Probing Exoplanets

Sometimes topics segue perfectly. With the recent buzz about habitable planets, followed by the raining on the parade articles we’ve had about the not insignificant errors in the detections of planets around Gliese 581 as well as finding molecules in exoplanet atmospheres, it’s not been the best of times for finding life. But in a comment on my last article, Lawrence Crowell noted: “You can’t really know for sure whether a planet has life until you actually go there and look on the ground. This is not at all easy, and probably it is at best possible to send a probe within a 25 to 50 light year radius.”

This is right on the mark and happens to be another topic that’s been under some discussion on arXiv recently in a short series of paper and responses. The first paper, accepted to the journal Astrobiology and led by Jean Schneider of the Observatory of Paris-Meudon, seeks to describe “the far future of exoplanet direct characterization”. In general, this paper discusses where the study of exoplanets could go from our current knowledge base. It proposes two main directions: Finding more planets to better survey the parameter space planets inhabit, or more in depth, long-term studying of the planets we do know.

But perhaps the more interesting aspect of the paper, and the one that’s generated a rare response, is what can be done should we detect a planet with promising characteristics relatively nearby. They first propose trying to directly image the planet’s surface and calculate the diameter of a telescope capable of doing so would be roughly half as large as the sun. Instead, if we truly wish to get a direct image, the best bet would be to go there. They quickly address a few of the potential challenges.

The first is that of cosmic rays. These high energy particles can wreak havoc on electronics. The second is simple dust grains. The team calculates that an impact with “a 100 micron interstellar grain at 0.3 the speed of light has the same kinetic energy than a 100 ton body at 100 km/hour”. With present technology, any spacecraft equipped with sufficient shielding would be prohibitively massive and difficult to accelerate to the velocities necessary to make the trip worthwhile.

But Ian Crawford, of the University of London, thinks that the risk posed by such grains may be overstated. Firstly, Crawford believes Schneider’s requirement of 30% of the speed of light is somewhat overzealous. Instead, most proposals of interstellar travel by probes generally use a value of 10% of the speed of light. In particular, the most exhaustive proposal yet created, (the Daedalus project) only attempted to achieve a velocity of 0.12c. However, the ability to produce such a craft was well beyond the means at the time. But with the advent of miniaturization of many electronic components, the prospect may need to be reevaluated.

Aside from the overestimate on necessary velocities, Crawford suggests that Schneider’s team overstated the size of dust grains. In the solar neighborhood, dust grains are estimated to be nearly 100 times smaller than reported by Schneider’s team. The combination of the change in size estimation and that of velocity takes the energy released on collision from a whopping 4 x 107 Joules, to a mere 4.5 Joules. At absolute largest, recent studies have shown that the upper limit for dust particles is more in the range of 4.5 micrometers.

Lastly, Crawford suggests that there may be alternative ways to offer shielding than the brute force wall of mass. If a spacecraft were able to detect incoming particles using radar or another technique, it is possible that it could destroy the incoming particles using lasers, or deflect it using a electromagnetic field.

But Schneider wasn’t finished. He issued a response to Crawford’s response. In it, he criticizes Crawford’s optimistic vision of using nuclear or anti-matter propulsion systems. He notes that, thus far, nuclear propulsion has only been able to produce short impulses instead of continuous thrust and that, although some electronics have been miniaturized, the best analogue yet developed, the National Ignition Facility, is, “with all its control and cooling systems, is presently quite a non-miniaturized building.”

Anti-matter propulsion may be even more difficult. Currently, our ability to produce anti-matter is severely limited. Schneider estimates that it would take 200 terrawatts of energy to produce the required amounts. Meanwhile, the overall energy of the entire Earth is only 20 terrawatts.

In response to the charge of overestimation, Schneider notes that, although such large dust grains would be rare, but “even two lethal or severe collisions are prohibitory”, but does not go on to make any honest estimations of what the actual probability of such a collision would be.

Ultimately, Schneider concludes that all discussion is, at best, extremely preliminary. Before any such undertaking would be seriously considered, it would require “a precursor mission to secure the technological concept, including shielding mechanisms, at say 500 to 1000 Astronomical Units.” Ultimately, Schneider and his team seems to remind us that the technology is not yet there and that there are legitimate threats we must address. Crawford, on the other hand suggests that some of these challenges are ones that we may already be well on the road to addressing and constraining.

