Planets and Gravitational Microlensing.

Peter R. Wallis

Most of the known extrasolar planets, those in orbits around other stars than our Sun, have been discovered by two different methods. One method is to observe the small reduction in the parent star's light when the planet transits the star's disc. Obviously only a small proportion of planetary orbits will be orientated for this to occur. The reduction may be easy to see if the planet is large like Jupiter, but may be missed if it is small like the Earth or if it orbits at a large distance from the star.

The other method observes the frequency shift of known lines in the spectrum of the star as it "recoils" from the planet's revolution. Again, it will be easier to detect a massive planet like Jupiter, which has 318 times the mass of the Earth, particularly if it is close to its parent star where it has a greater orbital velocity.

These methods are consequently biased in favour of finding hot massive planets in close orbits. They are not likely to present much possibility of hosting extrasolar life – at least life as we know it.

There is however a third method based upon Einstein's General Theory of Relativity, published in 1916. This predicted, amongst other things, that light passing near a massive body, such as a star, will be deflected. Sir Arthur Eddington led an expedition a few years later to confirm this; it was necessary to use a solar eclipse as only then would it be possible to observe the altered position of stars seen close to the sun. There was another consequence of the theory which Einstein himself reported [1] in 1936. If two stars happen to lie in the same direction, one a long way away and the other nearer, the gravitational mass of the nearer star will bend the light from the more distant star and act like a lens. When the two stars are precisely aligned, the light of the further star will be magnified and appear as a bright ring around the nearer star: this is called an "Einstein Ring". If the stars are slightly misaligned the ring may be split or distorted. The phenomenon is called gravitational microlensing.

Now when we use lenses in a microscope or a telescope, we use them to magnify or study the distant object. But in stellar microlensing it is not the distant star we are studying: we use that merely as a point of light. Our interest here is in the structure of the gravitational mass which forms the lens. For example, if the lensing star has a planet, the Einstein ring will be further distorted. We are now able to compute the mass distribution from the observed distortion.

We can now use such microlensing to study the probability of planetary systems around the stars in our own galaxy, called the Milky Way. The advantage of the system is that it can detect planets over a wide range of orbital distances from the parent star, typically 0.5 to 10 AU, and at any orientation. A disadvantage is that microlensing events are rare. Perhaps only one in a million stars has a lensing event at any one time, but there are many stars. We cannot pick and choose, but we can reach a general conclusion. A recent paper [2] reports on the conclusions from 7 years of microlensing events seen in the Milky Way (2002-2007) and concludes that on the average, every star hosts one or more planets in an orbital distance range of 0.5 – 10AU and mass from a few times Earth to Jupiter and more. For those of us who are interested in the possibility of extraterrestrial and extrasolar life, this is an encouraging result.

But there is more. The microlensing technique has also been used to study galaxies, not stars [3][4]. The lensing galaxy is an elliptical galaxy at red shift z=0.88 and is seen to have a bright Einstein ring. The background galaxy is at red shift z=2.06, imaged by the Keck telescope in Hawaii. The authors found that the distortion of the Einstein ring was caused by a dark satellite galaxy near the main lensing galaxy; it would not be possible to see it directly with current technology.

The objective of the authors in studying this lies in problems in understanding how galaxies form. The current theory is that galaxies form initially from dark matter and then they attract normal matter, which in due course coalesces into stars. But the theory seems to predict many more satellites around the Milky Way galaxy than we have yet found. So Vegetti and her team have demonstrated that it is possible to detect such elusive objects even at enormous distances where they could not possibly be seen visually. There is reason to think that such dark satellites are dominated by dark matter.

[1] Einstein, A "Lens-like action of a star by the deviation of light in the gravitational field" Science 84,506-7 (1936). Back

[2] Cassan, A et al "One or more bound planets per Milky Way star from microlensing observations", Nature 481, 167-169, 12 Jan. 2012. Back

[3] Vegetti,S et al,"Gravitational detection of a low-mass dark satellite galaxy at cosmological distance", Nature b>481, 341-343, 19 Jan. 2012. Back

[4] See also, 271-2, loc cit. Back

Editor's comment.

Doug Daniels

Just in case the conclusion reached by the authors in footnote 2 did not fully 'sink in', I should like to repeat it: "after 7 years of research using 'microlensing' techniques, the conclusion is that "every star hosts one or more planets". This is a profound piece of research and it has huge implications for the search for 'extra terrestrial life forms'. I have long held the view that if the present theories of star formation are correct, then it should be expected that the majority of galactic 'metal rich' population 1 stars ought to possess planetary systems. But then we learned that the first multiple planet system to be discovered was that surrounding a Pulsar: PSR 1257+12 – a most unlikely candidate for a planetary system. Recently, Kepler has discovered planets in orbit around the star KIC 05807617, a star that has passed the Red Giant phase – another unlikely candidate. All this adds considerable weight to this research which appears to push the boundaries even further by suggesting that 'every' star should host a planetary system. If this should prove to be correct, then in our own Galaxy alone there must be upwards of one hundred billion planetary systems, and in the observable Universe --- well, at the moment that's anyone's guess but it will be a lot!

For those of us interested in the existence of extraterrestrial and extra-solar life, Peter's observation that this is "an encouraging result" is something of an understatement! In a Universe consisting of billions of galaxies and trillions of stars and therefore by inference, trillions of planetary systems, the idea that just one planet orbiting an average star in an average galaxy could be the only abode of life is totally illogical and to quote the late Carl Sagan (Contact) it would seem to be "an awful waste of space".

Kepler's discoveries continue and recently it has detected a system containing two planets just 1.03 and 0.87 earth radii orbiting the G8 star Kepler-20. This system also contains previously detected planets of x2 and x3 earth radii. With diameters close to that of the Earth, Kepler 20e and Kepler 20f, could have similar compositions to Earth i.e. an iron core and silicate mantle, but at present an accurate determination of their masses has yet to be made and this determination will also indicate the chances of these planets possessing atmospheres and perhaps even liquid water.

Just two decades have passed since the first giant hot exoplanets were discovered. Now as our techniques become more refined we are beginning to detect the smaller rocky Earth sized planets and if the conclusions resulting from the technique of gravitational microlensing are correct, we can confidently expect to discover a whole lot more of them.

Doug Daniels

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