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.
My general impression of the Decadal Survey was that it was a dark energy party. Both the main space-based and ground-based missions are primarily focussed on dark energy research, with microlensing tagged onto WFIRST as an afterthought. While a space-based microlensing mission will definitely provide useful statistics about planetary systems (in principle such a mission should be sensitive to analogues of all the planets in our solar system with the exception of Mercury), it seems that exoplanet characterisation is not going to advance nearly so rapidly as might have been hoped for.
“…WFIRST, whose primary purpose is the study of dark energy…”
Dark energy is a theory only, and I’m sure that many researchers champion its study in favor of the also-important world-hunting. May be some future fame rides on this. It may be the camp which has the influences to see to it that things like SIM are abandoned to put resources to the likes of WFIRST.
Some at NASA suggested Jupiter/DIRECT as a viable, less expensive, and more available alternative to Ares. The officials went with the more expensive, to-be-developed system. Now there is a great lag as budgets are considered.
It reminds me of Sagan’s ‘Contact’, where the scientist is given little attention, and reduced funding, for her project… until it bears fruit, whereupon the boss begins to act as if he’d always been on top of it all…
Sad that science looses out in this world of excessive military spending and seemingly unlimited bank bail-outs.
How about private funding for missions like SIM, TPF and its European counterpart, Darwin?
Some very wealthy individuals, such as Gates and Buffett, are sometimes willing to fund philanthropic and humanitarian causes.
The private funding of a mission to detect, image and spectro-analyze earthlike planets would surely result in lasting fame for the funder as well.
What we need here is some good advocacy plus a few truly visionary moneybags.
Well, this is quite the negative counterpoint to the positive quotes from NASA’s Braun relayed here a few days ago.
SIM, as described, seems like a no-brainer mission to advocate soonest, at least with this audience. As valuable as abstract knowledge and the obtaining of a more complete statistical picture can be to science, and I admit freely theese things are mighty valuable, I’d personally much rather support programs that will provide us characterization of all known solar systems within 10pc.
There’s a post up on Cosmic Variance about what the future of US space-based astronomy looks like. The latter half of this decade is going to be quite bleak…
@Ronald
I think TPF/Darwin are basically dead for now. Not 100% sure though. Mind you there have been a lot of advances in coronographers of various kinds that can achieve similar results a lot cheaper so a pause there maybe is not too bad.
ESA has its analogue of SIM, albeit with less precision. It’s called Gaia and it will launch sometimes in 2012. It will operate until 2017 at least.
Besides mapping with high precision the position of 1 bliion stars, it should find all Jupiter analogue within 100 ly or more. I don’t remember the details, but I posted them here before.
Jupiter analogues are not Earths, but they might be an indication of earth-like solar systems.
As for private funding, this stuff is very expensive on the $1-2B mark. Also, any individual that would pay that much money would inevitably attract a lot of criticism for spending all that money like that instead of feeding Africa etc. etc.
Could the upcoming generation of huge ground-based telescopes with adaptive optics be used to image nearby(< 100 ly) earth-sized planets for study? If so it seems like that would be a much more economical way to learn about their environment than using a few dedicated space-based instruments.
In the late ’90s, NASA’s New Millennium Program had a proposed mission called Space Technology 3. ST3 was a SIM precursor that was supposed to test technologies for space interferometry such formation flying and laser metrology. Originally it had a main spacecraft and 2 independent smaller craft. As time went by, it went from 1+2 to 1+1 (and rechristened “Starlight”) to a single spacecraft with 2 separated mirrors to a laboratory testbed and then to nothing.
What really frustrates me about this decision is that NASA is wasting an opportunity to connect with the public on something that I believe people would really care about. Dark energy might be one of the current exciting topics in physics but in my opinion it only really appeals to a specific demographic while the exoplanet hunt has relevancy to everyone. If NASA gave the people what they wanted then maybe next time they wouldn’t have to cut so much.
I am afraid that Philip Horzempa has only a second hand perception of
what is going on in exoplanet ground and space projects.
Whether it is worth or not to launch a ~ 2B$ SIM to search for Earth-like planets
around the nearest ~60 solar type stars and measure their mass and orbits
is a matter of opinion.
Let’s look at the facts:
1/ For a 10 to 100 times lower budget radial velocity (RV) surveys will do the same
from the ground for at least 30 stars (see for instance Dumusque et al 2010
http://arxiv.org/abs/1010.2616 ), and probably more with infrared RV surveys
like CARMENES (Becerril et al. 2010 http://arxiv.org/abs/1007.3682 ).
