A planet that's unseen, but very much there

Planets orbiting distant suns can never be seen. This is because planets only reflect the

light of the parent star and at a distance of several light years, the planet cannot be made out in the glare of the star. Exo-planets are thus detected by indirect means. 

The first indirect method, which works for large planets, uses a slight wobble of the parent star, which the planet induces as it goes round in its orbit.

The back and forth movement of the star causes variations in the frequency, which is the colour, of the starlight that reaches the earth and these changes can be detected.

By measuring the colour shift, the speed and extent of wobble of the star, and hence the nature of the unseen planet can be worked out. This works best for large planets because small planets, whose size compares with the earth, do not cause appreciable wobble.

The other method uses a slight dip in the amount of light that comes to the earth from the star when the exo-planet passes in front of the star. The instruments that astronomers use are now sensitive enough to measure minute changes in the intensity of light and it is possible to use this method to detect even earth-sized planets, which is the kind we are most interested in.

NASA’s Kepler project is a space-based, ie orbiting telescope, launched in 2009, for carrying out this work most efficiently from a place that is outside the distortion that comes from the earth’s atmosphere. The telescope is about one metre in diameter and its light measuring arrangement is sensitive to one hundredth of a per cent, which amounts to measuring the dip in intensity of a car headlight when a fruitfly goes past.

This way of detecting exo-planets also reveals the size of the planet, from the extent of light dimming and the nature of the orbit, from the ‘time of transit’. But we can see that this method can be used only when the exo-planet transits the star along the line from the star to the earth. If the orbit is in a different plane, it will not block the light that reaches the earth and there is no ‘transit’. Such a planet would thus be ‘invisible’. While hundreds of exo-planets have been found using the method of transits, astronomers are conscious that they have found only a fraction of the planets that are there, waiting to be detected, but cannot be found by this method.

Finding the invisible
Sarah Ballard and others at the Harvard-Smithsonian Centre for Astrophysics report in a paper soon to appear in The Astrophysical Journal, that they have used the data from Kepler to work out the presence of an exo-planet that does not appear in transits. The method they used relies on the fact that the invisible planet, which does not pass before the star, still affects the motion of a sister planet that does. 

The star whose system was being studied is Kepler 19, a star in the constellation Lyra, 650 light years away. Its planet, Kepler 19b, is a world about twice as large as the earth and orbiting its sun every nine days and seven hours. As expected for such a fast speed of orbit, it is only some 15 million km away from its sun, unlike the earth which is over 150 million km away. And because it is so near the star, it heats up to a scorching 500˚C. But the interesting thing about Kepler 19b is that its time of orbit does not stay constant and keeps getting five minutes shorter or longer – the planet seems to speed up or slow down.

Now one of the great constants that we are used to is the time of revolution of planets. The earth takes 365 and a quarter days to go round the sun and while there may be a gradual slowing, over millennia, there is no variation from year to year. The thing that could be causing the variations in the case of Kepler 19b has to be another planet, which moves so that its force of gravitation becomes strong maybe every two revolutions, to bring about this alternation in the period of Kepler 19b.

This other planet, yet unseen, has been named Kepler 19c and its own period of revolution would also be uneven, being affected by the first planet. But as it has never been spotted, the orbit of Kepler 19c apparently does not cross the path between the star, Kepler 19 and the earth.

Detection of Neptune and Uranus
It was this method of deducing a planet’s presence that was first used back in 1846 to discover the planet Neptune. Just four times the size of the earth but 30 times further away from the sun, the planet is not visible to the naked eye and had not been sighted.

But there were variations in the orbit of Uranus, which led to the conclusion that there was neighbouring planet whose gravitational force was causing the disturbance. Neptune was actually sighted, subsequently, at almost the very place that had been worked out theoretically.

But the data of only the variation in the period of Kepler 19b is not enough tell us anything more about Kepler 19c, except that it exists. The orbit may be faster than Kepler 19b or it may be slower. It may be circular or it may be elliptical. The planet may be rocky or it may be a gas planet. As it does not affect the mother star to any appreciable extent, even its mass cannot be worked out. All we know is that it is there! 

But the fact that it has been detected is to show that methods that worked in the solar system are equally good at the greatest distances and a picture of the unseen cosmos may still be drawn. “This method holds great promise for finding planets that can’t be found otherwise,” says Harvard astronomer and co-author David Charbonneau.