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Warm ‘Saturns’ and Their Moons

Recent work from the Lick-Carnegie team has found that the M-dwarf HIP 57050 is orbited by a Saturn-mass world with an orbital period of 41.4 days. What catches the eye about this exoplanet is its temperature, some 230 kelvin or -43 degrees Celsius, warm enough to place it in the habitable zone of the star. Based on our knowledge of the gas giants in our own Solar System, it’s a natural supposition that this is a world with moons, and if so, their location in the habitable zone draws inevitable comparisons with fictional worlds like Pandora.

M-dwarf Habitable Zones

So what do we know about M-dwarfs that can help us with this system? For one thing, they’re exciting objects for radial velocity studies because of their low mass, making the signature of an orbiting planet more readily apparent than with larger stars. We also know that their low temperatures move their habitable zones in much closer to the star than in our system, ranging from 0.1 to 0.2 AU, corresponding to an orbital period of between 20 and 50 days. Finally, M-dwarfs are either less likely to have readily detectable planets, or the planets they do have are small enough compared to the planets of G-class stars like the Sun to make them more difficult to find.

As to the position of HIP 57050 b within the habitable zone, the verdict, based on 9.9 years of observations, seems clear. From the paper:

If we assume that the inner boundary of the habitable zone (HZ) of the Sun is at 0.95 AU (Kasting et al. 1993), and its outer boundary is at a distance between 1.37 AU and 2.4 AU, depending on the chosen atmospheric circulation model (Forget & Pierrehumbert 1997; Mischna et al. 2000), then by direct comparison, the inner boundary of the HZ of HIP 57050 would be at a distance of 0.115 AU, and its outer boundary would be between 0.163 AU and 0.293 AU. From Table 3, the perihelion and aphelion distances of HIP 57050 b are at 0.112 AU and 0.215 AU respectively, suggesting that this planet spends the majority of its orbital motion in the HZ of its host star.

HIP 57050b has an orbital eccentricity of 0.31, but this may not be a major issue for any interesting moons around the planet:

Although the planet makes small excursions outside the HZ, due to the response time of the atmosphere-ocean sysem (Williams & Pollard 2002; Jones et al. 2006), and the effect of CO2 cloud circulations (Selsis et al. 2007; Forget & Pierrehumbert 1997; Mischna et al. 2000), the times of these excursions are small compared to the time that is necessary for a significant change in the temperature of the planet to occur. In other words, the planet could hardly be more squarely in the HZ and will most likely maintain its habitable status even when its orbit is temporarily outside of this region.

A Problematic Habitat

Could a habitable moon exist here? Theoretically so, but the paper goes on to note that in our Solar System, on the order of 0.02% of the mass of the gas giants is found in their moons. Run the numbers and you wind up with a moon that is about 2 percent of Earth’s mass, or 1/5th the mass of Mars. That doesn’t sound particularly promising, but in an article in Scientific American, Darren Williams (Penn State) points out that larger moons could form on their own and be captured by a massive planet’s gravity.

We may be looking at that situation in our own system in Neptune’s moon Triton, which possibly arrived where it is today by being captured by Neptune, with a binary object pairing with Triton being ejected in the process. Williams, who has simulated the situation on objects as massive as the Earth, says that an Earth-size moon could form around a gas giant this way, with a secondary object the size of Mars being lost along the way. So while we’re a long way from discovering a moon around HIP 57050 b, we do at least have a world in the habitable zone of its star and the possibility of objects around it that are astrobiologically interesting.

But while this system continues to yield its secrets, don’t be surprised if we get the actual detection of an exomoon in the near future. CoRoT 9 b transits its star on June 17, and researchers will be using the Spitzer Space Telescope to look for evidence of rings or moons. And if this planet fails us, it’s also possible that the Kepler space telescope will be able to flag the presence of moons around some of the planets it finds.

