At first, due to limitations of the instruments at their disposal, astronomers were constrained to locating only very large planets--in some cases many times larger than our own Jupiter, which in spite of its dominance of our solar system is relatively puny among the gas giants of the galaxy. But as they have refined their instruments and their techniques, these exoplanetary bloodhounds have worked their way all the way down to being able to detect planets smaller than Saturn.
They determine the wobble by measuring the Doppler shift in a star's spectrum--the lengthening of its emitted wavelength--that occurs due to the gravitational pull of any orbiting planets. In fact, from this they can even ascertain facts about the planets themselves, starting with their number and carrying merrily on from there.
But in the catalog of known extrasolar planets, there have been only gas giants and more gas giants. It's nice to know they're there--with every extrasolar planet found, it becomes clearer that planetary formation is not an uncommon thing, and therefore the possibility for Earth-like planets, even ones harboring life as we know it, becomes all the more tantalizing--but even so, the presence of Earth-like planets per se remains a frustrating unknown and a point of speculation.
Two newly discovered extrasolar planets, however, are among the smallest to have yet been located, and are at last pointing in a direction other than a galaxy littered with monstrous (and uninhabitable in the familiar sense) gas giants. Mounting evidence suggests that it is only a matter of time, depending on continuing improvements in the detection capabilities of the planet-hunters, before the sizes of the planets they find creep into what some call a category of "Super-Earths", planets several times larger than our own but rocky nevertheless instead of swimming in dense, toxic atmosphere and oceans of hydrocarbons wrapped around an icy core. Such rocky planets would be one step closer to the ultimate goal of finding other Earth-like planets and, eventually, even assessing from afar whether they might harbor life.
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