The moon might be made of cheese, but Harvard astronomers have found a planet with the density of a marshmallow.

A team from the Harvard-Smithsonian Center for Astrophysics confirmed last week the existence of the largest planet detected and the lightest in weight for a planet of its size.

Using Hungarian-designed amateur telescopes, Harvard astronomer Gaspar Bakos led his group in the discovery of HAT-P-1, which stands for “the first planet detected by a Hungarian Automated Telescope.” With four-inch apertures, these miniature telescopes, some of the smallest in the world, are “just off-the-shelf big telephoto lenses,” said team member Robert W. Noyes, professor of astronomy.

Bakos could not be reached for comment yesterday.

After recording the change in light emissions from a star in the double star system ADS 16402, the team noticed a periodic dimming every four and a half days when the planet was transiting, or passing in front of the star. From this, they determined that the planet was 35 percent larger than Jupiter.

To measure the wobble of the star, or how much the planet tugs on the star, the team made observations from the Keck, which is the largest telescope in the world and has 10,000 times the collecting area of a HAT telescope, and the Japanese SUBARU, both of which are located in Hawaii.

What they found was an astronomical anomaly: Although it is the largest planet ever discovered, HAT-P-1’s mass is only half of Jupiter’s, rendering it very light for its size, or as Noyes called it, a “big, fluffy planet.”

“If you drop Jupiter into a huge bowl of water, it would sink. This planet would float high, like a beach ball,” Noyes said, explaining that HAT-P-1 has a density that is a quarter that of water’s.

It’s not the only low-density, large-mass planet—in fact, HD 209458b was detected in 2000—but HAT-P-1 turned out to be the most baffling. Noyes said that current methods of calculating planetary dimensions significantly underestimate the size of HAT-P-1.

“Now we have two oddballs, and we’re beginning to wonder if we’re missing something,” he said, adding that these findings are “sending theoretical astrophysicists back to the drawing board.”

Via a remote link, the team is continuing to observe the star’s wobble using the Center for Astronomy observatory in Arizona to understand HAT-P-1’s orbit better.

“The orbit may be slightly elliptical...that’s important,” Noyes said. “If it’s elliptical, then you can get tides on the planet induced by its star—that could cause some heating that could make it bigger.”

In the future, the team is hoping to make observations from space using the Hubble and the Spitzer telescopes to get better measurements on the transit curves.

With HAT generating additional planetary candidates, Noyes believes that more and more transiting planets will be detected.

“We and several other groups have now learned enough of how to search for planets with small telescopes,” he said. “We’re learning how to search for needles in haystacks.”

In the same week, Assistant Professor of Astronomy David Charbonneau led a team of Harvard astronomers in detecting a planet larger than Jupiter, also using four-inch amateur telescopes.

—Staff writer Lulu Zhou can be reached at luluzhou@fas.harvard.edu.