November 27 – Atmosphere on an Extrasolar Planet!

nPostednon November 27, 2015

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Havenyou ever heard of an extraterrestrial (E.T.) creature?

nAnnextraterrestrial creature would be an creature that is “not of thenEarth,” because one name for our planet is Terra. (The word terra,nlower case, means land or territory. And of course earth,nlower case, means soil or dirt. We tend to double-dip our words fornour planet!)

nWhatnabout extrasolar? What does that mean?

nAncreature who is extrasolar is “not of our solar system.” We oftenncall our local star the Sun, but sometimes we call it Sol. Anothernstar system would have its own star or stars, and perhaps somenplanets, satellites, asteroids, dwarf planets, and comets.

Fornas long as there have been humans, we have wondered if there werenother worlds. Part of the excitement of astronomy has been to confirmnthat some of the lights in the sky are, indeed, entirely separatenworlds (the Moon, Venus, Jupiter’s moon Io, and so forth). Verynencouraging, if we care about finding other forms of life.

nButnpart of astronomy has been to discover the enormous, enormousndistances in space. Huge gaps between worlds in our solar system, andneven huger – GIGANTIC – gaps between stars and tremendously hugernand amazingly gigantic gaps between galaxies.

nThosenhuge distances means that our chances of discovering other lifeformsnplummet from what we first imagined… Still, thanks to thencleverness of astronomers, we have been able to do what seemed almostnimpossible in my childhood:

nEvennthough they are so amazingly far away, we have been able to discovernextrasolar planets – planets circling other stars. Starting inn1992, scientists have used red- and blue-shifted light, light dimmednfrom transits, gravitational microlensing, and super-precisenmeasurements of stars’ positions to detect planets circling othernstars. We have even been able to directly image a few planets!

nAtnthis point, just a bit more than two decades into the exoplanet biz,nwe have already discovered thousands! n

nAsnyou have probably already guessed, we are especially interested inndiscovering extrasolar planets that are Earthlike – planets thatnare located in the Goldilocks Zone of their suns (you know, not tooncold and not too hot), and planets that are in Goldilocks-ishnsize-wise, too (not too large and not too small). Unfortunately, withnthe methods we have at our disposal, it’s much easier to detectnenormous gas giants that are far from their stars (not very friendlynto life as we know it) than it is to detect smaller rocky planetsnthat are warm enough for liquid water and, we hope, life.

nOnnthis date in 2001, scientists discovered the first atmosphere on annexoplanet. Utilizing data from the Hubble Space Telescope, theyndetermined that that the exoplanet often called Osiris has annevaporating hydrogen atmosphere with some oxygen and carbon.

nOsiris’snofficial name is HD 209458 b. But that official name is hard tonremember and not very colorful. So we are going to stick with thenunofficial “Osiris”!

nOsiris’snsun is HD 209458 – that star needs a cool name! If it were visiblento the naked eye from us Earthlings, Osiris and its sun would be seennin the constellation Pegasus. The star system is about 150 lightnyears from our solar system.

n(Thatnmeans that it takes light 150 years to travel from Osiris’s sun to usnhere on Earth.)

nBynthe way, Osiris is “the first” in several other categories, asidenfrom the “detectable atmosphere” category. It was the firstnexoplanet detected transiting its star, the first exoplanet detectednthrough multiple methods, and the first to have its orbital speednmeasured.

nBynthe way, I mentioned that it is easier to directly detect a planetnthat is far away from its star, but Osiris is really, really close tonits star. It whips around its sun in a much closer orbit thannMercury’s orbit around the Sun, so its year is just 3 and anhalf Earth days long! Being so to a star, the planet is prettynhot—about 1,800 degrees F (1,000 degrees C)!

nOsirisnis also enormous – it would have to be, to hold onto a hydrogennatmosphere! It’s larger than Jupiter:

Osiris (right) is larger than the largest
planet in our solar system, Jupiter (left).

nSince hydrogen is the lightest element in the universe, it doesn’t take much to blow it off a planet. The heat and stellar wind coming off Osiris’s sun are literally boiling away its atmosphere and, scientists have guessed, the escaping gas forms a tail rather like a comet’s tail. It will take a trillion years for the entire planet to evaporate.

nHere’s an artist rendition of what Osiris’s tail might look like, as seen from a rocky planet or moon:

nAnd here is another artist’s rendition of “the evaporating planet”:

nYou can see that Osiris is not a Goldilocks planet. It’s too big AND too hot, and we don’t expect that life-as-we-know-it could exist there. (Although it’s anyone’s guess when we consider life as we DON’T know it!)

nOur solar system doesn’t have any gas giants close to the Sun, but we have discovered several so-called “hot Jupiters” in other star systems. Some other terms for gas giants that circle their star innvery close orbit, aside from “hot Jupiters,” include roasternplanets, epistellar jovians, pegasids, and pegasean planets.

nAlsonon this date: