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?