Can planets orbit a blue star?

Another blue planet

An international team of researchers has succeeded for the first time in determining the color of an extrasolar planet. HD 189733b glows blue, but that is enough of its resemblance to the earth. The exoplanet being examined is a “hot Jupiter”, a gas giant that orbits its star in a narrow orbit. Scattering on silicate particles - a kind of rain made of glass in an atmosphere that is over a thousand degrees Celsius - is probably responsible for the blue color, according to the astronomers in the specialist journal "Astrophysical Journal Letters". While color determination was only possible via indirect measurements, a second team of researchers is presenting a real photo of a cool planet for the first time in the “Astrophysical Journal”. Gliese 504b is also a Jupiter-like gas giant, but it orbits its sun-like star in a wide orbit.

Gliese 504b

"We could see how the blue part of the spectrum dimmed when HD 189733b disappeared behind its star," said Tom Evans of the University of Oxford, recalling the observations made with the Hubble Space Telescope. “From this we were able to conclude that the planet itself must be blue, because the signal remained constant with the other colors.” And his team leader Frederic Pont from the University of Exeter adds: “It is the first time that the Color has succeeded in an exoplanet. We can now imagine what this planet would look like if we could see it directly. "

Of the more than eight hundred planets that have been discovered to date, astronomers have only been able to see a few of other stars directly. The vast majority of exoplanets only reveal themselves indirectly through small movements or fluctuations in the brightness of their central star. And the planets that have been photographed directly so far are all very young objects that are still glowing hot from their formation phase and are therefore easier to identify in the area of ​​thermal radiation.

Masayuki Kuzuhara from the University of Tokyo and his colleagues have now succeeded in photographing a cool planet with the Japanese Subaru telescope in Hawaii. The planet Gliese 504b has a temperature of about 240 degrees Celsius and therefore no longer shines itself in visible light. Gliese 504b has about four times the mass of Jupiter and moves its orbit around its star at 43.5 times the distance between the earth and the sun. Kuzuhara and his colleagues cannot say what color the planet is because they photographed it in the infrared range. They suspect, however, that its atmosphere is relatively free of clouds and therefore accessible to further spectroscopic studies.