By Sabine Stanley, Ph.D., Johns Hopkins University
Jupiter is not the only gas giant in the universe! There are many planets like Jupiter in size but differ in their conditions and orbital distance. They are too bright to be looked at, their winds are too strong for some planets to bear, and the temperatures sometimes reduce the mass significantly.
Exoplanets, hot Jupiters, and other Jupiter-sized planets are all too far away to get samples from and easily send missions to. Yet, there is always a way to know more about them. The first way to know an exoplanet is to see how it disturbs the flow of signals from the star it orbits. The second thing is the light.
Light and Its Spectrum
Light and its spectrum emitted from an object in space can reveal many facts about them and the objects on their way. Likewise, the interrupted light from a star can tell how the planets orbiting it are.
When the starlight is received, it is mingled with the one from the exoplanet itself. The transit spectroscopy method, however, can help with the transiting planets.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
The Transit Spectroscopy Method
When a transiting planet goes behind the star it orbits, its light is completely blocked by the star. Thus, the light we get is only from the star. We should subtract the spectrum of light from the star from the total spectrum to get the planet’s spectrum.
The emission lines in the planet’s spectrum are then detectable. The spikes show extra light at particular frequencies created when specific atoms or molecules change energy levels and release photons.
Transit spectroscopy can also determine if exoplanets have clouds since clouds reflect light and affect the albedo. Albedo refers to how much a planet reflects light. Venus has a very high albedo of 0.75 because of its sulfur dioxide clouds.
Osiris, a hot Jupiter, is extremely dark with its clouds of vanadium and titanium oxides. Clouds can also reduce emission lines in the atmosphere as they block the view of the lower atmosphere.
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Scientists can also get information about the atmosphere’s thickness. When a planet moves in front of the star, for a while, only the atmosphere blocks the starlight. The duration required until the planet begins blocking the starlight can determine the thickness of the atmosphere.
The next determinable characteristic is the average temperature of the atmosphere. The total light from the planet in the infrared shows the emitted heat.
33b in the Little Fox
A hot Jupiter called HD 189733 b, 33b for short, is located 63 light-years away in the constellation of Vulpecula or “little fox”.
33b is a Jupiter-sized planet with mass about 13% larger than Jupiter, an orbital period of 2.2 days, and orbital distance of ten times closer than Mercury to the Sun. 33b is very close to the dwarf star it orbits and is tidally locked to, and its 700°C-temperature creates a rain of molten glass.
The temperature map made for 33b shows that the temperature difference of its night and days sides is about 300° at most. The nigh side never receives starlight as the planet is tidally locked, but winds carry the heat there.
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Winds in Jupiter-Sized Planets
Jupiter-sized planets, like 33b, orbiting so close to their stars usually have extremely fast winds. Wind speed can be determined by looking at the Doppler shifting of spectral lines from 33b’s atmosphere.
In the case of 33b, the Doppler shift would exist even without the winds because it moves toward and away from us, and it rotates in addition to orbiting. However, there is a third Doppler shift created by the wind motion.
On 33b, there is a global-scale eastward jet stream moving at over 5000 miles per hour, which is 20 times faster than the fastest Earth-winds ever.
Light in Jupiter-Sized Planets
Can we look directly at an exoplanet, despite its significantly strong light? Rarely, yes. A detection method that is best when the planet is as far away from the star as possible helps to get direct images from exoplanets, at least in infrared.
All these gas giants have a size similar to Jupiter, but their masses differ. The denser the exoplanet, the more heat it radiates. The problem is that the starlight dominates the light from the Jupiter-sized planet by far.
The planet cannot even be detected if no technique is used to reduce the light from the star. It is similar to trying to see a fainter light while staring at a flashlight. It worked, and from 2008 to 2019, almost 20 exoplanets were discovered with this method.
These Jupiter-sized planets range from five to ten times the mass of Jupiter, orbiting their star from about 15 astronomical units to 68, 2000, and 2500 AU! Sometimes, exoplanets have a world much more enormous than our solar system.
Common Questions about Jupiter-Sized Planets
Yes. Normally Jupiter-sized planets radiate a strong light. Sometimes, it is too strong to look into directly.
Like looking directly into a strong flashlight, looking directly at hot Jupiters and some other Jupiter-sized planets is somehow impossible because of the very strong light they emit.
Not only do hurricanes form on Jupiter-sized planets, but also, they are sometimes significantly bigger than winds on other gas giants.