I think the evidence is about as good as it will get for the next few decades, until newer instruments can re-examine the current candidates and independently confirm their existence with different kinds of data. For instance, in November 1999 astronomers used the data for the planet orbiting the star HD 209458 to predict when the planet should be crossing the disk of that star as viewed from the Earth. Sure enough, on the predicted date, their photometric measurements showed that the star's brightness dimmed by 1.7 percent during the planet's transit ( see artist rendition above).
Independent confirmation such as this is pretty air-tight in making the case that the planet does exist, and that the spectroscopic measurements are really picking up bonefide planets and not just some oscillation in the star's surface. According to a news report,
On Nov. 5, one of the two teams that tracked the star's back-and-forth motion shared its findings with a colleague, Gregory W. Henry of Tennessee State University in Nashville. Two days later, at Fairborn Observatory in southern Arizona, Henry trained a small robotic telescope on the star. He did so during the time that the team had predicted the planet might pass in front of HD 209458, briefly blocking some of the starlight from reaching Earth. Seen from Earth, a planet can pass in front of its parent star once per orbit-but only if the orbital plane is aligned edge on with Earth. As luck would have it, this planet has that alignment, Henry found. Although the brightness of HD 209458 doesn't normally vary, it appeared to dip by 1.7 percent on Nov. 7. Henry and his colleagues report that the planet is immense, with a radius 1.6 times Jupiter's. The orbit's alignment with Earth indicates that the previously estimated minimum mass is the planet's actual mass. Henry, Butler, Geoffrey W. Marcy of the University of California, Berkeley, and Steven S. Vogt of UC, Santa Cruz detail the findings in a Nov. 12 circular of the International Astronomical Union. The planet's girth reveals that, like Jupiter, the object is gaseous rather than solid. In agreement with theoretical models, its proximity to its parent star exposes the planet to intense heat and radiation, which keeps it puffed up like a hot-air balloon, notes Adam S. Burrows of the University of Arizona in Tucson.
The press release from Tennessee State University notes that, "With the orbital plane of the planet known, the astronomers for the first time could determine precisely the mass of the planet and, from the size of the planet measured during transit, its density. Interestingly, while the planet's mass is only 63 percent of Jupiter's mass, its radius is 60 percent bigger than that of Jupiter. This fits with theories that predict a bloated planet when, as here, the planet is very close to the star. The density, about 0.2 grams per cubic centimeter, means it is a gas giant like Jupiter. However, such gas giants could not have formed at the distance this planet is from its star. "This supports the theory that extrasolar planets very near their star did not form where they are, but formed farther out and migrated inward," Henry said."
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