For the sake of argument, please assume the following:

  1. The planet in question is around the size of Earth
  2. The planet may contain native intelligent life, artificially engineered
  3. The planet is artificially constructed
  4. The civilization (composed of normal matter) constructing the planet or how they construct one is irrelevant
  5. There is an artificial magnetic shield on the L1 point of that planet, made of normal matter, shielding the planet from the local stellar wind (that would cook the planet by continuous matter-antimatter annihilation if the planet wasn't shielded)
  6. The magnetic shield only protects the planet within its shadow (the shadow's (magnetic equivalent of an umbra engulfs the planet up to around 1.2 times the radius of the planet), with a little opening that allows sunlight (just sunlight) to pass through the shield and light the planet
  7. The planet has a moon, about the size of our Moon, but it is composed of normal matter
  8. The civilization constructing the planet (and that put the magnetic shield in-place) is also constructing and operating large arrays of orbital defence system that diverts "normal matter asteroids or comets" from collision with the planet
  9. The star, is a normal G class star, comparable to our Sun


For the sake of experiences, a very advanced civilization constructed an earth-like planet, complete with local biospheres and native intelligent civilization, with a tech level comparable to Earth at the early age of the space race. They were about to start their first moon landing.

Due to the magnetic shield's presence, they could safely went into low orbit (only when they realize a the large magnetic shield is in-place, without knowing the function of the shield (that said, they don't know what kind of threat were shielded by the shield)), but not to space (they can't go to the moon without magnetic shielding, and only find that when they cross region beyond the shield's protection, they'll be cooked with sudden surge of gamma radiations.

The public perceives

  • The shield protects them from unknown evil forces that will beam up ships from going beyond the protection of the shield, with dangerous levels of gamma radiation
  • They know nothing (yet) about matter and antimatter

The questions

  1. What would their scientists postulate about the sudden surge of gamma radiation when they went beyond regions shielded by the magnetic shield?
  2. What would they think of the shield and its purpose, scientifically?
  • 4
    $\begingroup$ Note that a magnetic shield won't protect against neutral particles. $\endgroup$ Commented Aug 29, 2015 at 22:42
  • 2
    $\begingroup$ Wouldn't the natives of the planet notice the "large arrays of orbital defence system"[sic] around the planet? If not directly then by the large explosions caused by the defence system annihilating any incoming space debris? What happens when they get inquisitive and try to visit one? $\endgroup$
    – Steve Bird
    Commented Aug 29, 2015 at 23:14
  • 5
    $\begingroup$ Antimatter was discovered in 1932 (as positrons) so a civilisation identical to 1960 era humans in technological development should also know about them. $\endgroup$
    – March Ho
    Commented Aug 30, 2015 at 0:54
  • 3
    $\begingroup$ Okay, I have to ask. What gives! My world building question list is like 1/2 questions about antimatter! Did a fortnightly challenge about antimatter start? I've never seen so many questions on the same topic! $\endgroup$
    – Cort Ammon
    Commented Aug 30, 2015 at 2:52
  • 3
    $\begingroup$ Don't forget meteors! Tons of matter falls onto Earth every day. You need more elaborate isolation techniques. The reasons to creating this might play heavily on the particulars, so you ought to explore that and not leave it a black box. You can design a backstory that's not revealed in the story. $\endgroup$
    – JDługosz
    Commented Aug 30, 2015 at 3:45

4 Answers 4


You cannot avoid the problem with space dust if the planet is similar to earth. A nice article on cosmic dust shows a number of aspects of the matter hitting our plant daily. The total mass is expected to be somewhere in the range of 5-300 tons daily - yes the number is not very precise, but at least it gives us the general size of the problem. Even at the minimum 5 tons per day, that is a lot of matter.

The article states: Particles with diameters greater than about 2 millimeters produce visible “shooting stars,” but most of the mass of dust particles entering the atmosphere is estimated to be much smaller than this, so can be detected only using specialized meteor radars.

For a 2 mm sphere at an average density of 2.5 g/cm**3 the average mass per particle would be about 0.01 grams and the planet is being bombarded by something like 500,000,000 of these each day. Likely quite a bit more particles as most of the dust is quite a bit smaller than 2mm. Just how perfect do you think your planetary defense system is given the incoming velocities of about 38-248 thousand km/hr and diameter mostly less than 2 mm. 50,000 impacts per day would occur for a 99.99% effective defense.