Alpha Centauri Seven Years Too Late

Check the Scientific American article for good links to recent work, including studies by Lisa Kaltenegger (Harvard University) showing that the James Webb Space Telescope may be able not just to detect exomoons but to study their atmospheres. It’s interesting, too, to hear Sara Seager (MIT) talk about exoplanetary moons in light of recent films:

But if astronomers manage to turn up an extrasolar moon in the coming years, even a habitable one like those of sci-fi lore, some aspects of Pandora will remain firmly fictional. “What’s interesting is Avatar is out of date by about seven years,” Seager says. Astronomers have looked for the presence of giant planets in the habitable zone of Alpha Centauri, the nearby star system that is home to Pandora in the film, and have not found one. That’s not to say that Alpha Centauri doesn’t have a habitable world of some kind—it would just have to be a planet like our own, rather than a moon. “If they had called me or someone else in exoplanet astronomy,” Seager says, “we would have advised them to just put an Earth there.”

M-dwarfs and the Metallicity Connection

All the exomoon speculation is fascinating in its own right, but a major finding of the Lick-Carnegie paper is that the strong correlation between metallicity and expected planets — giant planets should be more likely with increasing metallicity of the host star and increasing stellar mass — may hold up with M-dwarfs despite earlier doubts. In fact, HIP 57050 has twice the metallicity of our own Sun, making it among the highest metallicity stars in our neighborhood. Indeed, “…planet-bearing M-dwarfs do appear to be systematically metal-rich, suggesting that there is no breakdown of the planet-metallicity correlation as one progresses into the red dwarf regime.”

The paper is Haghighipour et al., “The Lick-Carnegie Exoplanet Survey: A Saturn-Mass Planet in the Habitable Zone of the Nearby M4V Star HIP 57050,” Astrophysical Journal Volume 715, Number 1 (20 May 2010), pp. 271-276. Abstract and preprint available.


Comments on this entry are closed.

  • Bynaus May 11, 2010, 10:36

    Very interesting system, indeed.

    “on the order of 0.02 of the mass of the gas giants is found in their moons”

    0.02 %, or about 1:5000. It is clear from the context later on, but maybe you should correct that.

  • James M. Essig May 11, 2010, 11:27

    Hi Paul;

    It is kind of bummer that no gas giant planets have been found around Alpha Centauri. I still wonder what the prospects are for perhaps 2 to 5 Earth massed planets around which might orbit 0.2 to 0.5 Earth massed Moons. Perhaps even a 0.1 Earth massed moon could support life.

    Regardless, when considering huge interstellar space craft, the fictional account in the first “Aliens” movie provides for some whimsical mental imagery and possible future directions of 0.9 C capable star ships.

    I ran some numbers last evening regarding possibilities for achieving speeds in excess of 0.9 C using nuclear fusion fuel and for an M0/M1 ratio of 1,000,000, I obtained 0.928 C for an assumed Isp 0f 0.119 C. For M0/M1 = 10,000,000, I came up with 0.9578 C, and for M0/M1 = 0.975 C which corresponds to a gamma factor of 4.5.

    I assumed that some form of volumetrically extremely dense storage of Isp optimized fusion fuel would be developed as well as extremely light weight fusion fuel containment such as by monolithic multilayer graphene based confinements, or that perhaps some sort of carbon nanotube based or other extremely low mass extremely strong confinements could be utilized.

    The ship could start out with slow acceleration, ~ 0.03 to ~0.1 G and ramp up acceleration to perhaps 1 G to 2 Gs after the mass of the fuel was mostly depleated so as to avoid undue mechanical stress and deceleration might be accomplished by reverse thrust mini-magnetospheric-plasma-propulsion, reverse ISR thrusting, and linear magnetic induction breaking via large superconducting windings that could be deployed.

    Even though we might not have a Pandora to aim for, science fiction still gives us plenty of other whimsical scenarios.

    Regarding highly relativistic fusion powered star ships that carry all of their fuel onboard from the start of the mission, the good news is that we have loads of fuel right here in our own solar system, and presumably any stellar destinations would presumably have the same.