2/ Such a RV survey is already dedicated to the search for Earth-like planets around
alpha Cen A and B (Guedes et al. ApJ. , 679 , 1582), as correctly said by Paul Gister.
3/ RV surveys give only the product Mass x sin(orbit inclination).
But a very few subsequent direct images of the planet will draw the orbit
and therefore its inclination, so you have the true planet mass.
4/ The same direct images will also make spectro-polarimetric observations
of the planets to characterize their atmosphere (molecules, clouds etc),
climate etc, and hopefully biosignatures for a few of them.
Such projects do exist, if selected, for the 2020-2025 time frame:
– 2020: 1.5 m coronagraphic telescopes in space: ACCESS and PECO in USA,
SPICES in Europe (and later on ~3-4 m telescopes)
– 2025: the EPICS camara at the european E-ELT.
In ESA plans, Darwin, if any, is for after 2030.
5/ On the top of that, a new idea under investigation at NASA, has been
proposed by Olivier Guyon, which combines astrometry (with performnces
similar to SIM) with spectro-imaging with a single telescope: you have both techniques (astrometry and imaging) with one telescope. See http://www.naoj.org/staff/guyon/04research.web/30astrometry.web/content.html
Therefore the claim by Philip Horzempa that “The end of SIM means the end of looking for nearby Earths”
is just not correct.
OT slightly but has to do with nearby (nearest?) exo-planets, any news on Alpha Centauri A or B planet search?
Even a whisper? A rumour will do. Even if it is from “a friend of a sister’s, cat’s third cousin removed thrice who heard from a reliable source (ahem) that …”?
Hello, tesh,
The latest Paul had reported Dr Fischer’s work here was little more than a month ago:
“Any hint of how her radial velocity studies of the Alpha Centauri system are proceeding? I wouldn’t have expected any, I’ll admit, and Fischer says nothing about it, but the betting here is that we’ll have an announcement within the next year either by Fischer or Michel Mayor’s team either giving us a planetary discovery or sharply constraining the alternatives.” https://centauri-dreams.org/?p=14332
Dr Fischer is quoted in the article: “Further ahead I’d like to see tiny spacebots – smaller than your cell phone—travel outside our solar system to the nearest star system, Alpha Centauri. ” This is an admission of nothing, but it indicates she is not discouraged.
Consider the recent press about Gl 581g, which may not exist. Everyone involved would be very excited to have found the first Goldilocks Planet, as the Gliese incident shows. But the results are in doubt, and I am sure that Dr Fischer and the others scrutinizing the next system are taking due caution concerning this.
To be the first to announce a positive finding is quite an achievement, as it enhances reputations. The discovery of Neptune is a case of several people performing the work, with disputes afterward http://en.wikipedia.org/wiki/Discovery_of_Neptune
The hot Jupiters show themselves in comparatively short order. Here we are dealing with periods approximating our own Earth’s. Patience is needed above exuberance, but the latter stimulates researchers to continue their work.
tesh, Carl has this right — what he’s quoted me on is the latest I have. As soon as any news emerges, I’ll get it up on the site right away. Believe me, I’m watching the Alpha Centauri work very closely!
Thanks – Carl and Paul.
I saw the earlier post but sometimes I get impatient and in a childish fancy, I give in and try to squeeze out a bit more info from the ether… hasn’t worked once yet but as I always knew deep down, it was never going to work.
Has anybody considered the possibility that what we are calling “dark energy” and “dark matter” could be several different phenomena? It would be like 19th century scientists referring to ultraviolet, x-rays, gamma rays, radio waves as “dark energy”. I don’t know if that’s very useful.
On a site called “centauri-dreams”, I imigine we’ll get whatever there is to get on Centauri System. Place would sure be a good stepping stone for humanity (if there are any stones there).
I too am appalled at the cancellation of so many space-based astrometry missions over the past two decades, coinciding with the ascendancy of Dark Energy experiments.
Fortunately, as Enzo pointed out above, ESA’s Gaia mission is on schedule for a 2012 launch. Gaia has superior specifications than the SIM mission in many respects. The Gaia dataset will be ripe for exoplanet detection. Moreover, I also expect data from Gaia to throw the hypothesis of non-baryonic dark matter into deep distress and set the dominoes in motion that will bring on the eventual downfall of the entire Dark Energy / Cold Dark Matter (LCDM) paradigm. Future historians of science will contemplate the present malinvestment into Dark Energy research with much irony.
Does anyone know if a Cancellation Review is required as SIM is closed down?
