Which Exoplanet to Visit?
by Ian Crawford
Project Icarus is tasked with designing an interstellar spacecraft capable of exploring nearby stars and studying their planetary systems up close. The unmanned space vehicle—inspired by nuclear pulse propulsion studies by the British Interplanetary Society in the 1970s—will ideally reach its target star within 100 years after launch.
So the Icarus spacecraft is planned to take us beyond our solar system to neighboring stars, where should it go first?
The specific target star has not yet been selected, but its choice will be narrowed by a number of factors.
Given realistic propulsion options, and the requirement that the spacecraft can slow down to orbit its target, the target star cannot be more than 15 light-years from Earth. However, given that the project ideally wishes to complete the mission in much less than 100 years, it follows that the actual target will probably have to 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 uncertainties 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 dwarf stars.
Thirdly, perhaps surprisingly, there are still slight discrepancies between the catalogs of nearby stars. Probably the most authoritative recent compilation, and the one on which my number of 56 stars is based, is published by the Research Consortium on Nearby Stars.
The scientific objectives of Icarus include studies of the interstellar gas and dust lying between the sun and the target star, studies of planets orbiting the target star, and, most exciting of all, studies of any life forms that may have evolved on these planets.
Although studies of the interstellar medium do not depend on the precise choice of target star, the other scientific objectives clearly require that the target star has a planetary system.
In fact, two of the 56 stars within 15 light-years are already known to have planets. These are epsilon Eridani, a single K star at a distance of 10.5 light-years, and the 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 500, with an additional 1,235 candidates announced by the Kepler science team on Feb. 2) can be found in the Extrasolar Planet Encyclopedia maintained by Jean Schneider at the Paris Observatory.
The planet orbiting epsilon Eridani is a giant planet, with a mass about 1.5 times that of Jupiter. It has an orbit that 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 asteroids and 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 15 light-years, 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 Earths 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 means it’s only marginally interesting for Icarus.
Clearly it would be of great interest if planets were discovered orbiting stars that lie closer to the sun. Currently there have been no such planets discovered, but they are very likely to exist.
It has been estimated that roughly 30 percent of main-sequence stars will have planets with masses less than 30 Earth masses, based on the detection rate to-date, and allowing for the known biases in the detection methods. So 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), will greatly improve these estimates within the next few years.
The bottom line at present is that only further observations 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.
My own view is that the first interstellar space mission will be targeted at one of the very nearest stars, probably one of closest half dozen systems. Within this more restricted volume, by far the most interesting star system, given current knowledge, is the closest of all, namely the binary system 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 Centauri A), an investigation of this system would also permit close up studies of a star of a different spectra type (namely the K star alpha Centauri B). Moreover, given ingenious mission design, it might be possible to visit the nearby red dwarf star Proxima Centauri as well.
Although it clearly depends on what planets may be discovered in the coming years, my money is on the alpha Centauri system as the destination for humanity’s first interstellar probe.
This article first appeared on Discovery News.
Crawford’s paper Astronomical Considerations Relating To The Choice Of Target Star can be found in JBIS Vol 63 No 11/12 Nov/Dec 2010.
A list of Crawford’s publications can be found on his website.