Targets for Icarus: Planets within 15 light-years of the Sun
by Ian Crawford
The Icarus study is tasked with designing an interstellar space vehicle capable of making in situ scientific investigations of nearby stars. The specific target star has not yet been selected, but its choice will be constrained by a number of factors. Foremost among these are the design constraints that the propulsion mechanism must be based on a realistic extrapolation of existing technology (which essentially implies a fusion-based design, similar to that adopted by the earlier Daedalus study), that some deceleration is required at the target to maximise encounter time, and that the target star should be reached within a hundred years (and ‘ideally much sooner’). Taken together, these imply a maximum realistic range of 15 light-years from the Solar System. This would imply an interstellar cruise velocity of 15% of the speed of light (i.e. 0.15c) to reach within 100 years, which is probably close to the upper end of what is likely to be feasible with a fusion-based propulsion system extrapolated from current knowledge. Moreover, given that the Icarus study ‘ideally’ wishes to complete the mission in less than 100 years, it follows that the ideal target would actually be significantly closer than 15 light-years. Within 15 light-years of the Sun there are approximately 56 stars, in 38 separate stellar systems. I say approximately for several reasons. Firstly, at the outer boundary the errors on the distances can amount to a few tenths of a light-year, which could mean that some stars notionally just beyond 15 light-years might actually be closer (and vice versa). Secondly, not all stars within this volume may yet have been discovered, although this is only likely for the very dimmest red or brown dwarfs. Thirdly, perhaps surprisingly, there are still slight discrepancies between the catalogues of nearby stars. Probably the most authoritative recent compilation, and the one on which my number of 56 stars is based, is the RECONS (Research Consortium on Nearby Stars) list of the one hundred nearest star systems, available at: http://www.chara.gsu.edu/RECONS/TOP100.posted.htm There is also a useful tabulation, based mostly on the RECONS list, available on Wikipedia: http://en.wikipedia.org/wiki/List_of_stars_nearest_to_the_Earth Of these 56 stars, there is one star of spectral type A (Sirius); one F star (Procyon); 2 G stars (alpha Centauri A and tau Ceti); five K stars; 41 M stars (red dwarfs); 3 white dwarfs; and three probable brown dwarfs (the latter all members of multiple systems – there are no currently known free-floating brown dwarfs within this volume; these would be difficult to detect but in principle could exist). Two of these 56 stars are in fact already known to have planets, on the basis of radial velocity measurements. These are epsilon Eridani (a single K2 star at a distance of 10.5 light-years, and the M3 red dwarf GJ 674 at a distance of 14.8 light-years. There are also a couple of other stars, both red dwarfs (GJ 876 at 15.3 light-years, and GJ 832 at 16.1 light-years), which are known to have planets but which lie just beyond the 15 light-year limit considered here. An excellent summary of all known extrasolar planets (currently more than 400) can be found in the Extrasolar Planet Encyclopedia maintained by Jean Schneider at the Paris Observatory (http://exoplanet.eu/ ). The planet orbiting epsilon Eri is a giant planet, with a mass about 1.5 times that of Jupiter. It has a highly eccentric orbit, which brings it as close to its star as 1.0 AU (i.e. the same distance as the Earth is from the Sun), to as distant as 5.8 AU (i.e. just beyond the orbit of Jupiter in our Solar System), with a period of 6.8 years. Although this would span the habitable zone (i.e. the range of distances from a star on which liquid water would be stable on a planetary surface given certain assumptions about atmospheric composition) for the Sun, this orbit lies wholly outside the likely habitable zone for a K2 star like epsilon Eri. Also, being a gas giant, this planet itself it not a likely candidate for life, and its eccentric orbit wouldn’t help in this respect either (although it is possible that the planet may have astrobiologically interesting moons, perhaps similar to Jupiter’s moon Europa, which could in principle support sub-surface life). There is an unconfirmed detection of another planet in the epsilon Eri system, also a giant planet (although less massive at 0.1 Jupiter masses) in a very distant (40 AU) orbit. It is possible that the system contains lower mass, more Earth-like, planets, which might be more interesting targets for investigation, especially closer to the star than the giant planet that is known to exist. Epsilon Eri is also known to be surrounded by a disk of dust, which may be derived from collisions between small planetesimals (i.e. asteroids and/or comets), which is an indirect argument for smaller planets also being present. Only further research will tell how many planets actually reside in the epsilon Eri system, and whether any are of astrobiological interest. The existence of at least one planet, and the dust disk (itself of great astrophysical interest), would make epsilon Eri a high priority candidate target for Icarus if it were not for its distance of 10.5 light-years. Although within the 15 light-year radius considered here, this is still a very challenging distance for the first attempt at an interstellar voyage. The same is unfortunately true for the other known planetary system mentioned above: at 14.8 light-years GJ 674 is right on the limit! The planet orbiting this star is very different — with a mass of only about 12 Earth masses it is likely to be a giant rocky planet: a so-called ‘super-Earth’. It orbits its star every 4.7 days, in a moderately elliptical orbit at a mean distance of only 0.04 AU (one tenth of Mercury’s distance from the Sun!). Even for a red dwarf star, this is probably too close to be habitable. However, as one planet exists around this star it is possible that others will be discovered, perhaps in more habitable orbits, as observations continue. Only time will tell, but in any case the distance of this star probably renders it of marginal interest for Icarus. Clearly it would be of great interest if planets were discovered orbiting closer stars. Currently there have been no such planets discovered, but they are very likely to exist. Based on the detection rate to-date, and allowing for the known biases in the detection methods, it has been estimated that roughly 30% of main-sequence stars will have planets with masses less than 30 Earth masses. Thus, we might expect 16 or 17 of the nearest 56 stars to be accompanied by planets and, given the current lack of data on very low mass planets, it could easily be more. Although not targeted at any of the nearest stars, statistical results from the Kepler mission (which is looking for low-mass planets orbiting solar-type stars by the transit method, see http://kepler.nasa.gov), will greatly improve these estimates within the next few years. The ‘bottom line’ at present is that only further observational work will reveal how common planets actually are around the closest stars. The good news is that, long before we are able to build an Icarus-type starship, astronomical technology will almost certainly have reached the point where we will have a complete census of planetary systems within 15 light-years of the Sun. Not only will these instruments be able to identify which stars have planets, and calculate their orbital parameters, they will be able to make basic spectroscopic searches for biosignatures in their atmospheres. Thus, although currently we cannot identify an obvious specific target for Icarus, when the time comes to actually build a starship we will have a very good idea where to send it. It is my own view, discussed in more detail in an article currently ‘in press’ with the Journal of the British Interplanetary Society, http://www.homepages.ucl.ac.uk/~ucfbiac/Crawford_JBIS_Daedalus_paper.pdf that, out of necessity, the first interstellar space mission will be targeted at one of the very nearest stars (probably one the of closest half dozen systems – the most distant of which is GJ 65 (aka Luyten 726-8), a red dwarf binary at a distance of 8.7 light-years). Within this more restricted volume, by far the most interesting star system given current knowledge is the closest of all, namely alpha Centauri A/B at a distance of only 4.4 light-years. Not only does this system contain the closest Sun-like star (alpha Cen A), an investigation of this system would also permit close up studies of a star of a different spectra type (namely the K 0 star alpha Cen B) and perhaps, given ingenious mission design, the red dwarf star Proxima Centauri as well. Thus, although it clearly depends on what planet discoveries may be made in the coming years, my money is on the alpha Centauri system as the destination for humanity’s first interstellar probe. 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.
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