At 0.01 grams, the incoming particles will have a yields of a 0.5 kt bomb, with a very characteristic radiation signature. Most will be much smaller, but still very easily detected. Furthermore, since most of the cosmic dust will be in the ecliptic plane, there will be a pattern that these events are more common over the equator than the poles (due to the changing angle of incidence). Likewise there will be patterns of heavier activity that correspond to meteor shows. Because of the earth's orbital velocity (30 km/s) around the sun, the number of impacts at the leading edge will be significantly higher than the trailing edge and this will definitely be noticed i.e., the rate at dawn will be higher than at dusk.

The jig will be up long before spaceflight if this anti-earth had the same development history as us. In fact the bright flashes in the sky will make this a very well studied and obvious problem long before spaceflight, though without an understanding of particle physics will be inexplicable. In fact I believe that the light shows would drive an interest in astronomy.

The orbital defense grid needed to be 99.99% effective will be directly observable from the planets surface. You would need large arrays of Hubble-sized telescopes or smaller numbers of more massive telescope facilities for detection. Then you need the defensive equipment and power sources for the actual defense. These will be blatantly obvious to 1950's tech ground based telescopes.

As mentioned by bowlturner, the earth also loses mass -- about 250 tons per day (estimates vary), mostly hydrogen, a little helium and other traces. Some of this will result in noticeable effects from the planets surface too. However, since the bulk of this will be in the form of single atoms and molecules the effect will be much less noticeable than the daily light shows. Once noticed however, there will be an obvious correlation that the effect is a result of something coming from the planet.

I remembered learning about putting micro-meteoroid detectors in early spacecraft in school and found this article on the first one launched. On the Explorer 1 mission launched Feb 1 1958 detected 145 impacts during 11 days for an effective detector area of 0.075 sq. meters. This is 175 impacts per day per sq meter. The detector was designed to detect impacts of 0.01 mm diameter. The surface of the earth is about 510 billion sq. m, so the number of impacts per day is about 90 trillion per day. At a 0.01 mm level, the antimatter explosion would be about the same as a stick of dynamite so I expect quite a few microflashes would be visible in any night sky. An even worse situation for this particular scenario. There are good reasons that astronomers are fairly certain that no large scale antimatter objects exists in the universe.

For those of you that think a 0.5 kton explosion of TNT is not that large, watch the video of Operation Sail Hat.

I forgot about solar wind - but after doing the math, I expect less about 0.28 kg worth of solar wind striking the earth every second - ignoring the effect of earths magnetosphere, but what of course, the solar wind does not ignore the magnetosphere - I would expect the auroras to be supercharged.

I did overlook one other thing when I originally answered the question. As bulk matter the dominant annihilation would be that of proton/anti-proton or neutron/anti-neutron pairs. This type of radiation is not the gamma-ray characteristically produced by electron/positron annihilation that we tend to think of (511 keV). So, the radiation would be even higher energy levels than 511 keV, but the quark annihilations result very short-lived pions that further decay - but this decay also results in neutrinos which carry away over 20% of the original mass-energy meaning that the local explosive effect is somewhat less than expected from 100% mass-energy conversion.

With current satellite technology we do measure cosmic radiation resulting from positrons, anti-hydrogen, and even anti-helium. We do not detect cosmic rays that suggest there is anti-matter source equivalent to a anti-matter mirror-image source of material. I.e, the cosmic rays indicate that the anti-matter striking us is generated by high-energy physics style anti-matter generation, not the result of anti-stars exploding.

I found a New Scientist article announcing the first production of anti-helium in 2011. That should indicate how hard it is to produce heavier anti-elements via high-energy physics. The article also mentions the anti-lithium is "beyond the reach of today’s colliders" Even the theoretical Big Bang nucleosynthesis produces only small traces of any element heavier than Lithium (1 part in 1E15), Lithium itself being only 1 part in 1E9.

Making your own anti-matter planet will be a bit of a challenge too - a very advanced civilization that constructed an earth-like planet out of anti-matter would have really gone to a lot of effort as it appears that you have to make your own anti-matter from scratch.