New to CD, but on this I feel strongly: the real goal now must be to get the atmospheric spectrum of extrasolar planets, especially those in the Goldilocks zone. SIM wouldn’t do that. I think the emphasis should be on technology development, both ground-based and potential space-based telescopes to achieve the goal. Let it be an engineering race between possible Terrestrial Planet Finder and terrestrial architectures. Finding significant amounts of oxygen in the atmosphere of a roughly Earth-sized planet in the habitable zone would energize the public imagination for space exploration more than almost anything else I can imagine.
SIM versus Gaia:
– SIM was designed to have an astrometric precision of ~2 µarcsec.
The Gaia performances have been degraded (for budgetary reasons) down
to ~25 µarcsec, i.e. 10 times worse than SIM. That exludes the detection of
terrestrial planets with Gaia.
– Gaia will only observe stars fainter than visual magnitude 6 (to avoid
detector saturation), while SIM did not have such limitation. Therefore Gaia
will not observe alpha Cen A,B.
– SIM would have observed only ~ 60 stars (the nearest ones), while Gaia has
the capacity to detect giant planets around ~10,000 stars.
Dear Administrator (Paul),
What is interesting about discussions of possible planets orbiting Alpha Centauri A,B and tight binaries in general, is the diversity of scientific opinion regarding the kinds of planets that can form in such systems. I say ‘the kinds of planets’ because the more basic question of whether or not planets can form and exist in close binaries has been resolved; indeed, we know of several systems in which planets exist in binaries with average separations of less than 50 A.U. e.g. the gamma cephei system, Gl 86, NN Serpentis, DP Leonis, HW Virginis, HD 196885, HD 41004). However, all of the planets in the aforementioned systems have something in common: they are massive— no know examples of sub-Jupiter mass planets in close binaries exist to date. This absence of smaller planets in short-period binaries, planets that most likely form via core accretion, leads one to speculate that perhaps only large giant planets can form in these systems via a mechanism other than core accretion.
In any case, below are the titles of two very recent papers that give differing prospects for the likelihood of small core accretion formed planets existing around one or both of the alpha centauri stars:
“From Dust To Planetesimal: The Snowball Phase ?” Thebault et al (2010)
Key statement:
“For the specific case of close binaries such as ? Centauri, the role of snowball growth could be even more important. Indeed, it provides a safe way for bodies to grow through the problematic ?1 to 50 km size range for which the perturbed environment of the binary can prevent mutual accretion of planetesimals.”
“The first stages of planet formation in binary systems: How far can dust coagulation proceed?” Dullemond 2010
Basic idea:
Previous studies of planet formation have focused on the planetesimal stage, but this paper actually explores whether or not the initial dust can even come together to form large rocks and boulders, let alone bulky planetesimals in close binaries in the first place.
So, we have yet to settle the matter as to whether or not small planets can form via core accretion in close binary systems. I have several questions of my own regarding the matter:
1). Even if it turns out that planets cannot form via core accretion in short period binaries, what is the minimum mass planet that could form via, say, gravitational instability in such a system?
2). Alpha Centauri A and B, based on their determined metallicities should easily have had enough material to form large rocky planets around them. If the planet formation stage did not even progress beyond the dust phase, then why have we not found evidence of a mammoth infrared-emitting sterile dust or planetesimal disk around either one of these stars, as I thought colliding planetesimals continuously generate lots of dust? Is it because (i) the material could have spiraled onto the stellar surfaces, or (ii) because the material is indeed in the form of several inner terrestrial planets?
Lastly, I would like to add that the Kepler mission is looking not just at single stars for its planet search; therefore, it should provide interesting constraints on the prevalence and nature of planet systems around multiple star systems.
Excellent points, spaceman. We have much to learn about planet formation around close binaries. For those wanting to catch up with some of the recent work, I cover some of these issues in these stories:
https://centauri-dreams.org/?p=11083
This one discusses Ji-Wei Xie et al., “Planetesimal Accretion in Binary Systems: Could Planets Form Around ? Centauri B?” and
https://centauri-dreams.org/?p=11083
On Thébault et al., “Planet formation in the habitable zone of alpha Centauri B.”
The Thébault paper spaceman mentions is “From Dust To Planetesimal: The Snowball Phase ?”, available as a preprint at the arXiv site at
http://arxiv.org/abs/1009.4636
Dullemond et al., “The first stages of planet formation in binary systems: How far can dust coagulation proceed?” is likewise available as a preprint:
http://arxiv4.library.cornell.edu/abs/1010.4210
and both are in queue here for later discussion.
@spaceman:
Question is whether the apparent absence of low-mass planets in such systems is due to observational bias, since close binaries are more difficult to observe and obtain the required precision, plus you’ve got to correctly account for the radial velocity variations due to the companion star. They also seem to have been routinely avoided by most of the early exoplanet surveys. I’m really not sure that there is enough evidence yet to claim a genuine absence of low-mass planets around close binaries.
Furthermore you’ve listed two very different classes of systems: those with planets in circumbinary P-type orbits (NN Ser, HW Vir, DP Leo), and those in S-type orbits around one of the stars (? Cep, Gl 86, HD 196885, HD 41004). Definitely in the case of the P-type orbits the issue is observational accuracy: in each of these 3 cases the planets are found by timing the eclipses of the binary star. A Neptune-mass planet in a circular, edge-on orbit at 5 AU from NN Serpentis would cause a timing variation with a semi-amplitude of only 0.2 seconds over an orbital period of 13.9 years. Wider orbits give larger timing variations, but you have to wait longer to observe the full orbit.
I have enjoyed the comments, both pro and con concerning the fate of SIM. Let me address a few of the issues brought up.
I should have mentioned in my Space Review article that $600 million has already been invested in SIM. That is one major reason why I think that SIM should be allowed to proceed to Phases C/D and launch. Those funds have been used over the past 10 years to hone the design of the spacecraft. The team has focused on engineering risk reduction and has successfully accomplished that feat. I do not want to see about half of a billion dollars of taxpayers investment abandoned.
Now, one has to ask the question of whether it is logical for NASA to manage a project as it has with SIM. Is it logical to approve a project, see it through Phases A and B, sink over half a billion dollars in design and prototype hardware, then walk away from the effort? Is this a smart way to spend the public’s money? I believe that NASA needs to be steadfast in its support for a project that has successfully met milestone after milestone. It needs to “light this candle.”
“Schneider” makes some interesting observations. As far as I can tell from his sources, none of the Earth-based efforts will accomplish what SIM is designed to do, i.e., find Earth-mass planets. Some of them may find Super-Earths, but by definition, those are not Earth-clones like the ones that SIM will find.
“Schneider” also refers to some other interesting space missions that my equal what SIM can do. However, ALL of those missions exist merely on paper. If NASA can cancel an Exoplanet mission that has been approved, proceeded to the threshhold of Phase C, has had $600 million sunk into its development over a 10-year span, has built high-fidelity prototype hardware, then can we have any sliver of hope that it will ever approve any of those paper spacecraft?
That is why I am so adamant that NASA not abandon SIM. This project is real. It is not a concept, not a dream. Engineers have labored over its details for a decade. It is ready to go. If we, as a nation, turn our backs on this effort, will we ever muster the will to take up the task in 10 years or 20 years from now? Do we think that NASA will ever proceed to building a TPF or a Life Finder or a Planet Imager if it can so easily cast aside this present effort, an effort needed to find targets for those future missions?
If NASA decides to proceed with one of the paper spacecraft, then the earliest that this can occur is the year 2020. There are no plans to fund ANY Exoplanet effort in the next decade that will look for nearby Earths. If the next Decadal Survey of 2020 recommends such a mission, then after the time lag for development, its construction will probably not occur before 2025, with a launch by perhaps 2028 0r so. Remember though that there is no guarantee that the 2020 Decadal Survey will approve such an effort. That leaves us with hopes for the 2030 Decadal Survey.
Do you see where this is leading? If SIM is tossed aside, then we have a long wait for a similar mission.
“Schneider” does make some good comparisons between SIM and GAIA. GAIA is a great mission, but will be unable to find Earth-mass planets. I would quarrel with his last comparison. SIM will not merely look at ~60 stars, the nearest ones. Examine this detailed report on SIM’s science plan, “Taking the Measure of the Universe” –
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40906/1/07-4071.pdf
In this report, you can read that, in addition to its search for Earth analogs around the nearest stars, it will conduct a Broad Survey of a few thousand stars that will detect Super Earths and Jupiter-class planets. In addition, the report reveals that SIM will be able to detect planets with masses as low as 0.2 Earth mass, i.e., about the mass of Mars!
Elsewhere in this report, you can see that SIM will also revolutionize other areas of astronomy besides the field of Exoplanets. It will be an exciting mission!
I do not work for NASA. I am not seeking grant money from NASA. This allows me the freedom to comment on the saga, and politics, of the SIM mission. Most of those who are most familiar with the capabilities of SIM, however, do not enjoy such freedom. Their ties to NASA projects or funding limit what they can say. They risk professional ruin if they speak out. I hope that I have accurately described the wonders that SIM will reveal. The efforts of so many dedicated people now hangs in the balance. The readers of “Centauri Dreams” now know how the future of our dreams of other Earths also hangs in the balance.
Phil Horzempa
Schneider: Some of your “facts” seem not to be — references please?