# Can a planet orbit two stars, the first sun being like Earth's Sun, the second and larger star only visible from the planet around the horizon?

My planet orbits two suns. The orbit of the first sun is similar to Earths orbit around our sun. The orbit around the second sun however makes it appears only around the horizon. Is this possible?

• Each place on a planet surface has a different horizon.
– L.Dutch
Commented Sep 5, 2019 at 11:23

Here's another option: the second sun is actually the primary sun. It's a small red dwarf, to which the planet is tidally locked--but which it nevertheless orbits at a sufficient distance to receive insufficient warmth. For a large ring of the surface of the planet, that star will only ever be visible near the horizon. That whole star+planet system then orbits around a sunlike star much further away, which would rise and set normally everywhere.

• Is there any way to avoid the planet being tidally locked to one star? Commented Sep 6, 2019 at 9:35
• @AvengingEarth Not if you want the arrangement to be permanent. There are other ways to get two suns where one remains near the horizon in some region of the planet for "a long time", but what else will work depends on how long you need it to last and how much like our sun you want the moving sun to appear. Commented Sep 6, 2019 at 16:59

This is not possible for all locations on the planet, but might be possible at some locations. For example the Sun always appears very low on the horizon in the Arctic and Antarctic, but is sometimes directly overhead in the tropics.

The second sun would have to have an orbital inclination with respect to the plane of the ecliptic that exactly matched the planets axial tilt. In this way the second sun would appear to continuously move around the horizon as viewed from the poles, but would increasingly dip into a day and night cycle moving away from the poles and at the equator it would appear to cross the sky at a high angle.

A complicating factor is that the planet would orbit around the centre of gravity of the system as a whole (both stars) so the planets axial tilt might have to be adjusted to make this work.

There is also the issue of orbital stability. Such a system might prove to be unstable, depending on the exact masses of the planet, the stars and their respective orbital paths. Many such arrangements are unstable.

• The sun is at the horizon at the poles only during the equinox.
– L.Dutch
Commented Sep 5, 2019 at 11:30

Is a planet capable of existing in a two-star system? Sure.

However, a planet cannot orbit two stars simultaneously (edit: but they can orbit a binary star system, as pointed out by @M.A.Golding). Orbits are a result of gravity pulling on an object that is attempting to accelerate perpendicular to the direction of gravity, as shown quite neatly below:

For this reason, your planet cannot 'fall' in between two stars. The star with the greater gravitational pull will keep it locked. In the event your planet reaches a point where the resultant gravitational force is zero during its orbit, it will fly off at a tangent, and the star with the greater gravitational force at this point will pull it in. This is complicated astrophysics, and likely unsustainable.

EDIT: It's also worth saying a planet caught in between two stars would be torn in half. The gravitational fields would be opposite in direction, so you can also be certain a planet would never have naturally formed here.

To answer your question though, perhaps your first sun could be the larger, and your second sun could orbit this independent of your planet (i.e. they don't orbit each other). Hypothetical systems like Nemesis show how you could make this work, and with some thinking you could definitely position your orbits in such a way that the second, smaller sun was only visible on the horizon (at certain times). Again, you need to really think it through, but it's not beyond the realms of possibility. Good luck!

• A planet could be caught exactly between two stars, at their mutual L1 point, but only briefly (in astronomical terms). If something - like another planet - perturbed the system even slightly, it would inevitably be drawn towards one star or the other, to either orbit it, crash into it, or slingshot around it and be ejected from the system. Commented Sep 5, 2019 at 11:36
• If the planet had no linear momentum at that L1 point, you'd be right. I guess this could happen, but I'd write such an event down in the "hmm" pile for sure. Commented Sep 5, 2019 at 11:40
• L1 is notorious for not being stable. Commented Sep 5, 2019 at 14:52
• @mcRobusta Actually a planet can orbit around two stars, if they are close enough together, orbiting them would be similar to orbiting a single star. Tha tis called a P-Type or circumbinary orbit, and exoplanets have been discovered with such orbits. Of course a circumbinary orbit would not satisfy the OP requirements, and of course a figure 8 orbit switching from star to star would be highly unstable. Commented Sep 5, 2019 at 18:11
• @M.A.Golding Excellent point- I'll put an edit in my answer. Commented Sep 5, 2019 at 19:09

There is one way this could be true for a portion of your planet: if it's orbiting in/near a leading or trailing Lagrange libration point (L4 or L5), and is tide locked to the primary star.

There are some limitations on a Lagrange orbit, however: the primary must be at least about 80 times as massive as the secondary, and the planet must have negligible mass relative to the to heavier objects (easy, even if the secondary is an M9 barely red dwarf).

Where this falls down is that in order to be tide locked to the primary, your planet would generally need to have a close, short-period orbit, but that orbit must have the same radius as that of the secondary star, and these orbits are only in the habitable zone if the primary is a dwarf star with low luminosity. If that's the case, the 80:1 mass requirement pushes your secondary down into the brown dwarf or even sub-brown (planemo?) category -- "shining" only by radiating leftover heat of formation or from low-level, unstable deuterium fusion (short-lived by cosmological standards).

The other possibility is for a planet in this kind of orbit to have a "day" close to the length of its "year" by happenstance. Venus comes to mind; it's nothing like tide locked, but has a solar day of about half of its year (combine a sidereal day longer than the year, and retrograde rotation). You could pretty readily hand-wave a planet with sidereal day very close to its year, and the "horizon star" would very slowly circle through the sky, along with the sun, in a months-long day-night cycle.