31 Replies to “Probing Exoplanets”

  1. Terrawatt is a unit of power, not energy. Power is the rate of energy transformation.

    I’m very pessimistic on using antimatter for space propulsion. I think everyone knows that is has to be stored in “magnetic bottles” to prevent unintended contact with matter that is not intended for use in controlled annihillation of the fuel. Maintenance of a magnetic field will be from electromagnets, and would need uninterrupted power. One interruption, one large explosion. I don’t foresee the level of reliability to allow this, given the great leap of being able to make any significant quantity, which is not on the horizon.

  2. Production and confinement of anti-matter for an interstellar space travel would not be so practical to be accomplished. I still believe better option would be the aneutronic propulsion.

  3. The two main obvious problems with anti-matter are first in producing it and secondly in confining it. A tank with a metric ton of anti-matter is an enormous bomb if the mechanism for bottling it fails. This bottle must be such the anti-matter does not actually touch the walls of the bottle. It is really only possible to confine charged particles in a magnetic bottle, and even there one has holes at the end of bottle. Yet, a kilogram of unit charged protons would have an enormous electric potential. Producing anti-matter is very hard as well, where the total amount of anti-matter humanity has ever produced in high energy experiments is (as I recall) less than a gram. The only realistic way to get a relativistic rocket is to use some mechanism to violate baryon number. This might involve quantum black holes or some other physics which connects particle physics with quantum gravity in some way that is beyond our ability to prognosticate. This is possible, but a lot of theory needs to be worked through, then some experimental work must follow that, which might then be brought to engineering. This is likely 22nd or 23rd century technology.

    The most reasonable way is to use the photon sail, which I illustrated with connection to the Gliese 581g issue. I really think if we got ambitious about that we could get a gamma = 1.15 or v = .5c spacecraft. This involves focusing a collimated beam of solar radiation onto a large reflecting sail, where a large Fresnel lens in space is used to focus all this light. There is nothing particularly exotic required to do this. However, that does not mean it would be easy or cheap. Yet I think we could send this sort of craft to Gliese 581 and get data return in about 50 years. So newly minted PhDs who launch this spacecraft would likely be retirees by the time they see fruit of their work. This type of space science is somewhat analogous to the construction of cathedrals during the Middle Ages, which in some cases took a century.

    There are two ways of minimizing the problem of impacts. The first is not to send your spacecraft out near the plane of the solar system. This avoids sending your craft through the zodiacal dust lane. This is a problem with Gliese 581, for it is in Libra and on the zodiac. The other is to make your actual spacecraft needle shaped to avoid the cross section you present to any dust particle. The sail is of course a large disk and likely to be impacted. However, it is a thin sheet and any dust particle will largely just pass through it and leave a small hole of little consequence. BTW, it will require tests of material to insure this happens, where some semi-relativistic gun will have to fire 1-100 micron size particles into thin sheets.


  4. I see you mentioned project Daedalus. That study was done in the 1970s, and so is a bit out of date (not as out of date as I think we’d all like, but a bit). Project Icarus just began last year, and it is essentially a 5 year study intended to be an updated version of Daedalus with modern technology and improved assumptions.

  5. I seem to remember reading somewhere that it might be possible to passively confine antimatter in tiny amounts (basically using a material that forms into a lattice and has some *very* special properties, and storing the antimatter in the holes in the lattice. No idea how you’d get it out… or in for that matter).

    Has anyone seen any serious speculation about that idea?

  6. First is there any possible way in quantum mechanics to convert us into a bunch of photons that instantly travel al light-speed and somehow materializes us again at the right time when we arrive?

    Another way is if we somehow could influence the higgs boson influence so space becomes none-resistant or so. We would move at instantaneous light-speed.