  • $\begingroup$ I didn't see this coming. Though for history, the planet's age would be a couple of century or less. Guess in no time the first generation will pass their curiosity about astronomy to the younger generation. I believe it would be enough to speed up their astronomy advancement. $\endgroup$ Commented Aug 30, 2015 at 9:43
  • $\begingroup$ The radiation from a matter-antimatter collision (even at microdust scales) is a burst of HARD gamma rays (500Kev for the electrons, multiple 2Gev from the protons and neutrons). The planetary surface will be STERILIZED. $\endgroup$
    – user79911
    Commented Nov 16, 2020 at 9:36
  • $\begingroup$ @marvin the atmosphere will absorb almost all the gamma rays. 52 meters of air at surface density will absorb 50% of 500 kEv gamma radiation. The whole depth if the atmosphere is roughly gamma shielding to 5 feet of lead. Of course so will still get through. This is why long flying commercial flights result in a radiation does, you are above the denser ground layer of atmospheric protection. $\endgroup$ Commented Nov 16, 2020 at 13:53
  • $\begingroup$ image the effect on one larger meteor, 10kg meteors hit the planet every few centuries. that's a 240 megaton yield. $\endgroup$
    – John
    Commented Mar 27, 2023 at 21:17

I think the first problem is that if the magnetic shield didn't cover the entire planet, where ever it wasn't protected annihilation's would be happening bombarding the planet with gamma radiation. Without the shield between the planet and the sun, it would look like a magnesium flare as the planet 'slowly' annihilated away.

However, there is plenty of matter our there and the planet is moving 'across' much of the path of the solar wind. So even if there isn't enough actual gamma radiation to roast the planet, they would be able to detect the shield and it's possible shape from what does react. On top of that the Earth 'loses' mass out into space as it travels, and there would be a 'gamma ray' trail following along behind this planet as anti-hydrogen are left behind and annihilated.

The first craft they send out past the protection of the shield is going to be a large bright fireball as the solar wind immediately starts to to interact with the antimatter of the vessel/probe (this is partly because the 'shield' will be concentrating the matter in the solar wind as a boat hull pushes water into a wave as it travels through the medium) . I am not sure how much mass is in the solar wind but considering how much energy a few grams of interaction can cause, it is possible that it could cause lethal levels of radiation in it's line of site and and possibly other electromagnetic disturbances.

one gram of antimatter annihilating with one gram of matter produces 180 terajoules, the equivalent of 42.96 kilotons of TNT (approximately 3 times the bomb dropped on Hiroshima - and as such enough to power an average city for an extensive amount of time)

  • $\begingroup$ Basically, anything get pass the shield will cook half the planet? How if the shield were bigger? $\endgroup$ Commented Aug 30, 2015 at 6:09

The magnetic field needs to be a good amount stronger than Earth's to keep out the antimatter, so it doesn't annihilate upon impact with the atmosphere and bombard the surface with gamma rays. The people will believe that the gods created a barrier around the planet, to protect them from the "evil demons" that burn and mutate anything and everything. As for the scientists, they'll be a point where they realize that they need a powerful magnetic shield to take people safely into space. Don't know when, but there will be the theory, and there will be the test runs.


You actually have a fundamental flaw in the shield.

Passage of light is in the form of normal photons being absorbed by “antimatter” matter of the planet.

Essentially the sun side of the planet is being cooked by the star continuously for the entire day,

Then it is cooked by the reflected light from the moon and the incident star light. The entire surface of the planet would be radioactively un-inhabitable

The amount of solar energy hitting the plant is approximately equal to 20,000 times the current consumption of the entire human race.

This planet would literally be hell on earth.

As a consequence of this, plants would not be able to convert solar energy into food, as well as all exposed flesh of any animal would literally be destroyed in the resulting antimatter/matter annihilation taking place, so there is no food, and you’re cooked in your own skin.


  • 6
    $\begingroup$ Uhm, photons have themselves as their antiparticles thus there would be no adverse effects. Protons, OTOH, would annihilate, but then we are speaking about solar wind, not light - and that is deflected by the magnetic field. $\endgroup$ Commented Aug 31, 2015 at 12:00
  • 1
    $\begingroup$ You have a fundamental flaw in your answer. Photons != Protons. $\endgroup$
    – bowlturner
    Commented Aug 31, 2015 at 14:52
  • $\begingroup$ @bowlturner, you're correct spell check does wonderful things, but the ultimate responsibility still lays with the monkey, will be corrected. $\endgroup$ Commented Aug 31, 2015 at 16:49
  • $\begingroup$ @Radovan Garabik, researched, and agreed, I withdraw the answer and thank you. $\endgroup$ Commented Aug 31, 2015 at 16:54

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .