There is a big planet far away.
It is very hot. It stays near its bright star. This makes a year very short. It has a lot of gas.
Can you imagine a world that hot?
There is a giant planet far away.
It is made of gas. This planet stays very close to its star. Because it is so close, the heat is very strong. A year there lasts only three and a half days.
The planet also has a superstorm. This storm has very fast winds. It moves gas from the hot side to the cool side. Scientists even found air around the planet. This world is a very busy place.
There is a giant planet far away in the Pegasus constellation.
Its name is HD 209458 b. Some people call it Osiris. This planet is a gas giant. That means it is made of gas instead of rock. It is much bigger than Jupiter. It is also much heavier.
This planet stays very close to its star. Because it is so close, it is very hot. A year there lasts only 3.5 Earth days. Scientists found the planet by watching a transit. A transit happens when a planet moves in front of its star.
When this happens, the star looks a little dimmer. This is because the planet blocks some light. We can also study the planet's air. This air is called an atmosphere. Scientists found oxygen and carbon in the air. They also think there might be water vapor there. A huge superstorm moves gas across the planet. The winds in this storm are very fast. They move gas from the hot side to the cool side.
There is a huge planet far away in the Pegasus constellation.
Its name is HD 209458 b. Scientists often call it Osiris. This planet is a gas giant. This means it is made of gas rather than rock. It is much larger than Jupiter. In fact, its volume is about 2.5 times greater than Jupiter's. It is also very heavy. Its mass is 220 times that of Earth.
This planet orbits a star very closely. Its orbital radius is only one-eighth the distance of Mercury's orbit. Because it is so close, the planet is very hot. The surface temperature is about 1,000 degrees Celsius. A year on this planet lasts only 3.5 Earth days. The planet's close path causes it to be quite large. This is called inflation. Intense heat from the star makes the outer atmosphere swell up.
Astronomers first found this planet in 1999. They used a method called a transit. A transit happens when a planet passes in front of its star. This causes the star to look a little dimmer. On September 9 and 16, 1999, a team led by David Charbonneau saw a 1.7% drop in brightness. Another team led by Gregory W. Henry also saw a transit. They published their work at the same time. This made HD 209458 b the first known transiting extrasolar planet.
Scientists have learned a lot by studying the planet's atmosphere. This is the layer of gas around the planet. In 2001, the Hubble Space Telescope found sodium in the air. This was the first time anyone measured an atmosphere outside our solar system. In 2005, the Spitzer Space Telescope measured light from the planet directly. This helped scientists know the temperature was at least 1,000 degrees Celsius.
Researchers also found oxygen and carbon in the atmosphere. Some models even suggest there might be water vapor there.
This planet is a very active place. In 2010, astronomers measured a massive superstorm. The winds in this storm move at 7,000 meters per second. These winds carry gas from the hot day side to the cooler night side. The planet is also losing part of its atmosphere. Hydrogen gas escapes into space because the star is so hot. This creates a long tail of gas behind the planet. This tail is about as long as the planet's diameter.
HD 209458 b is a remarkable exoplanet located in the constellation Pegasus.
It orbits a solar analog star named HD 209458. This planet belongs to a specific class of worlds called hot Jupiters. These are gas giants that orbit very close to their parent stars. Because of its close orbit, the planet is extremely hot. Its estimated surface temperature is about 1,000 degrees Celsius. It is also very large and heavy. Its mass is 220 times that of Earth, or 0.69 Jupiter masses. Its volume is approximately 2.5 times greater than Jupiter's.
The planet's orbit is incredibly tight. The orbital radius is only one-eighth the radius of Mercury's orbit. This proximity causes a very short year. A single year on HD 209458 b lasts only 3.524736 Earth days. This close distance also causes a phenomenon called inflation. The intense heat from the star causes the outer atmosphere to swell. This makes the planet's radius about 35% larger than Jupiter's. Some scientists believe tidal heating also helps inflate the planet. This happens because of the eccentricity, or the non-circular shape, of its orbit.
Astronomers discovered this world using several different methods. In 1999, spectroscopic studies first revealed the planet's existence. Shortly after, two separate teams detected a transit. A transit occurs when a planet passes directly between its star and Earth. This causes the star's brightness to drop temporarily. On September 9 and 16, 1999, David Charbonneau's team measured a 1.7% drop in brightness. Another team led by Gregory W. Henry observed a partial transit. Both teams published their findings in the Astrophysical Journal at the same time. This made HD 209458 b the first known transiting extrasolar planet.
Scientists have used advanced tools to study the planet's atmosphere. In 2001, the Hubble Space Telescope detected sodium in the atmosphere. This was the first time an atmosphere outside our solar system was measured. In 2005, the Spitzer Space Telescope achieved the first direct detection of light from an exoplanet. It did this by subtracting the star's light to see the planet's own light. This confirmed the temperature was at least 1,000 degrees Celsius. In 2007, researchers used spectroscopy to look at the chemical makeup of the air. Spectroscopy is a method of studying how light interacts with matter.
The chemical composition of the atmosphere has surprised many researchers. In 2007, a team led by Jeremy Richardson found unexpected results. They observed a peak at 9.65 micrometres, which suggested clouds of silicate dust. They also found a peak at 7.78 micrometres that had no known explanation. While some models suggested water vapor might be present, other studies found very dry atmospheres. As of 2021, different instruments show inconsistent results. This might mean the atmosphere has a low metal content or unusual chemistry. The atmosphere also contains elements like oxygen and carbon.
HD 209458 b is a very violent and active world. In 2010, astronomers measured a massive superstorm in its atmosphere. The winds in this storm reach speeds of up to 7,000 meters per second. These winds move carbon monoxide from the hot day side to the cooler night side. The planet is also losing its atmosphere to space. This is called an evaporating hydrogen atmosphere. The extreme heat creates hydrodynamic drag. This process blows hydrogen, carbon, and oxygen away from the planet. This creates a massive hydrogen tail that is as long as the planet's diameter.
This planet has provided many important milestones for science. It was the first planet detected through more than one method. It was also the first gas giant to have its superstorm measured. Astronomers even used it to measure orbital speed to determine mass directly. The planet is sometimes called Osiris. This name was proposed by A. Vidal-Madjar and A. Lecavelier des Etangs in 2003. They compared the evaporating atmosphere to the Egyptian god who lost part of his body. While the IAU has acknowledged the name, it is not yet an official proper name. Studying HD 209458 b helps us understand how all planets orbiting close to stars behave.
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