Astronomy/instance

Advertisement

Expert: - 2/15/2004


Question
My professor posed the question, that supposing there is a planet twice the mass of the Earth orbiting another star exactly like our Sun at distance of 1 AU. What would the orbital period of the planet be?

Is the answer still 1 year like earth? It seems to me even double the mass of the earth is still negligible to the Sun's mass. Am i correct?

Thanks  

Answer
Hello.

Yes, you're correct. The new period is only insignificantly larger than 1 yr.

The Newtonian casting of Kepler's 3rd or harmonic law has the form:

G(M + m)/ 4 (pi)^2 = a^3/P^2

where M denotes the mass of the Sun, and m, the planet.

Since M >>>> m (to be precise, M ~ 330,000 Earth masses)

then the period would be 1 year.

For 2m, this would only change negligibly, e.g. period very marginally longer than 1 yr. (By 1.00003x if you work it out, e.g. using ratios, treating the other factors as constants).  

Astronomy

All Answers

Answers by Expert:

Ask Experts

Volunteer

Philip Stahl

Expertise

I have forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics as applied to astronomical investigations.

Experience

Astronomy: more than forty years experience starting with construction of my own simple telescopes. Worked at university observatory in college, doing astrographic measurements. M.Phil. degree in Physics/Solar Physics and more than ten years as researcher.

Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union

Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.

Education/Credentials
B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research

©2012 About.com, a part of The New York Times Company. All rights reserved.