I have heard that we cannot know the masses of the extrasolar planets exactly. If this is the case, how can we be sure that they are planets?

Dear Suma,

You are right that the mass of planets orbiting other stars cannot be
determined exactly. When astronomers quote a mass of a planet, they are
really quoting the minimum mass that planet could have. The math equation
for determining the mass is:

M(actual) = M(minimum)*sin i

Where i is the "inclination angle", or the angle between the observer and
the pole of the planet's orbit. If we are viewing an orbit edge-on, then
i=90 degrees and M(actual) = M(minimum). This occurs because we see the
maximum possible velocity of the parent star when the orbit is viewed
edge-on.

When we have a sample of many candidate planets, we can begin to see what
an average mass might be via statistics. We expect that the orbits are
randomly oriented in the sky. There then must be an average angle of
inclination (this can be calculated using statistics. I don't know the
value off the top of my head, but I would guess it is either 45 degrees,
30 degrees or 60 degrees). If the majority of planets in the sample still
have planetary mass using this average inclination angle, then we know we
are seeing planets in a majority of the cases.

So, we may not be able to point to a specific planet candidate and say
"This is a planet", but we can say with almost absolute certainty that we
are seeing at least some planets.

Hope this helps, and thanks for writing.

Sincerely,

Kurtis Williams

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