I'm designing a hypothetical newly discovered planet in our solar system that has an ecosystem comparable to Earth's that supports intelligent life. The explanation given for the planet remaining undiscovered for so long is that a lot of dust has accumulated in the planet's L1 Lagrange point and this has had the effect of obscuring it from Earth telescopes. In the current design, the planet's ecosystem is a universal desert with the minimum water required to sustain a water cycle. The planet has an orbit that is perpendicular to the ecliptic plane at the same distance from the sun as Mars and exactly 90 degrees to Mars' orbit, in orbital resonance with the red planet so that they never collide. The planet has a strong magnetic field, and the gravity to retain its atmosphere, which resembles Earth's, but with more greenhouse gases so as to retain more heat - it is further from the sun than Earth after all.

My question is - would the accumulation of dust in the L1 Lagrange point also block the sun's heat to the point the planet would become too cold to support life? To plausibly avoid detection from Earth until recent history, would it have to be even further away from the sun as well has having an eccentric angle of orbit? Would it work better to make it an undiscovered desert planet closer to the sun than Earth, with an accumulation of dust in the L2 Lagrange point?

  • $\begingroup$ You migth want to ask this question in the worldbuilding stack exchange. $\endgroup$ May 26, 2022 at 17:15
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    $\begingroup$ There would be no need to see the planet to detect its presence, its effect on the motion of other planets would be enough to detect it anyway, even without visual confirmation. $\endgroup$ May 26, 2022 at 20:30
  • $\begingroup$ Shouldn't the Kozai mechanism eject this hypothetical planet due to its high inclination? $\endgroup$
    – WarpPrime
    May 27, 2022 at 12:02
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    $\begingroup$ This is incidental to the question, but a planet doesn't have an L1 Lagrange point. Lagrange points are defined for two bodies orbiting each other. So the Sun-Earth system has Lagrange points, or the Earth-Moon system has Lagrange points. The Earth alone doesn't have Lagrange points. $\endgroup$
    – zephyr
    May 27, 2022 at 20:20

1 Answer 1



The L1 point is unstable and dust doesn't accumulate there.

A dust cloud could not be large and dense enough to obscure a planet without being so massive that it would contract under its own gravity, and form a new moon.

Moreover, dust is not invisible. If there was a huge dense cloud of dust, it would reflect the sun light and we would see it. If it was perfectly black it obscure the stars behind it and we would see it.

  • $\begingroup$ Additional, also a dust cloud in, say, Venus' L1 point would not stop us from detecting Venus due to the phase angle $\endgroup$ May 26, 2022 at 7:35
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    $\begingroup$ Ah I see. I presume as well that a close in planet orbiting at a strange angle to the ecliptic would be easy to spot. At what distance would a planet orbiting perpendicular to the ecliptic be overwhelmed by the lights of the galactic plane? $\endgroup$ May 26, 2022 at 9:02
  • $\begingroup$ There are quite a lot of unknowns! A planet comparable to the Earth, at about 10AU (similar to Saturn) would be close to the limit of naked eye visibility, and would be unlikely to have been discovered prior to 1781 (when Uranus was discovered) and could conceivably have remained undiscovered until systematic photographic surveys in the early 20th century. It is challenging to design a "habitable" newly discovered planet $\endgroup$
    – James K
    May 26, 2022 at 16:55
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    $\begingroup$ However this is no longer anything really about "Astronomy" so perhaps further questions could be asked on Worldbuilding @Emeraldminer $\endgroup$
    – James K
    May 26, 2022 at 21:05
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    $\begingroup$ Ah OK. There are definitely other questions about planet design in Astronomy though. The question about habitable zone limits is definitely Astronomy. I notice that there is disparity in opinions about what the limits of the habitable zone are. A recurring theme is that planets with uniform desert ecosystems are habitable both closer in to the sun and further out as well. $\endgroup$ May 27, 2022 at 9:39

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