Hubble Makes First Direct Measurements of Atmosphere on World Around Another Star

Hubble Telescope ^ | November 27, 2001

[The_Victor]

Hubble Makes First Direct Measurements of Atmosphere on World Around Another Star

1. How did they detect the atmosphere?

Astronomers discovered the atmosphere by watching how starlight dimmed slightly when the planet crossed in front of its star, an event known as a transit. During the transit, a small amount of starlight passed through the planet's atmosphere on its way to Earth. Hubble's spectrograph collected the light and dispersed it into the colors of the spectrum, which yielded clues about the atmosphere's chemical makeup. When astronomers analyzed the spectrum, they found the telltale "fingerprint" of sodium.

2. What does sodium reveal about the planet?

Astronomers expected to find sodium in the planet's atmosphere. Discovering sodium does not mean that life exists on the alien planet. In fact, astronomers don't think the planet can sustain life. It is a Jupiter-sized planet made up mostly of gas and is 20 times closer to its star than the Earth is to the Sun. The planet is so close to its star that its atmosphere is heated to a torrid 2000 degrees Fahrenheit (1100 degrees Celsius).

The astronomers, however, actually found less sodium than scientists had predicted for a Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere may have blocked some of the starlight. The astronomers discovered the sodium by analyzing the starlight that passed through the planet's atmosphere.

3. What are the conditions for life?

A key ingredient for life as we know it is oxygen. The most suitable planets for life, where oxygen may be abundant, are small, rocky planets like Earth that orbit at comfortable distances away from their stars. Finding these planets and probing their atmospheres for signs of life is beyond the scope of current telescopes and detection techniques, including the transit method used in this Hubble observation. So far, astronomers have been successful at discovering a parade of alien planets. But all of them are Jupiter-sized giants that are much larger than Earth. Some of them orbit perilously close to their stars, like the planet whirling around the star HD 209458.

[steve-b]

C. Looked at the whole spectrum and picked out absorption lines that exhibited a Doppler wobble with respect to the emission lines as the planet moved around the star.

Er, no. If the planet is directly between the star and us, its radial velocity is close to zero unless it's in a very eccentric orbit.

[Physicist]

Well, in order to look for a wobble, you have to look at the whole orbit, not just one part of it. The point is that as the planet swings around the star, its spectral lines will move back and forth periodically with respect to the lines of the star. The radial velocity with respect to us at any one point doesn't matter; it's the change in radial velocity over the course of an orbit that you'd measure.

That's not the method they used, of course. I was shooting from the hip before I read the article.

[leadpenny]

The APoD for 28Nov01.

[The_Victor]

Link here to a few details on how they did it from Science@NASA

[Neuromancer]

There is only one kind of gravitational field, and the more intense it is, the more the effective slowing of the speed of light. Gravitational lenses only have a positive index of refraction; there is no way to get a negative index of refraction. Gravitational fields can't give you a shortcut.

Einstein's special theory of relativity predicts that nothing can exceed the speed of light. But special relativity applies when spacetime is flat. When spacetime is curved, the theory applies only "locally"--that is, over regions of spacetime small enough to be considered flat. Consider the analogy of a plane that is tangent to a sphere. The flat geometry of the plane is a good approximation to the geometry of the sphere when the size of the plane is very small compared to the sphere's radius of curvature.

In curved spacetimes, when we compare two observers at large separation, we can no longer use the "locally flat" approximation. In the plane-and-sphere analogy, this situation would correspond to comparing two observers on the sphere separated by a distance comparable to the sphere's radius of curvature. Although each observer could approximate the geometry in his or her local region as a plane, there is no single plane that would be applicable to both observers. Consequently, the two observers in curved spacetime can each apply special relativity in their own local region, but not globally.

A similar situation arises in an expanding universe. Here one should not think of the galaxies as moving through space, but rather that the space between the galaxies is expanding. Einstein's general theory of relativity, on which such models are based, imposes no restrictions on the rate at which the expansion of space can drive the galaxies apart. But special relativity still applies locally, in the sense that a particle chasing a light ray can never catch up to it. An analogy is to imagine bugs crawling on a rubber sheet. By stretching the sheet we can make the bugs recede from each other at arbitrarily high speeds, but no bug can crawl across the sheet faster than a light beam.

In serious proposals for "warp drive," such as the Alcubierre warp bubble, space is flat inside the bubble and special relativity applies. In this region, nothing can travel faster than light--relative to observers inside the bubble. Outside the bubble, spacetime is also flat and no particle can travel faster than light--relative to observers outside the bubble. But because of the large expansion and contraction of the spacetime in the wall of the bubble, the inside of the bubble can move faster than light relative to the outside. This would also be true of light rays inside the bubble; they would be carried along by the spacetime warp, too. What causes this mismatch of the two flat spacetime regions is the large spacetime curvature in the bubble wall that separates the regions.

Thank you both for the opportunity to discuss such esoteric subjects.

[RadioAstronomer]

such as the Alcubierre warp bubble

I have been reading about those. :)

[RadioAstronomer]

Where the rub is, how do we make such a spacetime discontinuity? Spacetime is "stiff"! We observe curvature due to mass but overall, it is an extremely flat universe we inhabit. Where would the energy for such a "bubble" come from? And should we be able to create such energies, could we direct them in a meaningful way to obtain the results we desire?

[Joe Hadenuf]

Something tells me that they couldn't detect the atmosphere of an Earth size planet that way. Or even detect the planet for that matter. Boggles the mind that they can do this anyway.

Actually it would depend upon how far the star system is from earth. If it was close by, it would likely be detectable and even more likely if the planet had an atmosphere that extended a sizable distance from the planet.

I am confident that in the future, new telescope designs for Earth based telescopes and Earth orbiting telescopes will actually be able to detect surface details on planets orbiting other star systems many light years away from Earth.

[<1/1,000,000th%]

Its been awhile, but I remember that SR only predicted that objects couldn't travel at the speed of light. Objects with imaginary mass (factored by square root of negative one) could travel faster than the speed of light. I just have no idea what that means.

Maybe this is the physical property of the universe that allows us to imagine that objects can travel faster than the speed of light.

OK. I'm just goofing around, but you guys were getting too technical for me.