Challenge - A problem in astronomy

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    Astronomy Challenge
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Discussion Overview

The discussion revolves around a hypothetical scenario involving two earth-sized planets that orbit each other and a star, exploring various astronomical questions related to their characteristics and dynamics. The scope includes theoretical modeling and conceptual exploration of celestial mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that several questions (Q2, Q3, Q4, Q5, Q6) depend on the semimajor axis, either of the planets to their center of mass (CM) or the CM to the star.
  • There is a suggestion that the star would belong to the same stellar class as our sun, with one participant proposing G5 and another correcting it to G2 V.
  • Regarding the bonus question about a moon's orbit, some participants argue that the initial conditions would determine the moon's path, with specific locations like L4 or L5 leading to an elliptical orbit, while other conditions could result in different orbital behaviors.
  • One participant mentions that the CM of both planets would revolve in a manner similar to Earth, suggesting a 24-hour rotation period, and questions whether the double mass of the system would affect its orbit around the star.

Areas of Agreement / Disagreement

Participants generally agree on the need for more information to answer several questions, but there are competing views on the specifics of the star's classification and the moon's potential orbits. The discussion remains unresolved regarding the exact dynamics and characteristics of the system.

Contextual Notes

Limitations include the lack of specific values for the semimajor axis and other parameters, which are crucial for answering the posed questions. The discussion also reflects uncertainty about the implications of the planets' mass on their orbital characteristics.

Messiah
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Given -
1) Twin earth-sized planets rotate about each other in a solar system.
2) The plane of their mutual orbit is the same plane in which they rotate about their star
3) Their star provides (on average) the same amount of energy as our sun

Questions -
1) What is the size and nature of their star?
2) What is the average distance from the center of their orbit to the star?
3) What is the distance between the planets at mutual apogee and perigee?
4) What is the period of rotation about each other?
5) What is the period of rotation about the star?
6) What is the likely period of their individual revolution?

BONUS POINTS -
Would a moon orbit around both planets
a) in an elipse
b) in a figure '8'?

PS: The winner gets to name the planets
 
Last edited:
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It seems to me that you haven't given enough information.

Q2, 3, 4, 5, & 6 all depend on the semimajor axis, either of the planet to the CM or the planets' CM to the star.

For the bonus question, it would depend on the initial conditions. If the moon started off at either L4 or L5, it would stay in an elliptical orbit, following the rotation of the planets about each other. Other initial locations and velocities would result in different orbits, including a possibility of an alternating posigrade/retrograde orbit about one of the planets, but not the other (passes close to L1, but doesn't reach it, then turns around).

EDIT: Q1 would be the same stellar class as our sun, but I'll be danged if I can remember what that is... G5?
 
Last edited:
Originally posted by enigma

EDIT: Q1 would be the same stellar class as our sun, but I'll be danged if I can remember what that is... G5?

G2 V.
 
Originally posted by axeeonn
G2 V.

Thanks. I was sure there was a 5 in there SOMEwhere. :wink:
 
Originally posted by enigma
It seems to me that you haven't given enough information.

Q2, 3, 4, 5, & 6 all depend on the semimajor axis, either of the planet to the CM or the planets' CM to the star.

For the bonus question, it would depend on the initial conditions. If the moon started off at either L4 or L5, it would stay in an elliptical orbit, following the rotation of the planets about each other. Other initial locations and velocities would result in different orbits, including a possibility of an alternating posigrade/retrograde orbit about one of the planets, but not the other (passes close to L1, but doesn't reach it, then turns around).

EDIT: Q1 would be the same stellar class as our sun, but I'll be danged if I can remember what that is... G5?

CM of both planets is same as Earth - revolving @ 24 hours (earth time). Rotation about the star - as close to earthly as possible. With the double mass, wouldn't its orbit be further from the star?
 

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