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4

It would not be a stable configuration, but it wouldn't be too far from the stability. The are 5 points where a third body could have a semi-stable orbit in a 2-body system: The Earth is the blue point. The "anti-Earth" on the other side would be in the third Lagrange point ($\rm{L}_3$). However, this is not a stable orbit. Any small deviation, including ...


1

Yes, the reference plane for exoplanet orbital inclination is our sky plane. From the NASA Exoplanet Archive documentation: The Observed Inclination is the orbital inclination with respect to the plane of the sky.... 0 degrees correspond to an orbit in the plane of the sky, face on with respect to our line of sight. 90 degrees corresponds to an orbit ...


3

There are many definitions for the "Goldilocks zone" or Habitable zone. If you want liquid water on the surface then that can happen for a wide variety of atmospheric pressures (totally unknown for the vast majority of exoplanets) and temperatures. In fact Mars is considered to be in the habitable zone using only this criteria. More strict criteria exist ...


4

From the Wikipedia article Circumstellar habitable zone, which is just another name for the Goldilocks zone: Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this ...


15

It is possible in a Trojan configuration: In the place of the "Planet" on the image, also a small star could exist. The third star would be at $L_4$ or at $L_5$. This configuration could be made stable. However, as this link shows, In unnormalized units, this criterion becomes $$\frac{m_2}{m_1+ m_2} < 0.0385$$ We thus conclude that the $L_4$...


9

Systems of three stars can exist, but a system of three stars in a triangle is unstable and won't exist in reality. There are configurations of three stars that are stable, for example, two stars in a close orbit about their common centre of gravity, and a third star in a distant orbit. Planets can exist in such a system, they could orbit around the distant ...


4

The year length depends on the distance between the planet's centre & the Sun's centre, not the Sun's surface. So if the Sun merely expands, Earth at 1 AU will still take a standard year to perform 1 orbit. However, when the Sun becomes a red giant, it won't just expand. As Wikipedia mentions, red giants shed a considerable amount of mass in the form of ...


3

Addressing @Allure's comment below @ JohnHoltz's excellent answer, the synodic period is simply a function of the two periods. It will return something like the average value between two successive events where the planets would line up if they orbited in the same plane, but it does not predict the exact times as pointed out in that answer. In addition to ...


1

Gravity. Specifically, the gravity of the mass of stars in the disk. As we move upward above the disk, we are slowing down. The amplitude of the vertical displacement is about 70Pc, or about 110LY up, 110LY down and back again over about 66 million years. In addition, we move from perigalacticon (8,130PC, 26,100LY) to apogalacticon (around 9,040Pc 29,...


4

Mercury's orbit is highly eccentric: 0.21 according to Wikipedia. Therefore, the actual time between repeating occurrences will vary depending on the year. If you were to perform your calculations for many periods, the average should approach the value given by Stellarium. The theoretical synodic period, using the sidereal period of Earth and Mercury, is ...


0

Not really an answer to that specific question, but I think its neat that we could actually measure this reflection on exoplanets. There is a neat astrobite about a recent paper discussing ocean glint on exoplanets. They calculate how this reflection changes the overall spectrum of an exoplanet (with Earth as an example): However, we probably have to ...


1

Sure it does, and it's beautiful! Here's a GIF from the YouTube video Earth from Himawari-8 satellite which I found in but any geosynchronous Earth observation satellite imagery will show this, such as the video Planet Earth in 4K found in this now-deleted answer. See also: this answer to Could a binary system of two planets with oceans reflect each other?...


14

Close approach seems to be the prevailing term for an event when two objects independently orbiting the Sun pass a minimum distance from each other. JPL and ESA use this term with near-Earth asteroids even if the approach is not especially close. Since real orbits are not perfectly concentric or coplanar, this generally does not occur at exactly the same ...


5

perihelion. Definition from Google the point in the orbit of a planet, asteroid, or comet at which it is closest to the sun. From Wikipedia: Apsis denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit of a planetary body about its primary body (or simply, "the primary"). Also from Wikipedia: For generic situations ...


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