  7. From a layperson’s perspective, it seems we’re at least a century from developing the appropriate spacecraft technology to explore even the nearest star system. LBC’s photon sail proposition seems on the surface to be a reasonable idea, yet the large sail may present a problem. IMHO, the only realistic solution for reaching Gliese 581 is to develop a propulsion system that has yet to be invented. Perhaps if some of the brain power that has been dedicated to the development and production of smaller and faster computer hardware were diverted to this challenge we may see a suitable craft in less than 50 years. The problem is, no one to this point has figured out how they can realize a profit from this sort of venture. We need another Jobs or Gates dedicated to star system travel and meaninful exoplanet discovery.

  8. Interstellar probes can be propelled at reasonable speeds via fission or fusion reactions if we used nuclear pulse drives. The chief problem is the limitation of all rockets – the Tsiolkovski equation, which means mass-ratios rise exponentially with speed. One possibility is to beam the propellant to the vehicle as a series of small pellets of fuel. That way the limitations of laser-sails can be avoided.

  9. I realize this isn’t the topic of the article, but this:

    You can’t really know for sure whether a planet has life until you actually go there and look on the ground.

    doesn’t conform to any science I know of.

    If we try to quantify the claim and apply it on science we find that all we need to do to make sure within the context of a theory is to test its predictions. And to make sure for all theories, test “all applicable” ones. (Granted, generally a hard task to discern and do.)

    As an analogue, always risky but sometimes worthwhile, the above claim would be socially equivalent to claim that we can never know for sure that someone has been murdered unless we witness it firsthand (whatever that means). it isn’t enough to find a corpse with a deadly knife cut in the back.

    In the same way we would never be able to rule people guilty of murder without “first hand accounts”. All the rest of the evidence gets thrown out of court, because we have to “look on the ground”.

    Perhaps the papers gets into this, but a certain prediction of a biosphere is a temperate climate combined with a thermodynamically unbalanced atmosphere on the order of Earth’s oxygen atmosphere. Photosynthesizers were mainly unicellular, the oxygen catastrophe happened well before the advent of multicellular organisms, so it will be a general characteristic of biospheres.

    [In fact, photosynthesizers are still mainly unicellular, the oceans still stand for ~ 70 % of Earth oxygen production AFAIK. In the same way that most biomass is still in biofilms. I, for one, hail our Unicellular Galactic Overlords!]

    Photosynthesizers were early too, the first signs of life at 3.5 Ga was photosynthesizers containing stromatolites. The oxygen catastrophe was at ~ 2.5 Ga ago IIRC, so it will also be a general characteristic of observations of inhabited planets, ~ 70 % of young biospheres @ 4 Gy, and rising to virtually 100 % of them around long lived M stars.

    [I know Pamela Gay claims the energy content of M stars light is too weak for photosynthesizers, but I haven’t seen anyone crunch the numbers. This year a 5th chlorophyll pigment, chlorophyll f, was found in ancient bacteria that adds a layer of IR photosynthesizers living below (either in films or as water depth) the common ones. And our biosphere isn’t even trying to niche into IR in general, due to the plentifulness of visible light!]

    And to test this theory we don’t need sun-sized telescopes. Characterizing the gas content of exoplanet atmospheres is ongoing with todays equipment. Similarly no other mechanism is known, AFAIU, that with temperate climate (temperature, UV) gives the same prediction!

  10. For kicks: Wikipedia’s M star classification is surface temperature 2,600–3,850 K. Unless I’m mistaken chlorophyll f max absorption @ 706 nm corresponds to a max emission black body emitter @ 3830 K.

    I can’t see why at least some M stars inhabited planets will have fully oxygenated atmospheres.

    My d’oh! above: “young biospheres @ 4 Gy” – young biospheres < 4 Gy. (Not really pertinent anyway, because we can add star/planetary system age to star UV emission and planet surface temperature to possible observations or rough estimates we can make, to complete the necessary set when testing the biosphere theory.)

  11. A lot of this stuff is years off but there is harm in discussing it.

    Ideally what we would need is a anti-matter production facility on Mercury.
    Mercury is near the Sun and could use solar power to provide the large power requirements. Lots of particle accelerators I think are the only known way of making anti-matter, so the facility would be a collection of solar power stations and particle accelerators.

    Mercury is also remote location in case of containment failure.
    Another option may be to use an asteroid orbiting close to the Sun, same benefits except you don’t lose a planet if something goes wrong.

