'Warm Neptune' Exoplanets May Have Lots of Helium

An anonymous reader writes: Phil Plait reports on new research into exoplanets that came to an unexpected and non-obvious conclusion. Throughout the galaxy, astronomers have been finding exoplanets they call "warm Neptunes" — bodies about the size of Neptune, but which orbit their parent star more closely than Mercury orbits the Sun. When astronomers looked at spectra for these planets, they found something surprising: no methane signature (PDF). Methane is made of carbon and hydrogen, and it's generally assumed that most large, gaseous planets will have a lot of hydrogen. But this class of exoplanet, being significantly smaller than, say, Jupiter, may not have the mass (and thus the gravity) to hold on to its hydrogen when it's heated by the close proximity to the star. The result is that the atmosphere may be largely made up of helium instead. If so, the planet would look oddly colorless to our eyes, very unlike the planets in our solar system.

Helium

[Translation Error]

The result is that the atmosphere may be largely made up of helium instead. If so, the planet would look oddly colorless to our eyes,

and have a really squeaky, high-pitched voice.

Re:Helium [Drab planets]

[Tablizer]

TFA: If so, the planet would look oddly colorless to our eyes, very unlike the planets in our solar system. [Emph. added]

Venus is pretty drab to the human eye. It's bland light-grey with a slight tinge of yellow or tan, perhaps with faint wisps of lighter streaks on occasion. It's one of the reasons why the early probe, Mariner II, didn't have cameras.

Around the mid 60's it was discovered that in ultraviolet light, diagonal bands can be seen fairly clearly, and later probes had ultraviolet cameras.

Uranus i

Re:

[gstoddart]

Which means anyone from that planet visiting elsewhere would have a voice lower than Michael Clark Duncan [wikipedia.org], and would therefore be a total badass.

All of their singers would immediately make Barry White sound like a squeaky teenager.

The swooning would be incalculable. Knickers would be removed from several star systems away.

You could actually discover the brown note.

Re:

[zerosomething]

umm... well... If it reflects all colors then it would be white, so that's not colorless. If it absorbs all colors it'd be black which means it also reflects no color. Reflecting no color would be colorless then. So then it looks like what people think a black hole looks like, and even blacker place in the sky. Yea that's it.

Re:

[WalksOnDirt]

It is transparent to all colors. What's wrong with calling that colorless?

Re:Colorless? (The Fine Article Says...)

[willworkforbeer]

"helium planets would be somewhat colorless, gray or white"

Maybe you'd prefer 'not colorful'?

Re:

[shaitand]

It looks like whatever is on the other side of it.

Semi-off Topic Dr. Plait recommendation

[willworkforbeer]

If you have a younger family member interested in an introduction to astronomy, check out his work on the CrashCourse series sponsored by PBS. Very well done.

Molecular weight that matters, not atomic weight.

[erice]

From TFA:

The next most abundant element in giant planet atmosphere is helium (four times heavier, per atom, than hydrogen). That means that these warm Neptunes would have proportionately a lot more helium than planets in our solar system, and in fact may have air that’s mostly helium!

But this is not the right math. Mono-atomic hydrogen is nearly a hypothetical construct. Under nearly all circumstances, hydrogen comes bound in pairs. H2: molecular weight of 2. This is still less than helium (atomic weight of 4), which is nearly always mono-atomic but it not a simple thing that helium stays because it is heavier.

Earth has plenty of hydrogen because it binds so readily with heavier elements. Methane (CH4) has molecular weight of 16. Water is 18. 4 is not enough. Earth's primordial helium escaped long ago. All the helium on earth now is due to alpha particles spit out from the radioactive decay of heavier elements.

Venus is thought to have lost its hydrogen due to intense solar radiation breaking apart complex molecules in the upper atmosphere, allowing the freed hydrogen to escape. It is an interesting balancing act to consider a planet where the radiation is strong enough to strip all the hydrogen from a Neptune size planet yet not strong enough to heat it enough for the helium to also escape.

Hydrogen can be monatomic in low pressure

[PeterM from Berkeley]

and high energy conditions.

Such as in the high, upper, tenuous atmosphere of one of these giants. The H2 molecule, which is the rule in higher pressures, can be disassociated by light, and the monatomic H can persist a while because of the low pressure and low collision rates.

So escape of monatomic hydrogen could indeed be the significant loss mechanism for these planets. H-H bond is 4.5 eV, which requires an ultraviolet C photon, or multiple photons of lower energy. Quite doable close in to a star.

--PM