  • Zen Blade May 11, 2010, 13:01

    “-33” degrees C… habitable zone? did I read that correctly.
    Water freezes at 0 degrees C. I suppose the idea is that the moons would be warmer?
    Any planetary scientist out there care to help me understand why -33 degrees C is a good thing?

  • Administrator May 11, 2010, 13:30

    Bynaus writes:

    0.02 %, or about 1:5000. It is clear from the context later on, but maybe you should correct that.

    Now fixed — thanks.

  • Administrator May 11, 2010, 13:40

    Zen Blade wonders about the temperature of this world. The figure cited from the paper refers to the planetary effective temperature (Teff), which is defined in the Wikipedia this way:

    The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation.

    And here’s another bit that’s to the point:

    The effective temperature for Jupiter is 112 K and 51 Pegasi b (Bellerophon) is 1258 K. The actual temperature depends on albedo, atmosphere, and internal heating. The actual temperature from spectroscopic analysis for HD 209458 b (Osiris) is 1130 K, but the black body temperature is 1359 K. The internal heating within Jupiter of 40 K combined with the effective temperature of 112 K results in a total of 152 K as the actual temperature.

    But someone else weigh in who knows more about how these temperatures are calculated. The actual figure, by the way, looks to be -43 degrees Celsius — typo on my part in making the conversion.

  • kurt9 May 11, 2010, 13:48

    Any habitable moon around that warm Saturn is going to have wild seasons. The moon rotates, revolves around the Saturn, and the Saturn revolves around its sun. Also, that moon may have axial tilt with regard to both the Saturn and the sun. The Saturn swings outside the habitable zone during part of its orbit around the sun. The weather on the moon is going to depend where it is in its orbit around the Saturn when the Saturn moves outside its habitable zone, and this will vary from year to year as well. It will have complex seasons.

    What about the Van Allen belts around the Saturn? Our own Jupiter and Saturn have nasty radiation belts around them. Presumably this is a feature of all gas giant planets. The moon could be habitable because of its atmosphere, but the local space could be hazardous in the extreme. Imagine the auroras at night on such a moon, should be quite impressive.

  • andy May 11, 2010, 13:48

    CoRoT 7 b transits its star on June 17, and researchers will be using the Spitzer Space Telescope to look for evidence of rings or moons

    With an orbital period of less than a day, CoRoT-7b transits its star at least once every day.

    CoRoT-9b on the other hand…

  • Tulse May 11, 2010, 14:12

    The temperature guesstimates for such moons are made all the more difficult because of the possibility of tidal heating, which is why someplace like Enceladus can have liquid water geysers. I think we should take “habitable zone” estimates for these kind of worlds to be minimums.

  • Administrator May 11, 2010, 15:01

    andy writes:

    With an orbital period of less than a day, CoRoT-7b transits its star at least once every day.

    CoRoT-9b on the other hand…

    This was clearly not my morning. Anyway, typo corrected.

  • andy May 11, 2010, 15:44

    The temperature guesstimates for such moons are made all the more difficult because of the possibility of tidal heating, which is why someplace like Enceladus can have liquid water geysers

    Tidal heating is definitely something to consider for a potentially-habitable moon in a red dwarf system. Main issue is the strength of the stellar tide (scaling as m/d³, I estimate this would be roughly 35 times the lunar tide on Earth) and also the fact that the nearby star will likely induce significant perturbations on the moon’s orbit around the planet, increasing its eccentricity and thus causing planet-induced tides as the moon librates.

    Intense volcanic activity definitely seems like something that could be on the cards.

  • kurt9 May 11, 2010, 15:57

    I forgot to mention another thing. All of the large moons around Jupiter and Saturn are tidally locked. Their rotation and revolution around the Gas Giant is the same, which ranges from 3 Earth days (Europa) up to 16 Earth days (Ganymede) . Any habitable moon orbiting that warm Saturn will likely be tidal locked as well, meaning that it will have really long days (up to 14 Earth day “days”) as well. The day to night temperature swings must be quite impressive.

    Imagine a 14 day “day” and a 41.4 day “year”. A three “day” year. I don’t think this place would be very pleasant to live.

  • Eric May 11, 2010, 18:45

    I’m just amazed to be living at a time when we’re on the cusp of these kinds of discoveries. The variety of possible worlds out there is really thrilling!

  • Bounty May 11, 2010, 20:10

    I can’t wait for our ability to model these things improves. I can’t fathom how nice or terrible a 14 day “day” there would be. I’d guess it depends on how much heat you’re getting from the star v.s. tidal heating, albedo, atmosphere density, all kinds of things. I always find the difference between Venus, Earth and Mars fascinating. The temp and pressure of Venus for example.

  • Carl May 11, 2010, 20:56

    “HIP 57050b has an orbital eccentricity of 0.31, but this may not be a major issue for any interesting moons around the planet…”

    Mercury’s figure is roughly 0.2. The hypothetical exomoon would experience varying apparent solar diameters with shorter periods than Mercury. Depending on the orbit around the gas giant, and assuming a tidal lock to the planet, the course of the sun in the moon’s sky may be fascinating.

    If Kurt’s figures are used, the year can be, in one instance, 2.957 times the length of the day. Close to three, as he wrote. Some days will start with the bloated sun, and others not. And when the solar orbital velocity is at its peak, the sun appears to retrograde…

  • Phil May 11, 2010, 23:40

    It might be worth saying that there are indications of a second body in this system with a 16 day period. It will need folloup observations of course. I suspect this putative planet might be a little toasty…


  • tesh May 12, 2010, 12:40

    I wonder if the potential second planet, once it is ressolved, lead to a reduction in the e value for the first planet. Hasn’t this been the case for other systems?

  • Dave Moore May 12, 2010, 20:58

    Kurt9, the climate could be modeled on that of a tidally-locked planet in a 14 day orbit around a red-dwarf star. Modeling by Merlis and Schneider indicate that the climate on a tidally locked planet isn’t as extreme as imagined.
    Their models indicate that the subsolar point of a tidally locked Earth analog is in the 30-40 deg. C range; although, that’s the good news. You also get about 500 inches of rain a year.
    In the case of a 14 day orbit around a giant primary, the hot, wet spot would walk around the equator every two weeks.

  • amphiox May 13, 2010, 18:27

    re: Zen Blade

    If I recall, the planetary effective temperature of earth, at present, is – 20 something Celsius, and would have been even lower in the past when the Sun was less luminous.

    So -43C for this planet is excitingly close to home.

    Imagine a 14 day “day” and a 41.4 day “year”. A three “day” year. I don’t think this place would be very pleasant to live.

    For us, having adapted for our particular ratio, maybe. For something that evolved on such a world, hard to say. Maybe they would find earth’s 8.82 year “year” and 0.07 day “day” terrifyingly uncomfortable. Such extended seasons, such short daylight duration. Like trying live under a perpetual strobe light….

  • Adam May 16, 2010, 21:54

    Just a point to consider but the combined angular motions of an orbitting moon going around a star means the sol, s – time between sunrises – is related to the sidereal day, d, and the orbital year, y, by the following…

    1/s = 1/d – 1/y

    …which depends on the sign of the rotations as to how long the sol is. Assuming they’re rotating in the same direction we get, for 14 days d and 41.4 days y an s of 21.1 days.

    On Mercury it means the solar day is actually twice the year because the sidereal day is 2/3 of the year i.e.

    1/s = (3/2 – 1)*(1/y) …thus s = 2y.

    On Venus because the sidereal day is retrograde it means the sol is less than the sidereal day – it’s just 116.7 days versus 243 days i.e.

    -1/116.7 = -1/243 – 1/224.7

    …thus a sol shorter than the sidereal day.