The rings of Uranus are narrow and faint and not observable from Earth. Pluto not only is small, but it is on the outer fringes of the solar system, making it equally difficult to view its atmosphere directly.
Elliot relied on an indirect technique called stellar occultation, in which astronomers watch a planet or other astronomical object very carefully as it passes in front of a star. But the technique requires immaculate planning, precise timing and careful observation. There are no second chances.
In 1977, Elliot, Edward Dunham and Douglas Mink — all three then at Cornell University — flew over the Indian Ocean in the Kuiper Airborne Observatory to use its 36-inch telescope to observe an occultation of a bright star by Uranus. Elliot had the foresight to turn the instrument on about an hour in advance.
To the team's surprise, the light from the star winked out briefly several times before it was eclipsed by Uranus, then again several times after the eclipse. Those symmetrical drops in the star's brightness were a clear indication that Uranus possessed rings, something that astronomers had not suspected.
The rings were subsequently viewed directly by the Voyager 2 spacecraft.
In 1988, Elliot and his colleagues observed Pluto — which was then considered a planet but which has since been demoted to the status of dwarf planet — by the same technique. They observed that instead of winking out sharply, as had been expected, the light from the star faded gradually, then reappeared gradually once Pluto had fully passed in front of the star.
That was a sign that the celestial body had an atmosphere, albeit a thin one. Elliot's team plans to do similar occultation observations in June.
James Ludlow Elliot was born June 17, 1943, in Columbus, Ohio. He received his bachelor's degree in physics from MIT in 1965 and his doctorate in astronomy from Harvard in 1972. He was a postdoctoral fellow at Cornell, then joined the faculty until he returned to MIT in 1978, spending the rest of his career there.
He was especially supportive of women in astronomy; more than half of his students were women. At a 2010 celebration of his life and work at MIT, called the "Jimboree," former students spoke about his gift for engaging them deeply in his research and then sending them off on their own, trusting that they could do the research themselves.
Students wrote their memories on white index cards because Elliot always had such cards and a pen in his pocket to jot down thoughts as they came to him.
Elliot is survived by his wife of 44 years, the former Elaine Kasparian; daughters Lyn of State College, Penn., and Martha of Brooklyn; sisters Suzanne Elliot of New London, Conn., and Martha Bureau of Piedmont, Calif.; a brother, Tom, of Arlington, Va.; and one granddaughter.