Thursday, August 06, 2015

A one-way ticket to an unsuspecting Kepler 452b?

There has been a bit in the science press about the newly discovered exoplanet, Kepler 452b. This related to the observation that it is in the “habitable zone” of a sun-like star.  The big news, as always, is that this planet is completely unlike anything in our solar system. If it is solid, it has three times the mass than every rocky body in our solar system combined. If it is not solid, then it is one of the sub-neptune planets common everywhere but around our star.  But there are two points in particular which have been ignored- or at least not appreciated, which I would like to expound on.

Firstly, we can see them, but they can’t see us.  There are two main techniques used for detecting planets around stars: Radial velocity, and transits.  The motion of the planet around the star pulls the star backwards and forwards, in proportion to their relative masses.

With the radial velocity method measures the very small Doppler shift in the light of the star created by this pull. However, in order to see this motion, the orbit of the planet around the star needs to be somewhat edge-on as seen from earth. If we are looking town down on the orbit, then the star doesn’t move towards or away from us; it just goes in a circle (or ellipse). And sideways motion in the sky is much harder to detect that motion towards of away from Earth.

With transit detection, the crossing of the planet across the face of the star (as seen from Earth) causes the light from the start to dim a little bit in a periodic fashion. This requires Earthly observers to be in the same plane as the orbit of the planet- For a Earth-like planet orbiting a sun-like star, the planet will only transit if the Earth is within a half degree of the planet’s orbital plane.  The Kepler mission is a transit mission; all the planets it detects are systems which are edge-on as seen from Earth.

For aliens trying to detect us using the transit method, they need to be viewing us from a star that lies in the ecliptic. Basically, if they want to see the Earth pass in front of the sun, to detect its transit, then from our point of view, the sun needs to cross in front of their star.

However, the Kepler primary mission* field of view is nowhere near the ecliptic. It is, fore the most part, more than 60 degrees from the ecliptic. This makes transit detection of the Earth in front of the sun impossible from any star systems in the original Kepler field of view. And due to the high angle, radial velocity measurements of the Earth’s pull on the sun will be less than half as effective as our radial velocity measurements of their planets.

So the Kepler mission isn’t just a telescope.  It is a spy satellite, peering down on a thousand planets circling hundreds of distance star, all of whom are blissfully unaware of our planet’s existence.  If there are aliens on Kepler 452b- or any other planet Kepler discovers, they aren’t waving at us, because assuming technological parity, they can’t possibly know that we are here.

Of course, we know that they are there.  And it might be that one day,. given a modest technological advancement, someone could sent a colony ship on a hundred thousand year mission to visit them.  However, the visit could easily overstay its welcome.

Kepler 452b is probably not an Earthlike planet. However, if it does have an Earthlike composition, then it is a gigantic hunk of rock and metal three times more massive than every rocky-metal planet in our solar system combined. Due to gravitational self-compression, this planet would have a mass six times that of Earth.  At 36 hellagrams, it is just under half the mass of Uranus. The surface gravity would be a crushing 2.3 times greater than on Earth.  No rocket we currently have could even leave the launch pad under suck crashing gravity.  And even if it did, the velocity required to achieve orbital velocity, 15.5 km/s, is almost twice what is required on Earth.

Although technology is sure to advance if we are to get the ability to launch colony ships, such a huge planet would trap any rocket conceivable with current technology.  Kepler 452b is, in essence, a gigantic Hotel California, from which no-one can ever leave.  As a result, any short-lived visitation attempt would inevitably become a permanent stay.

So don’t be too disappointed if the locals on Kepler 452b don’t wave back.  They are blissfully unaware that we are staring at them.  And if they did know, the fact that we would wear out our welcome upon visiting by a factor of infinity is unlikely to cheer them up.

 * The secondary mission, however, is observing on the ecliptic.

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