Possible Discovery of an “Earth-Like” Planet orbiting GJ 581: Implications for Project Icarus
by Ian Crawford
In recent days there has, justifiably, been much excitement over the reported discovery of an “Earth-like” planet orbiting within the “habitable zone” of the relatively nearby (20.5 light-years distant) red dwarf star GJ 581. The announcement was made in a paper by Steven Vogt (University of California), and colleagues, which has been accepted for publication in the Astrophysical Journal. A preprint of this paper may be found at: http://arxiv.org/abs/1009.5733 As the Icarus study is tasked with designing an interstellar space vehicle capable of making in situ scientific investigations of nearby stars, it is natural to ask what implications this discovery may have for Icarus, and whether it affects the considerations of target star selection that were discussed in a previous article: http://www.icarusinterstellar.org/blog/targets-icarus-planets-15-lightyears-sun/ By all accounts, GJ 581 is a very interesting planetary system. Even before the publication of Vogt et al.’s recent results, it was widely thought to contain at least four relatively low-mass planets, with one (GJ 581c) at the inner edge, and another (GJ 581d) at the outer edge, of the “habitable zone” (usually defined as the range of distances from a star within which liquid water will be stable on the surface of an orbiting planet). The new results add two more planets, bringing the total number to six, one of which (GJ 581g) appears to be well within the habitable zone. The architecture of the GJ 581 planetary system, as elucidated by Vogt et al., is summarised in Table 1.
Table 1 only gives the minimum mass of these planets, because the unknown orbital inclination means that it is not possible to determine the actual masses from the radial velocity measurements used to detect them. Vogt et al. present arguments for believing that the actual masses are unlikely to exceed these values by more than a factor of 1.6. Thus, apart from the Neptune-mass planet GJ 581b, all fall within the so-called ‘super-Earth’ mass range (i.e. between 1 to 10 Earth masses). Note that the whole planetary system would fit within the orbit of Venus in our Solar System! Before proceeding any further it is necessary to add a word of caution regarding the planetary candidates identified in Table 1. None of these planets have been directly imaged, and their properties are inferred from an analysis of about 240 separate radial velocity measurements of GJ 581 as it is tugged back and forth by its orbiting planets. The new planets (GJ 581 f and g), while apparently statistically significant, are close to the limit of what can be reliably determined from this data set, and Vogt et al. are at pains to point out that additional observations will be required to confirm their interpretation. Such observations will likely be obtained within the next few years. Assuming for the present that the orbital parameters of GJ 581g are as given in Table 1, some caveats remain regarding its habitability. Although it has been widely reported that GJ 581g lies within the “habitable zone” of GJ 581, it is important to realise that, strictly speaking, stars per se do not have habitable zones. This is because the distance at which liquid water is stable on a planetary surface (the usual definition of a habitable zone) depends on properties of the planet as well as those of the star. In particular, the density and composition of the planetary atmosphere is crucially important, as this determines the strength of any atmospheric greenhouse effect. In fact, in the absence of a greenhouse effect, the ‘equilibrium temperature’ of GJ 581g would be 228K (–45ºC), so the presence of liquid water on the surface will depend on the greenhouse effect provided by an as yet uncharacterised planetary atmosphere. Only if future observations demonstrate the existence of such an atmosphere will we be justified in claiming that liquid water will be stable on the planet’s surface. It also needs to be pointed out that GJ 581g is sufficiently close to its star that it is almost certainly tidally locked to it, with one hemisphere permanently pointed towards the star, and the other permanently pointed away. Such planets may still have habitable surfaces, but again this largely depends on the role of the (currently unknown) atmosphere in redistributing heat around the planet. For these reasons, some recent assertions regarding the habitability of GJ 581g are premature. Future observations, perhaps by the next generation of large space telescopes able to detect and characterise exoplanet atmospheres, will be required to remove this uncertainty. On the other hand, the usual definition of a ‘habitable zone’ does not address the possible presence of life below the surface. For example, Mars is at the outer edge, or perhaps just beyond, the Sun’s ‘habitable zone’ as usually defined, but conditions deep within the crust may still be suitable for microbial ecosystems. Similarly, Europa is well outside the ‘habitable zone’ by any definition, but it may nevertheless posses a habitable sub-surface ocean. From this perspective, all five of the super-Earth planets orbiting GJ 581 (and any moons of the Neptune-mass GJ 581b) could be of interest from an astrobiological point of view. So, how does this affect Project Icarus? The first thing to note is that, interesting though the GJ 581 system certainly is, it is not the closest known extrasolar planetary system. As noted in my earlier article referenced above, that distinction goes to epsilon Eridani at a distance of 10.5 light-years – half the distance of GJ 581. Planets are also known to orbit GJ 674 (14.8 light-years), GJ 876 (15.3 light-years), and GJ 832 (16.1 light-years). Details of these, and all other extrasolar planets identified to-date, can be found in the Extrasolar Planet Encyclopedia maintained by Jean Schneider at the Paris Observatory (http://exoplanet.eu/ ). Thus, GJ 581 is far from being the closest possible target star with a known planetary system and, in the absence of any compelling evidence that any of its planets are any more habitable than those orbiting closer stars, it is difficult to justify giving it a higher priority. It also lies significantly beyond the ‘nominal’ 15 light-year maximum range of Icarus. While this distance limit is not set in stone, and could certainly be over-ridden if compelling new observational evidence regarding the scientific importance of more distant planets came to hand, it does represent the outer envelope of what is considered realistic for a fusion-powered interstellar vehicle meeting the constraints imposed by the Project Icarus Terms of Reference (i.e. deceleration at the target system and a total travel time of under 100 years; http://www.icarusinterstellar.org/TOR.pdf ). Given that it would take over 170 years to reach GJ 581 at the Daedalus velocity of 12% of the speed of light, this planetary system, interesting though it is, appears just too distant to be the target for humanity’s first interstellar mission. The real significance for Project Icarus of the recent results for GJ 581 is what they say about the probability of habitable planets around closer stars. As Vogt et al. point out in their paper, very few of the stars closer than GJ 581 have been studied as carefully as this star, and so planets similar to GJ 581g very likely exist around closer stars but have not yet been discovered. Indeed, based on their observations of GJ 581, and allowing for the observational biases in their method, Vogt et al. estimate that the fraction of Sun-like and cooler stars (i.e. spectral types G to M) possessing Earth, or super-Earth, -mass planets within the conventional definition of the ‘habitable zone’ to be at least 20%. On this basis, at least 11 of the 56 stars (belonging to 38 stellar systems) within the 15 light-year nominal range of Icarus would be expected to have planets more or less similar to GJ 581g. Only further observations of nearby stars will confirm or refute this expectation. And, as noted above, observations of the presence and composition of atmospheres around such planets will be needed to inform any judgement regarding habitability. Currently this information is lacking for any of the planets found orbiting nearby stars, and obtaining it will probably require a new generation of space-based astronomical instruments. To my mind, these considerations demonstrate the wisdom of the current Project Icarus policy of not selecting a target star prematurely. As additional observations are gathered over the coming years, it is likely that planetary systems as, or even more, exciting as the GJ 581 system will be identified around even closer stars. —————– Ian Crawford is a Reader in Planetary Science and Astrobiology at Birkbeck College, University of London (http://www.bbk.ac.uk/es/staff/Ian_Crawford), and Lead Designer for the Icarus ‘Astronomical Target’ module.