For all the excitement the Kepler mission has generated, we sometimes forget its limitations. Kepler is engaged in a transit hunt for exoplanets that will help us identify not just gas giants but planets the size of our own. But it’s a brute-force method, looking at a huge number of stars to identify the few whose planetary systems are aligned properly for us to see transits. And the necessary limitation is that when we do find terrestrial-sized worlds, we’ll be unable to do much by way of follow-up, because most of those planets will be thousands of light years away.

This is not to diminish Kepler’s critical work (nor that of CoRoT), for in no other way are we currently gaining this kind of overview of the planetary environment around a wide range of stars. But Philip Horzempa reminds us in a recent post on The Space Review that we have follow-up missions in the pipeline that are now losing their funding. Specifically, the Space Interferometry Mission (known as SIM Lite in its last incarnation), received no backing from the Astronomy and Astrophysics Decadal Survey, recently released by the National Research Council. If SIM still hangs by a thread, it is getting to be a very slim thread indeed.

The Next Phase of the Planet Hunt

I want to talk about SIM (and I’ll try to avoid elegaic tones) because it illustrates the kind of questions we face as we try to press beyond finding exoplanets to eventually bringing Earth-class planets into close scrutiny. In its various incarnations, SIM was a radically different concept than Kepler, one that would have used an interferometer to combine the light from separate telescopes to obtain high-resolution data on the positions of stars. So exquisitely sensitive would SIM’s instruments have been that they could have detected Earth-class planets as well as the familiar ‘hot Jupiters.’ But more important still was that SIM would have brought the terrestrial planet hunt closer to home by looking for nearby Earths that circle Sun-like stars.

Detecting the telltale ‘wobbles’ of planets around the stars closest to us is an obvious priority and whether we have SIM or not, we still need a plan that gets a comparable result. Let me quote Horzempa on what he thinks SIM’s significance would have been in terms of future missions that will look for biosignatures in the atmospheres of exoplanets:

The key will be finding the “exoEarths” that are close to us, meaning within 30 light-years. Only those Earths that are close enough to our solar system will reflect sufficient light from their parent Sun to allow telescopes, of any design, to examine them in detail. SIM is the only mission capable of detecting those nearby Earths. SIM will be a guide, a “GPS” for those who seek other Earths. Without SIM, all future endeavors to examine and chart those nearby Earths will stall.

I disagree with that last statement, although I do think SIM would have put us on a faster track. And the main reason for that is what Horzempa goes on to say:

In addition, SIM will be a pathfinder for the use of interferometers in space. This is a vital technology for future projects such as the planned Life Finder and Planet Imager missions, which will use arrays of space telescopes. Those arrays will use the technique of interferometry to combine their light and produce exquisite data, and maps, of those nearby Earths. Without SIM’s pioneering effort, those projects will be delayed for decades.

Losing SIM in the Budget

Horzempa is blunt about what happened to a SIM project that was already highly developed and ready to enter its implementation stage. What he calls the ‘big-ticket’ items like Constellation and the James Webb Space Telescope had precedence in the mid-2000s, with work on SIM reduced to a low but continuing level as ground testing of hardware proceeded. Given all that, what has him exercised is the fact that the Astro2010 report did not include SIM in its list of recommended missions for the next decade. The WFIRST mission is proposed instead, consuming the funding that might have made SIM a space-borne reality. Horzempa again:

The decision of Astro2010 to eliminate nearly all traces of funding for planet-hunting space telescopes is breathtaking. They did offer a token to those studying exoplanets by adding a microlensing capability to the WFIRST infrared telescope. Microlensing can detect exoplanets by detecting subtle brightening of stars. However, microlensing will only find exoplanets that orbit stars located 20,000 light-years away. Distance, as noted above, is crucial to any follow-up missions. Microlensing, like the Kepler mission, can detect Earth-sized planets, but they will be so far away that it will be almost impossible to locate the stars around which they orbit. Like Kepler, microlensing will return data on the statistics of planet sizes, but will lead us no closer to finding a warm, water-bearing Earth twin.

Should NASA put $2 billion in WFIRST, whose primary purpose is the study of dark energy? This is Horzempa’s primary question, noting that the already developed SIM would take about half as much to get it ready for launch, leaving sufficient resources for a smaller mission with a dark energy focus. The key point is that while WFIRST exists as a concept, SIM existed as the work of a team that had reached all technical milestones required of it and had built hardware.

Looking Toward Nearby Stars

Without SIM, the question becomes ‘where do we stand on identifying and characterizing nearby planets’? WFIRST’s microlensing capabilities will be used to detect exoplanets tens of thousands of light years away, but what about the stars whose atmospheres we will eventually study for life signatures? Shouldn’t these stars be NASA’s logical focus after Kepler?

I pulled out the decadal survey again to look at its treatment of the terrestrial-planet hunting process. The report goes from the Kepler transit survey directly to WFIRST’s microlensing studies, followed up by improved radial velocity measurements on existing ground-based telescopes, looking for planets a few times more massive than Earth as targets for future missions. It calls on the James Webb Space Telescope to study the atmospheric or surface composition of small planets orbiting the coolest red stars. The missing link here is the precision that a SIM mission would bring to what the survey assumes will be ground-based follow-ups.

That precision could help us greatly in identifying small planets around stars close to the Sun. Right now we have three ongoing studies of the Alpha Centauri system, looking for planets around Centauri A and B. We’ve just been through a wave of public interest about a nearby planet that may not even exist — Gliese 581g — because it was thought to be in the habitable zone of its star. Nearby stars exert an understandable fascination because of the prospect of closer study.

The other missing link is the technology of space-based interferometry. Budgetary realities are what they are, and the decadal survey process has served us well for many years at prioritizing science. Yet thankfully, there is a sense in which good mission ideas never really die. To get those biosignatures we hope to find and, one day, actual images of a distant planetary surface, we’ll tap the interferometric expertise that is SIM’s legacy. But the betting here is that a true terrestrial planet finder using these methods is still decades away.