Can a planet have a day that's always longer than night?

This question has been rewritten to incorporate all clarifications.

On Earth, half the planet is illuminated at any time (let's ignore eclipses). Axial tilt lets day lengths vary, but over the course of a year, every location is illuminated half the time.

It's easy to make a planet where, over a year, everywhere is illuminated more than half the time. Use a binary star.

But is there a naturally occurring, stable solar system that satisfies the more restrictive requirement that the planet is always more than half illuminated?

In the general case, if it orbits one star of a binary, there will be a point in its orbit where the other star passes behind the one the planet orbits. If it orbits both stars, there will likewise be a point where all three are in a line. And note that, even if the planet's orbit is inclined relative to the plane containing the stars' orbits, a collinear situation is still possible... barring some resonance that prevents it.

Constraints:

Note that I'm only talking about solar system geometry. Cloud cover means you can't see the sun all the time (though light gets through). Atmospheric refraction and diffraction extend visible light onto the 'night' side; this gets extreme with a dense atmosphere like Venus. I know this, so I'm not asking for answers involving that. All solutions must work for a vacuum world. My purpose is to explore the geometry of solar systems.

The planet must satisfy both of "At any time, >50% of the surface is illuminated" and "At any location, illuminated >50% of the year."

Approximate scale of the effect: Let's say that "more than half" means at least 195/360 of the surface (IE, an extra hour in an Earth day). It must also be light providing meaningful illumination, not just technically visible. Let's say that said area is illuminated to a level at least 1/40 of (should it be "the brightest illumination it receives" or "the brightest illumination Earth receives"?).

Before asking this question, I thought of a Trojan planet of a binary star. I then saw a figure of a minimum mass ratio of 25 for two bodies to generate stable L4/L5 points. With stars, luminosity is roughly proportional to mass to the 3.5 power. This means one star must be at least 78000 times brighter than the other, and the planet is equidistant from them. Given that full moonlight on Earth is about 1/400000 of full sunlight, this is hardly better, nowhere near enough to count as "day". That's why I asked the question.

posted almost 6 years ago

Tristan Klassen‭

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