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.