Starship Asterisk*

https://apod.nasa.gov/apod/ap070417.html

How long can the water and atmosphere have been "rapidly evaporating"
from this planet? Even though it could have an excentric orbit, just a few passes would completely boil it away.... Their theory does not make sence... Certainly it could not have been in its orbit for long.

I am tempted to ask how many billions of years has its atmosphere been rapidly evaporateing away

Now that really is the question isn't it.

Good one. Two Thumbs Up to yah.

Norval

"Rapidly" is a relative term here. Even if millions of tons per year are evaporating away, that may be trivial. In fact, the earth alone has 330 million cubic miles of water, which is 1.5 quintillion tons. We could lose a 100 million tons of water every year for 10 billion years and still have some left over. Of course because of our distance and strong magnetic field, in reality we lose very little water or atmosphere.

This planet is much bigger, and although as a gas giant it wouldn't have oceans, it could still have as much or even far more total water as earth. I believe it is also thought to be younger than the earth, perhaps significantly, so even losing water at the huge rates listed above is not at all unreasonable. It is believed that gas giants can not form so close to a star, so the theory is that this one formed in a Jupiter-like orbit and migrated inward later on.

To figure out if this is reasonable, the next step (which some researchers probably have already undertaken) is to determine how much energy is required to evaporate this much water and heat it up enough to expand beyond the planet's gravitational reach. Then you compare that to the amount of heat believed to be absorbed by the planet from its star, based on the known distance and brightness of the star and the estimated diameter and reflectivity of the planet.

The entire atmosphere of this planet is gradually boiling away, and water is just one of the many compounds they've observed by spectroscopy.

http://antwrp.gsfc.nasa.gov/apod/ap070417.html

Ok, so even if it has quite a bit of water, and quite a bit of water is (probably) still on the planet, How much water are we really seeing?

I am imagine we just need to compare the strength of the frequencies associated with the absorption of light in water with the strength of the rest of the light we receive from that star.

Does anybody have any solid numbers?

Good question. I think your suggested method is probably as good as could be done, but I don't know numbers.

One of the links in the caption is to the research paper. I just read the abstract, which didn't give any mention, but there's a chance there may be some analysis of the quantity in there. However, it sounds like the paper doesn't just discuss water, but all of the significant molecules detected.

I made estimate that HD209458b lost about 4% of its mass till now. I did not publish it, I made similar aproch as:
Roche lobe effects on the atmospheric loss
from “Hot Jupiters”
N. V. Erkaev, Yu. N. Kulikov, H. Lammer, F. Selsis, D. Langmayr, G. F. Jaritz, and H. K. Biernat