    As to if anti-matter can ever be made in large quantities and stored and used without blowing something, I think no one will know unless we try it, but lets try it somewhere other than Earth.

  12. Missing a word in the first line.
    A lot of this stuff is years off but there is ‘NO’ harm in discussing it.

  13. There is a quantity involved with rocket propulsion called specific impulse. This is the velocity of the gas in the rocket plume, or reaction mass, divided by g = 10ms^2, s = v/g. The specific impulse of chemical rockets is around 500 seconds. This means the hot gases escaping the rocket nozzle of a modern launch vehicle such as the Delta or space shuttle is 5000m/sec. This is rather impressive, but it only gets you so far. A chemical rocket can reach about 20km/sec top speed. A nuclear propulsion system does better with s = 1000 to 2000 for a solid core reactor. Such a craft could get to about 100km/sec. Liquid and gas core reactors, where a gas core reactor is very tricky and dangerous —- it borders on being a controlled nuclear bomb, can get to 5000 and maybe 10,000 respectively So we have a max v = 500km/sec. Well this only about v = 0.0017c, or .17% light speed. A nuclear propelled VASIMR propulsion can have s = 10,000 or more, and could maybe get to .01c. Fusion might get to .1c. However, if you shoot photons out the back of your rocket the plume velocity is the speed of light and s = 3,000,000. Now you are “cooking with gas,” and a photon rocket can reach respectable relativistic velocities.

    Getting that photon rocket requires converting mass into energy with almost 100% conversion. Nuclear power is only about 1% conversion, and chemical is less than a thousands of that. That is the key, and to get to relativistic velocities with a rocket you need huge conversion factors. This is where antimatter comes in to play. You either must have that or some means to violate some basic quantum number of particle field physics, such as the baryon number. A proton, p, has baryon number one, and the antiproton, p-bar, has baryon number -1. So p plus p-bar has total baryon number of zero. To make antimatter you have to generate pairs of particles, such as p plus p-bar from some high energy process. So it is not an energy source, but more of an energy storage system. To really make this work we would need some way of violating the baryon number so that protons, a big fat composite QCD particle with quarks etc, can be converted to an anti-electron plus photons.

    Is this possible? — Maybe. When a proton collides with another proton it creates quark-gluon plasma, a QCD plasma. It turn out this has black hole features. This is particularly demonstrated for the collision of heavy ions. The QCD plasma might be thought of with some other dimension with a length L, as setting at the floor at 0, and splashing upwards some. There is a net force, say gravity from the top at L to the bottom, some this splash tends to puddle out a bit, and then it decays by Hawking-like radiation. Now at the top at L you have actual black holes, and as our blob splashes at high enough energy it reaches closer to the top, from the floor to the ceiling. This might begin to have features more related to black holes, which include baryon number violation.

    Making this into a working technology is very far into the future — believe me.


  14. So LC would we need a superconducting magnetic energy storage system beyond anything we have ever made or designed, which in itself would be challenging to accomplish, or would we need to invent a completely new form of energy storage?

  15. I vote for LC’s method. Anti-matter or fusion engines seem a long way off in comparison to the photon sail suggestion. Nuclear Pulse propulsion seems inefficient.

  16. Into the Future… Hang on! Here we go!

    I’ll wager that we will have interstellar travel within a half century! I’ll base that claim on several points. First and foremost, our computers and software are evolving rapidly and will allow explorations into new realms of theoretical physics… especially electrodynamics concerning the properties of magnetic field generation and confinement. (The serendipitous discovery of crystalline shapes in plasma aboard the Space Station by the Russians comes to mind here!)

    Secondly, in the sheer number alive on the Planet, odds are there is/will be a genetically enhanced individual(s) born who has the mental capacity to design/create a breakthrough in technology.

    Third: The possibility that humanity has reached a ‘critical mass’ and point in time where Cosmic Consciousness becomes the next part of our evolution.

    Fourth; We MAY stumble upon interstellar travelers who share their technology.

    And lastly… we might expect a miracle~

  17. AQUA: “And lastly… we might expect a miracle~”

    You’re not going all religious on us, are you? 😉

  18. Using magnetic fields to bottle up charged particles is the problem. The problem is independent of the means by which you generate the magnetic field. The reason is because the magnetic field has no monopole charge —- at least not under ordinary conditions. The poles for magnetic fields are N and S and they always come in pairs, or as a dipole. On case of a dipole is a solenoid, which has inside the solenoid magnetic field that is fairly uniform. A particle which heads directly along the axis of the solenoid will pass through without being deflected by the Lorentz force F = qvxB, q= charge, v the velocity of charge, x = cross product and B = magnetic field.


  19. My “Expect a miracle” comment was made for my own personal gratification in that I would definitely like to travel to the stars and poke around a bit! And a miracle would appear to be just about the only way I’ll be able to do that…

  20. @ Aqua

    Secondly, in the sheer number alive on the Planet, odds are there is/will be a genetically enhanced individual(s) born who has the mental capacity to design/create a breakthrough in technology.

    Not even someone like Sheldon Cooper would be sufficient, I fear. Maybe an offspring of the “Shamy” could have that capacity, but is that likely?

    😉 (Btw: The new season of TBBT is just awesome!)


    I was thinking more along the lines of a child prodigy born to a dirt poor farm family somewhere in India or Uzbekistan but will probably be Chinese instead… its in the numbers.

  22. Thats a good point Aqua, history has always underestimated the contribution of a single individual, maybe there is a young einstein out there formulating a means to go FTL. I hope she/he doesn’t keep it to themselves.

  23. Yes, I know that, and I didn’t mean to offend, in case someone is thinking I was. It was merely supposed to be a joke. 😉

  24. I think there is an idea we can take away from all this:

    We should start thinking seriously about interstellar travel.

    Likely, it’s only a matter of time before a planet very similar to Earth can be resolved from it’s host star. What then? Is it more expensive to observe it in detail from here?

    We may need to physically get there to solve some of the greatest questions ever asked.

    A lot of comments here have different ideas about how to get there. Yet lets face it, most are on a technological scale, many orders of magnitude greater then present-day propulsion technologies. Many may physically be possible – inefficient – yet possible.

    Perhaps it’s a bit like having a garage full of vehicles and deciding which transport is best to hit up your distant friend’s pad. Should I take the bike? The scooter? Skateboard? Segway? Walk? Or Maybe drive that fancy new car you’ve been building?

  25. Sheldon Cooper eh? I had to look him up…I’m not a big fan of commercial TV. Worth a watch, eh?

  26. @ Aqua

    The Big Bang Theory worth a watch? Yes. YES! I have such a good time watching it. It’s absolutely hilarious and, more importantly, the science is correct! They have a real science advisor and the people shown could actually exist (and I guess they do in one way or another…). If you get a chance, take a look! I’m quite sure, you won’t regret it.

  27. It is unlikely we can ever send anything beyond 50 light years out, and 25 ly is probably a more realistic limit. Warp drives, wormholes and the like are in my opinion ruled out. If we send probes to a number of stellar systems in a 35ly radius there are close to 2000 stars to choose from, which is a tiny fraction of the stellar systems in our galaxy. Our galaxy is one out of 100 billion observable on our past light cone — which in turn are only 10^{-22} of the total! So we would be looking at a tiny sample. So if we sent 10 probes out to nearby stars these would be chosen for their proximal configuration for an Earth-like planet and in order to get an idea of stellar systems at various stages of evolution or different configurations.


  28. Earth’s recent 1970’s radio and tv wave signals, are now passing through more then 2,000 nearby star systems within just 35 light years. These planetary life sign emissions are what our probes can search for to detect intelligent life in nearby star systems. Eventually we’ll be able to detect earth sized inner planets from earth. Current technology was limited to nepturne sized planets with outermost orbits. Recent breakthroughs at ua discovered the smallest and innermost exoplanet of all time named beta pictoris b. still though jupiter sized. the scientist used his own unconventional mathematical approach to model the aprodizing phase plate by an intricate etched phase pattern that blocks out central starlight, allowing exoplanets to show up. Eventually a more intricate pattern etch on the APP may reveal small inner earth like planets, and even a way to search for tv radiation !

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