Can a planet float on water?

While some headlines proclaim that TrES-4 could float on water, doing so would violate several laws of physics. See more pictures of space exploration.
Image courtesy © Photographer: Mat Monteith | Agency:

In early August 2007, scientists representing the Trans-Atlantic Exoplanet Survey (TrES) announced the discovery of the largest known planet in the universe. The planet is called TrES-4 because it was the fourth planet discovered by the TrES, a network of three telescopes operating in the Canary Islands, Arizona and California. TrES scientists initially spotted the planet in the spring of 2007 in the Hercules constellation, 1,435 light years from Earth [source: BBC News].

Space Exploration Image Gallery

TrES-4 orbits a star called GSC02620-00648 and takes only 3.55 Earth days to finish a revolution. The planet is mostly made up of hydrogen and is classified as a gas giant, like Jupiter, Saturn, Neptune and Uranus in our solar system.


The excitement about TrES-4 is not just because of its size -- it's because some scientists believe the planet is so large yet so light that it shouldn't exist at all. TrES-4 is 70 percent bigger than Jupiter, our solar system's largest planet, but has 25 percent less mass [source: National Geographic]. Georgi Mandushev, the lead author of the journal article announcing the planet's discovery, said that "theoretical models of planets cannot explain this one" [source: ABC News Australia]. It's been called part of a group of "fluffy planets," but no planet so large and yet so lacking in density has ever been found [source: Scotsman].

Mandushev told National Geographic News that the planet's density of 0.2 grams per cubic centimeter was similar to balsa wood, which is known for being extremely light and is used in model airplanes [source: National Geographic]. TrES-4's low density has led some to say that the planet could float on water [source: The Guardian]. (Water's density is 1 gram per cubic centimeter, while Jupiter's is 1.33 grams per cubic centimeter.)

But notions of a planet floating on water are only theoretical. In actuality, it would be impossible for an entire planet to literally float on water. First, TrES-4 is a gaseous planet; one researcher said that it likely doesn't have any solid areas on its surface [source: Scotsman]. If TrES-4 somehow crossed paths with a less massive planet, TrES-4 would probably swallow the smaller planet up, while a larger planet could, owing to its greater gravity, tear TrES-4 apart. Even so, the concept of a planet floating on water highlights how unusual TrES-4 is.

Scientists from the Transatlantic Exoplanet Survey speculate that TrES-4 has such a large size relative to its mass because of its high temperature -- around 2,300 degrees Fahrenheit. Further study of the planet will focus on how it can be at once so large and so light while closely orbiting its parent star.

On the next page, we'll look at how scientists find planets like TrES-4 and learn about a planet as baffling as TrES-4.


Planet Hunting

Using this artist’s rendition as a guide, you can see why some scientists still debate what distinguishes a small brown dwarf from a large planet. From left to right, this image depicts our sun, a very cool star, a warm brown dwarf, a cooler brown dwarf and Jupiter.
Image courtesy Robert Hurt/NASA

More than 220 exoplanets -- planets outside of our solar system -- had been discovered around the time of the announcement of TrES-4. Teams of scientists, often called planet hunters, use networks of telescopes around the world to seek out undiscovered celestial bodies.

There are two techniques used by planet hunters. Most exoplanets have been discovered using the wobble method. Astronomers using the wobble method look for stars' "wobbling" due to the gravitational pull of orbiting planets. The technique also allows scientists to deduce a planet's mass.


The second technique is known as the transit method. As a planet passes between Earth and the path of light from its parent star, the visibility of that light is partially disrupted. Scientists take note of these disruptions and use them to figure out the locations of planets. The transit method allows scientists to learn much more information than they could through the wobble method. Besides finding out a planet's mass, scientists can learn information about a planet's size, chemistry and orbit. TrES scientists used the transit method to find TrES-4. (For more information about planet hunting, see How Planet Hunting Works.)

Although TrES-4 is the biggest planet ever discovered, it's not the most massive. That honor belongs to XO-3b, an orb that's 13 times more massive than Jupiter. Like TrES-4, XO-3b puzzles scientists. It has a very short, elliptical orbit -- rather than the expected circular orbit -- and completes a revolution in less than four days, meaning it's very close to its parent star. In fact, no other planet this big has been found orbiting so close to a star.

Some scientists question whether XO-3b is a planet at all. It may actually be a brown dwarf. The issue is contentious because brown dwarf classifications are a matter of debate, at least regarding what distinguishes a very large planet from a small brown dwarf. Stars are generally considered to be any body more than 80 times the mass of Jupiter, or any body able to perform hydrogen fusion. Scientists generally define brown dwarfs as bodies that are less massive than stars but more massive than planets -- or at least 13 Jupiter masses. But some say that mass is not the most important determining feature of a brown dwarf [source:]. The way a brown dwarf develops -- as part of a grouping of planets or independently -- may be the most important factor.

If XO-3B is indeed a brown dwarf, the title of most massive planet likely passes to another planet residing in the Hercules constellation. HAT-P-2, which was discovered through the transit method, weighs as much as eight Jupiters.

For more information about exoplanets, planet hunting and other related topics, please check out the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • "Gas Giants." Sol Company.
  • "Jupiter." World Book Encyclopedia. NASA.
  • "Space oddity: astronomers discover giant planet." Times of India. May 31, 2007.
  • "Team finds largest exoplanet yet." BBC News. Aug. 7, 2007.
  • Almond, B.J. "XO-3b: Supersized planet or oasis in the 'brown dwarf' desert?" Rice University. EurekAlert!. May 30, 2007.
  • Bryner, Jeanna. "Oddball Planet Puzzles Astronomers." May 30, 2007.
  • Edward, Rhiannon. "Universe's biggest planet is found by scientists." Aug. 9, 2007.
  • Highfield, Roger. "Alien planet 'could float on water.'" The Telegraph. Aug. 7, 2007.
  • Johnson, Bobbie. "Bigger than Jupiter, less dense than water." The Guardian. Aug. 9, 2007.
  • Miller, Barbara. "New monster planet 'could float on water.'" ABC News Australia. Aug. 8, 2007.
  • Minard, Anne. "Largest Known Planet Found, Has Density of Cork." National Geographic News. Aug. 8, 2007.