61. An Earthlike Planet. In January 2006 , astronomers reported the discovery of a planet comparable in size to the earth orbiting another star and having a mass of about $5.5$ times the earth's mass. It is believed to consist of a mixture of rock and ice, similar to Neptune. If this planet has the same density as Neptune $\left(1.76 \mathrm{~g} / \mathrm{cm}^{3}\right)$, what is its radius expressed (a) in kilometers and (b) as a multiple of earth's radius? Consult Appendix $F$ for astronomical data.
