WASP-17b: anunciado em Agosto de 2009. Tem metade da massa de Júpiter, pode ter 2 vezes o tamanho de Júpiter, e um período orbital de 3,7 dias. Encontra-se a cerca de 1000 anos-luz da Terra.
O WASP-17b é o “Júpiter Quente” menos denso descoberto até hoje, com uma densidade bastante inferior à da água, o que faz com que possa flutuar num oceano terrestre (não fosse pelo seu tamanho).
E é o maior planeta descoberto até hoje!
Finalmente, o WASP-17b tem uma órbita retrógada, isto é, orbita a sua estrela no sentido contrário ao normal. Se virmos os sistemas planetários de cima, todos os planetas orbitam a estrela no sentido contrário ao do ponteiro dos relógios. Este é o primeiro planeta a orbitar a estrela no sentido do ponteiro dos relógios – ou seja, orbita a estrela no sentido contrário ao de todos os outros planetas conhecidos até hoje! Orbita a estrela no sentido contrário à da rotação da estrela.
O planeta orbita ao contrário provavelmente devido a uma colisão com um enorme objecto ou então devido a ter ficado tão próximo de outro planeta que lhe provocou um “slingshot” gravitacional – um incremento de velocidade tipo fisga – que o fez “saltar” para outra órbita e numa direcção inversa.
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universetoday.com…
Encontrou-se sódio na atmosfera deste exoplaneta 🙂
http://www.universetoday.com/79749/the-atmosphere-of-wasp-17b/
“One of the greatest potentials of transiting exoplanets is the ability to monitor the spectra and examine the composition of the planet‘s atmosphere. This has been done already for HD 18733b and HD 209458b. In a new article by a team of astronomers at Keele University in the UK, absorption spectroscopy has been applied to the unusual exoplanet WASP-17b, which is known to orbit retrograde.
Not only does the spectra tell astronomers the atmospheric composition, but can also give an understanding of the the composition, but can also be indicative of how the atmosphere absorbs the light from the star and how heat is transferred around the planet. Additionally, since the atmosphere will absorb differently at different wavelengths, this gives differences in the timing of the eclipse and can be used to probe the radius of the planet more tightly as well as potentially examining the layering of the atmosphere.
(…)
Applying these spectroscopic techniques to WASP-17b, the team discovered the presence of sodium in the atmosphere. Yet the absorption wasn’t as strong as expected based on models using formation mechanisms from a nebula with solar composition and forming a planet with a cloudless atmosphere. Instead, the team describes 17b’s atmosphere as “sodium-depleted” similar to HD 209458b.
An additional observation was that the depth of seeing dropped off when using certain filters with different bandwidths (ranges of allowed wavelengths). The team noted that at bandwidths greater than 3.0 Å, the amount of sodium absorption seen nearly disappeared. Since this property is related to how much atmosphere the light travels through, this allowed the team to speculate that this may be indicative of clouds in the upper layers of the atmosphere.
(…)
Lastly, the team speculated as to the reason on the lack of sodium in the atmosphere. They proposed that energy from the star ionizes sodium on the day side. The motion of the atmosphere carrying it to the night side would then allow it to condense and be removed from the atmosphere. Since giant exoplanets in such tight orbits would likely be tidally locked, the sodium would have little chance to return to the day side and be brought back into the atmosphere.”