A mirror is a reflective surface that creates an image of objects in the environment. Unlike diffuse surfaces that scatter light, mirrors have the unique property of reflecting waves in a particular direction (as a flat white piece of wall). Objects reflected by a plane mirror are not distorted in size, position, or color; they appear “real,” although their outlines may be warped. The reflected image, however, is laterally inverted (like the left hand in the right glove).
Similarly, when an exoplanet passes in front of its host star, a mirror-like effect causes the planet to look as if it were sitting above or below the star. This illusion, called a transit, allows astronomers to determine the presence of an exoplanet’s atmosphere.
Astronomers have spotted the first rocky planet that appears to shine as brightly as Venus, the most reflective planet in our Solar System. This strange world, more than 260 light-years from Earth, is the most reflective planet ever observed outside our Solar System. Using the European Space Agency’s exoplanet-probing Cheops space telescope, astronomers have found that LTT9779b, which orbits its host star every 3.5 days, reflects 80 percent of the light from that star. This remarkable brightness is equivalent to a world five times its size.
Astronomers modeled the chemical compositions of the planet’s clouds to analyze the data. They identified a potential composition of aluminum oxides such as corundum, the material of rubies and sapphires; silicon oxides, known as quartz; sulfides of manganese or zinc, which exist as minerals on Earth; and organic hydrocarbon compounds. The scientists also analyzed temperature maps of the planet’s surface. They discovered that its hottest region is shifted to the east of the star, meaning extreme winds blow hot gases into the planet’s night side before they can be reradiated back toward the star.
The team’s observations were published today in Astronomy & Astrophysics. The research was primarily funded by the ESA and NASA.
The planet’s extraordinary reflectivity was made possible by its cloud cover, which is thought to consist of droplets of titanium. The metal particles act like a mirror, absorbing light from the star and reflecting it away. Because a Sun-like star is a billion times more intense than the reflected light from a planet orbiting it, it can be challenging to detect the existence of a planet’s atmosphere using direct imaging methods alone. However, a planet’s atmosphere can significantly reduce the contrast between its reflected light and that of its parent star. This can dramatically increase the signal-to-noise ratio in a planet’s spectrographic data.