Exoplanet Phases Seen in Optical Light
In this artist depiction, CoRoT-1b can be seen in several phases as it orbits its sun. Because the planet is tidally locked, it has a permanent dayside and nightside.
Credit: Leiden University

For the first time, astronomers have observed the phases of an extrasolar planet in visible light, as the world orbits around its star.

The planet, CoRoT-1b, was the first planet discovered by the French CoRoT (Convection Rotation and Planetary Transit) satellite about 2.5 years ago. It is about 1,600 light years away in the constellation Monoceros.

CoRoT-1b is a so-called "hot Jupiter." These planets are the size of Jupiter but orbit very closely to their stars (CoRoT-1b orbits its star in just 36 hours). Hot Jupiters are expected to be tidally locked, with one side always facing their stars, the other permanently dark (our own moon is tidally locked with the Earth, only showing its "near side" to us).

If these planets do have permanent daysides and nightsides, astronomers expect to see very large temperatures differences between the two hemispheres, depending on which one was in view from Earth's vantage point.

Astronomers had taken infrared measurements of these light and dark phases, but the CoRoT-1b measurements mark the first time they have been seen in the optical wavelengths, and they show that the nightside of the planet is completely dark, while the dayside is strongly heated by the star up to about 3,600 degrees Fahrenheit (2,000 degrees Celsius).

"So we see a very large difference between the dayside and the nightside," said study author Ignas Snellen of Leiden University in the Netherlands.

The measured effect is very much like the changing phases of objects in our own solar system, such as Earth's moon, when the sun shines on the moon from different directions while the moon revolves around the Earth. Although in the case of our moon this is reflected sun-light, while for CoRoT-1b it is probably heat radiation.

The observations, detailed in the May 28 issue of the journal Nature, also shed light on the atmosphere of the exoplanet and suggest that there is not a lot of transfer of heat from the dayside to the nightside of the planet. Another observed exoplanet had less of a heat difference between its two sides, suggesting that wind was transporting the incoming solar energy, but "for this planet, that is apparently not the case," Snellen told SPACE.com.

Instead, CoRoT-1b may have certain molecules in its atmosphere that absorb and re-radiate the light coming from the star on the dayside before that energy can be transferred to the other side of the planet.

Optical observations can also tell astronomers whether or not any of the light they are seeing is reflected starlight, which would suggest the exoplanet had clouds. Unfortunately, the researchers could not tell if any of the light from CoRoT-1b was reflected, Snellen said.

To determine whether or not the light is reflected, the astronomers will need observations of the planet at multiple wavelengths, Snellen said.

Snellen and his colleagues have already started to look at observations of other exoplanets to glean more information on the phases and atmospheres of hot Jupiters.

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