ULTRAHOT JUPITERS ACT LIKE STARS

Their at­mos­pheres are in­tense enough to rip wa­ter mol­e­cules apart

Sky at Night Magazine - - BULLETIN - www.unibe.ch/in­dex_eng.html

A pair of re­cent stud­ies into ultrahot Jupiters have re­vealed that they are su­per-heated to the point of act­ing act like stars, with at­mos­pheres so in­tensely hot they can tear wa­ter mol­e­cules apart, and va­por­ise iron and ti­ta­nium.

Ultrahot Jupiters are gas gi­ants that are tidally locked to their star, mean­ing that one side is con­stantly in day­light. As th­ese worlds are also in a tight or­bit, tem­per­a­tures can reach 4,000°C , which is hot­ter than many stars.

The spec­tra of th­ese worlds – which in­di­cate what el­e­ments and com­pounds are in their at­mos­pheres – has con­fused as­tronomers for many years. To help un­der­stand what’s go­ing on in the at­mos­pheres of th­ese odd worlds, one group of re­searchers re­cently sim­u­lated the at­mos­phere of ultrahot Jupiter KELT-9b.

“The re­sults of th­ese sim­u­la­tions show that most of the mol­e­cules found there should be in atomic form, be­cause the bonds that hold them to­gether are bro­ken by col­li­sions be­tween par­ti­cles that oc­cur at th­ese ex­tremely high tem­per­a­tures,” says Kevin Heng from The Univer­sity of Bern, who led the study.

This means that met­als would be va­por­ised by the heat, mak­ing them de­tectable. This was in­deed con­firmed when as­tronomers made fol­low-up ob­ser­va­tions of KELT-9b and found ti­ta­nium and iron in the at­mos­phere. Con­sid­er­ing the plan­e­tary at­mos­phere to be more like that of a star’s than a planet’s led an­other group, headed by Vivien Par­men­tier from Aix Mar­seilles Univer­sity, to un­cover a dif­fer­ent mys­tery re­gard­ing ultrahot Jupiters. As­tronomers ex­pected them to be rich in wa­ter, but in­stead they ap­pear al­most com­pletely arid. This study found that the ex­treme heat of the star­lit side would tear apart the wa­ter mol­e­cules in the at­mos­phere, cre­at­ing oxy­gen and hy­dro­gen. How­ever, the dark sides of ultrahot Jupiters are thou­sands of de­grees cooler and the dis­par­ity cre­ates pow­er­ful winds that carry th­ese el­e­ments into an at­mos­phere that’s cold enough for them to re­form. As the dark side is con­stantly in shadow, this can’t be con­firmed di­rectly through ob­ser­va­tion, but it could ex­plain why traces of wa­ter have been spot­ted along the ter­mi­na­tor be­tween day and night. “With th­ese stud­ies, we are bring­ing some of the cen­tury-old knowl­edge gained from study­ing the as­tro­physics of stars to the new field of in­ves­ti­gat­ing ex­o­plan­e­tary at­mos­pheres,” says Par­men­tier.

Ultrahot Jupiters have one side that al­ways faces their sun, which is one rea­son they heat up to such ex­tremes

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