March 7, 2012 | 3
When astronomers talk about methods for finding exoplanets the list is relatively short. There is the radial velocity, or ‘wobble’ technique, which senses the motion of a star around a common center-of-mass with its planets. There is the transit technique, employed with great success by NASA’s Kepler mission, and there are direct imaging and phase-photometry techniques – challenging observations that seek the light being actually emitted or reflected from a planet. And then there is gravitational microlensing, the chance magnification of the light from a distant star by the distortion in spacetime due to the mass of a foreground star and its planets – with distinctive ‘blips’ or cusps of brightness due to any worlds aligned close to the right place in the star’s lensing field.
This form of gravitational distortion of the pathway of photons is called ‘micro‘ because the typical arrangements and masses of stars results in tiny images; while the light of a background object may be greatly magnified we can’t see its distorted image directly, its light merges with that of the ‘lens’ stars, mere thousandths of a second of arc from it. The gravitational effect of a planet around the lens star is effectively amplified when it is close enough to the zone of maximum magnification, the Einstein ring, but its effect is also only seen as an additional and asymmetric boost in photons arriving at our telescopes.
But the key phrase here is ‘typical arrangements’. Given the rarity of alignments between two stars separated by great distances, caused by the endless motions of all objects within our galaxy’s gravity well, the majority of such events that we see occur between stars that are both very distant from us – perhaps more than halfway between here and the center of the Milky Way. At these enormous distances (many thousands of light years) we cannot measure the motion of a lens star (or its potential lensing victims) relative to others, and so have no idea when or if any given star will magnify the light of something aligned directly behind it. The situation is rather different however for much closer stars. Not only can we obtain their ‘proper motions‘ with careful high-precision astrometry, the zone around them that is optimal for magnifying a background object is that much bigger in angular diameter, it is ‘meso’ not micro.
Thus, gravitational mesolensing opens up a number of intriguing possibilities. First, as discussed in a triplet of wonderful recent papers by Lepine and Di Stefano, and Di Stefano, Matthews, and Lepine, it becomes possible to predict when a nearby lens star may move close enough to the position of a distant object to magnify it, and the larger lensing angle may cause a directly measurable shift in the apparent position of that background star as well as its brightness. If there are also planets around the lens star the predictability of the event may allow us to snag the early or late lensing signature of worlds on larger orbits that we might otherwise miss. The larger scale of the lensing angles also offers a unique probe of the effect of any close-in, very short orbital period planets. And the icing on the cake is that nearby stars are much more amenable to the detailed astronomical measurements necessary to estimate their true masses – the true strength of the lens.
So can this be done? The authors point out a specific case; the object VB 10 is a low-mass star (perhaps a tenth the mass of the Sun) a mere 19 light years away. VB 10 ‘scoots’ across the sky at about 40 kilometers a second in transverse velocity, a miniscule but measurable angular shift per year (see the animation to the left here). Earlier Hubble Space Telescope images reveal something small and faint in its path – a distant background star headed for the lensing zone of VB 10 in late 2011/early 2012. Has VB 10 produced gravitational mesolensing? We’re awaiting the authors’ report on their efforts to observe any possible event. It’s not an optimal case, a nearby lens moving very fast doesn’t provide the best combination of factors, but it really is a pioneer in what might just become a new tool for hunting exoplanets – welcome to the fold gravitational mesolensing.