Every Day Is Special: November 27 – Atmosphere on an Extrasolar Planet!

November 27 – Atmosphere on an Extrasolar Planet!

Posted on November 27, 2015

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

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

What about extrasolar? What does that mean?

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

For as long as there have been humans, we have wondered if there were other worlds. Part of the excitement of astronomy has been to confirm that some of the lights in the sky are, indeed, entirely separate worlds (the Moon, Venus, Jupiter's moon Io, and so forth). Very encouraging, if we care about finding other forms of life.

But part of astronomy has been to discover the enormous, enormous distances in space. Huge gaps between worlds in our solar system, and even huger – GIGANTIC - gaps between stars and tremendously huger and amazingly gigantic gaps between galaxies.

Those huge distances means that our chances of discovering other lifeforms plummet from what we first imagined... Still, thanks to the cleverness of astronomers, we have been able to do what seemed almost impossible in my childhood:

Even though they are so amazingly far away, we have been able to discover extrasolar planets – planets circling other stars. Starting in 1992, scientists have used red- and blue-shifted light, light dimmed from transits, gravitational microlensing, and super-precise measurements of stars' positions to detect planets circling other stars. We have even been able to directly image a few planets!

At this point, just a bit more than two decades into the exoplanet biz, we have already discovered thousands!

As you have probably already guessed, we are especially interested in discovering extrasolar planets that are Earthlike – planets that are located in the Goldilocks Zone of their suns (you know, not too cold and not too hot), and planets that are in Goldilocks-ish size-wise, too (not too large and not too small). Unfortunately, with the methods we have at our disposal, it's much easier to detect enormous gas giants that are far from their stars (not very friendly to life as we know it) than it is to detect smaller rocky planets that are warm enough for liquid water and, we hope, life.

On this date in 2001, scientists discovered the first atmosphere on an exoplanet. Utilizing data from the Hubble Space Telescope, they determined that that the exoplanet often called Osiris has an evaporating hydrogen atmosphere with some oxygen and carbon.

Osiris's official name is HD 209458 b. But that official name is hard to remember and not very colorful. So we are going to stick with the unofficial “Osiris”!

Osiris's sun is HD 209458 – that star needs a cool name! If it were visible to the naked eye from us Earthlings, Osiris and its sun would be seen in the constellation Pegasus. The star system is about 150 light years from our solar system.

(That means that it takes light 150 years to travel from Osiris's sun to us here on Earth.)

By the way, Osiris is “the first” in several other categories, aside from the “detectable atmosphere” category. It was the first exoplanet detected transiting its star, the first exoplanet detected through multiple methods, and the first to have its orbital speed measured.

By the way, I mentioned that it is easier to directly detect a planet that is far away from its star, but Osiris is really, really close to its star. It whips around its sun in a much closer orbit than Mercury's orbit around the Sun, so its year is just 3 and a half Earth days long! Being so to a star, the planet is pretty hot—about 1,800 degrees F (1,000 degrees C)!

Osiris is also enormous – it would have to be, to hold onto a hydrogen atmosphere! It's larger than Jupiter:

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

Since 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.

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

And here is another artist's rendition of "the evaporating planet":

You 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!)

Our 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 in very close orbit, aside from “hot Jupiters,” include roaster planets, epistellar jovians, pegasids, and pegasean planets.

Also on this date: