-40 with that received at the present time set as 1.
    He mentions that at 2.5 billion years (3.956) that the vastly greater complexity of eukaryotic metazoans developed. this is exactly 1/4 of the radiation from K40 at 4.5 billion years (11.88). At 500 million years the radiation has dropped again by 1/4 to 1.316 and we see the Cambrian Explosion! I just wonder what type of simple organism developed the ability to survive in high radiation environments, since many have been discovered.

    • Alex Tolley July 20, 2016, 12:51

      The radiation levels of K-40 have dropped by less than 1/2 since the Cambrian explosion. But we know that the radiation tolerance for organisms is highly variable, and spans orders of magnitude. Life would be so close to the damaging effects of radiation that any increase might destroy it, yet we do not see such effects from natural or artificial increases in radiation levels, e.g. around Chernobyl. Therefore Walker’s idea that K-40 abundance in any way affected the evolution of life makes no sense at all as a theory.

      • Mike Fidler July 20, 2016, 21:56

        “The new Earth was radiating radioactive energy and heat producing isotopes such as potassium-40, uranium-238, uranium-235, and thorium-232 (Jordan 1979; Robertson 2001; Turcotte & Schubert 2002). In the early history of Earth, these heat producing isotopes would have been at full strength (Turcotte and Schubert 2002), thereby destroying all biological molecules, proteins, and naked DNA; but not any of the many microbial species and viruses which are radiation resistant.”

        Potassium Ion Channels: Could They Have Evolved from Viruses?

        • Mike Fidler July 20, 2016, 22:31

          The Chemistry of Evolution: The Development of our Ecosystem
          R.J.P Williams, J.J.R Fraústo da Silva
          See page 174-175 – via Google Books

        • Alex Tolley July 21, 2016, 21:57

          In the early history of Earth, these heat producing isotopes would have been at full strength (Turcotte and Schubert 2002), thereby destroying all biological molecules, proteins, and naked DNA; but not any of the many microbial species and viruses which are radiation resistant.”

          If the radiation destroys proteins and DNA, then how exactly is te repair mechanism going to cope? This is trying to suggest some Red Queen race where repair can happen fast enough to mitigate radiation damage. If it could do this, then the high radiation isn’t an issue. As I said, some species are highly radiation resistant, e.g. cockroaches, so the theory just fails, as the radiation resistance is far greater than the difference between the initial conditions and the decayed level. For example, if the decay range is 10x, but the radiation tolerance is 1000x, then the decay is of no consequence.

          • Mike Fidler July 22, 2016, 10:25

            Let see if you can understand what I’m saying, viruses can tolerate
            extremely high radiation levels and could propagate thru space to earth. Could these simpler life forms develop more complicated life as the radiation from K40 decreased, since it is a Beta emitter. The more advanced life was using potassium in voltage-gated ion channels and as evolution advanced, in nerves. So less Beta (electrons) decay better function in voltage regulation!

            • Alex Tolley July 25, 2016, 13:20

              Firstly we don’t think viruses evolve to more complex organisms, like bacteria. So that pathway to higher life forms is not plausible.
              Secondly, you have completely ignored the evidence of the wide variety of radiation tolerance in organisms. If cockroaches have high radiation resistance, then the issue of K-40 levels on their nerve signaling is irrelevant. They can tolerate radiation levels from the early earth, so no need for K-40 levels to fall. Radiation levels from decay have no bearing on the timing of evolutionary events.
              Lastly, the rate of K-40 decay is so slow that it has no effective impact on voltage regulation in any type of cells, nerves or otherwise.

              • Mike Fidler July 26, 2016, 2:02

                Biggest Virus Yet Found, May Be Fourth Domain of Life?
                2. Cockroaches have been on earth for less than 1/10 the age of the earth (320 million years)
                3. Has anybody fed cockroaches with newly made K40 isotopes to see what it does to them?
                4. Show me the evidence for what you say is true!

                “The saddest aspect of life right now is that science gathers knowledge faster than society gathers wisdom.”
                ― Isaac Asimov

                • Mike Fidler July 26, 2016, 2:25

                  Potassium is the principal positively charged ion (cation) in the fluid inside of cells, while sodium is the principal cation in the fluid outside of cells. Potassium concentrations are about 30 times higher inside than outside cells, while sodium concentrations are more than ten times lower inside than outside cells. The concentration differences between potassium and sodium across cell membranes create an electrochemical gradient known as the membrane potential. A cell’s membrane potential is maintained by ion pumps in the cell membrane, especially the sodium, potassium-ATPase pumps. These pumps use ATP (energy) to pump sodium out of the cell in exchange for potassium (Figure 1). Their activity has been estimated to account for 20%-40% of the resting energy expenditure in a typical adult. The large proportion of energy dedicated to maintaining sodium/potassium concentration gradients emphasizes the importance of this function in sustaining life. Tight control of cell membrane potential is critical for nerve impulse transmission, muscle contraction, and heart function.
                  Oregon State University
                  Linus Pauling Institute
                  Micronutrient Information Center

                  • ` July 26, 2016, 10:56

                    As a biologist, I don’t need a lecture on cell membranes. Your reference is correct, but irrelevant. K40 has a half life of 1.251e9 years and is less than 0.01% (1/10,000th) of the Earth’s potassium. Putt the numbers in context. Assume a cell has to stay viable for 1 year, that means that about 0.1 trillionth of the potassium in the cell will decay during its lifetime. IOW, the cells electrical balance will be changed by 10e-13 over that period. You can play with the numbers all you like, but the reality is that K-40 decay was, and remains, irrelevant to cell function. You could make a better case for lightning.

      • Mike Fidler July 21, 2016, 9:59

        We are talking about an element that is in the very structure of cells, not something outside – Potassium-40.
        Chernobyl, Ukraine
        Home to one of the world’s worst and most infamous nuclear accidents, Chernobyl is still heavily contaminated, despite the fact that a small number of people are now allowed into the area for a limited amount of time. The notorious accident caused over 6 million people to be exposed to radiation, and estimates as to the number of deaths that will eventually occur due to the Chernobyl accident range from 4,000 to as high as 93,000. The accident released 100 times more radiation than the Nagasaki and Hiroshima bombs. Belarus absorbed 70 percent of the radiation, and its citizens have been dealing with increased cancer incidence ever since. Even today, the word Chernobyl conjures up horrifying images of human suffering.

  • spaceman July 20, 2016, 11:57

    Still amazes that we are able to detect planets this small.

    • DJ Kaplan July 20, 2016, 13:20

      Indeed. I think a lot depends on their sun being small as well.

      I wonder what is the lower bound on planet mass able to hang onto to an oxygen-rich atmosphere with liquid water on the surface? These are pretty small spheres glittering in the void.

  • andy July 20, 2016, 17:36

    As noted on the Extrasolar Visions II forum, many of the stellar parameters including those for K2-3 and K2-72 are taken from a paper (Huber et al. 2016) using models that systematically underestimate the radii (and apparently also the masses) of low-mass stars. The insolation on the planets of K2-3 and K2-72 are almost certainly substantially higher than the values given in the Crossfield et al. (2016) paper.