One of the great difficulties of directly detecting extra-solar planets with
space-based infrared nulling interferometry is the requirement to have long
baselines. Nulling interferometry works by taking the signal from two or more telescopes and combining them in such a way as to cancel out the light comes from the star that the planets are orbiting around. The baseline is the distance that separates each pair of telescopes and the longer the baseline the more localized and deeper the nulling pattern is around the star while at the same time not diminishing the light from nearby planets. This is important because stars are (depending on the wavelengths used) anywhere between a million and a billion times brighter than the planets which orbit them and the most interesting planets to study are the ones that, just like the earth, lie in the so-called "habitable zone" close to the star.
The difficulty is that in order to achieve the long baselines that are required (more than 100 metres) current plans for space-based nulling interferometers envisage that a separate spacecraft would be needed in order to carry each telescope and that all of these spacecraft would need to fly in precision formation. While this approach is a very flexible arrangement and offers many benefits, the difficulties involved in getting this to work mean that the actual precision flying techniques are still ... (ahem) ... " currently being developed " so it may be quite some time before we see the working version. NASA is still considering its technology options at this time with the plan to launch its Terrestrial Path Finder (an array of five spacecraft) in 2012 at the very earliest.
There is hope, however, that a simpler approach to planet detection via nulling interferometry may be possible. A recent paper suggests that it may be possible to detect planets with a two telescope system using a baseline as little as 12 metres. This is short enough to consider mounting two telescopes on a single rigid structure (the International Space Station, by way of comparison, is 108.5 metres across and 88.3 metres long). The short baselines can be achieved because even though the shorter baseline results is a wider fringe pattern of nulled light reducing the effective brightness of the planets that fall within it, if one takes into account the increased infrared intensity of planets that are close to stars (due to their heating by the star) there may be just enough light to break through the nulling effect and enable detection.
With a modest baseline of only 12 metres it should be possible to detect 7 of the known 100 extrasolar planets but with a slightly larger baseline of between 20 and 30 metres it should be possible to fulfill all of the basic goals of the Terrestrial Path Finder mission. These goals include analyzing the composition of planetary atmospheres, an essential part of the search for planets with life.