How big is TrES-4b?

TrES-4b is a giant gas planet that orbits a star about 1,700 light-years from Earth. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. A 2015 study found TrES-4b to be about 260,000 kilometers (160,000 miles) across, about 1.8 times the width of Jupiter. When its discovery was announced in 2007, it was the largest known planet with a measured size.

Size

TrES-4b's size comes from its transits. Every 3.55 days, the planet passes in front of its star as seen from Earth, and blocks about 1% of the star's light. How much light it blocks shows how wide the planet is compared with the star: about one-tenth as wide.

To turn that into kilometers, astronomers also need the size of the star, which is harder to pin down. Estimates of the planet's width therefore differ.

The 2007 discovery team found 1.67 times Jupiter's width, about 239,000 kilometers. The 2015 study found 1.84 times, the 260,000 kilometers given above. NASA's exoplanet catalog lists 1.61 times, about 230,000 kilometers.

Size comparisons

TrES-4b is about 1.8 times as wide as Jupiter. It has about six and a half times Jupiter's volume.
About 21 Earths lined up side by side would reach across it.
Proxima Centauri, the next smaller object in the explorer, is a star, yet it is only about four-fifths as wide as this planet.
TrES-4b's width is about two-thirds of the average distance from Earth to the Moon, the next larger object in the explorer.
If Jupiter were shrunk to the size of a basketball, TrES-4b would be about as wide as the ring of a basketball hoop, about 45 centimeters (18 inches) across.

How it was found

TrES-4b was found by the Trans-atlantic Exoplanet Survey, or TrES, which searched for planets by watching for transits. Between May and August 2006, telescopes at Lowell Observatory and Palomar Observatory watched a patch of sky in the constellation Hercules. Both caught the same star dimming again and again in the way a passing planet would cause. Measurements of the star's motion with the Keck I telescope then ruled out other explanations, such as a pair of stars eclipsing each other.

A puffed-up planet

TrES-4b is so wide for its mass that astronomers describe it as bloated, or inflated. Its mass is uncertain: the discovery team measured about 0.84 times Jupiter's, while the 2015 study found about half of Jupiter's. Either way, its average density is very low, between about one-thirteenth and one-sixth of Jupiter's. In 2015 it was the second least dense hot Jupiter known with a measured size.

A hot Jupiter is a giant planet that orbits very close to its star. TrES-4b circles its star about 7.7 million kilometers out, about one-twentieth of Earth's distance from the Sun. Its temperature is estimated at about 1,500 degrees Celsius (2,800 degrees Fahrenheit).

Astronomers think several effects can puff up hot Jupiters, including heat from their stars. Even so, TrES-4b is larger than those effects predict.