EARTH II: How to Find Earth-like Planets

There has been a lot of press lately about the discoveries of many “Earth-like” planets outside of our solar system orbiting other stars, otherwise known as terrestrial exo-planets. When I read press releases about these things, I picture exotic worlds filled with oddly colored vegetation, some animal like creatures, and maybe skinny humanoid biped extraterrestrials

with some intelligence and language. I also wonder how long before humans can colonize.

Then I remember that I am probably getting way ahead of myself and I start asking annoying questions like:

• What is the definition of an “Earth-like” exo-planet?
• What technologies are scientists using to discover exo-planets?
• What technologies are scientists using to discover the nature of these planets?

What do scientists mean when they call a planet “Earth-like”?

Historically, an Earth-like, or terrestrial, planet has been characterized as a rocky planet like Earth or Mars, as opposed to a gas giant like Jupiter. However, recent news articles seem to be adding an additional element to the vernacular connotation of “Earth-like”; they seem to be talking about rocky planets that are the same distance from their star as we are from our sun. The basic implication here is that scientists are trying to find environs that might be adequate or ideal to support life; like the our home planet Earth.

The most interesting thing to note about the definition of “Earth-like”planet is that it is a very broad definition. A planet exactly like Mars in another solar system would easily fall under the definition. As we all know, Mars is hardly lush with tropical rainforests.

What technologies are scientists using to discover exo-planets?

Right now, scientists can’t see these planets directly through conventional light telescopes, not even with the amazing Hubble Space Telescope without first knowing exactly where to look. This is because the planets are not bright or big enough relative to their cosmic surroundings to stand out. In the same way that ambient city lights make it difficult for us to distinguish stars in the night sky, bright stars make it very difficult for us to distinguish nearby planets in the night sky even with high powered telescopes. Because of this, scientists need to be creative when discovering distant non-star celestial bodies.

Most exo-planet discovery has been accomplished with a technique called gravitational microlensing. Gravitational microlensing harnesses one of clever Albert Einstein’s equations predicting the nature of light and gravity. Einstein predicted that light observed from a distant source, like a bright star, would bend whenever a massive object passes near the light beams between the source and observer. Not surprisingly, Einstein was right and scientists can use these light bends to determine when planets are passing between stars and us. Depending on how big the star is, scientists can use the degree of light bending to determine how big an orbiting planet is.

What technologies are scientists using to discover the nature of exo-planets?

Basically, gravitational microlensing can indicate the presence of a planet and a little bit about its mass. Once we know where a planet is, we can point high powered telescopes at it in order to catch a glimmer of its reflected light. Once we can observe the reflected light, we can learn more about a planet’s orbit using another indirect observation technique called radial velocity analysis.

To explain radial velocity analysis, we will talk a little bit about the nature of light. Light travels in waves with specific wavelengths for each color of light. Longer wavelengths look redder and shorter wavelengths look bluer. Another one of Einstein’s clever insights was that if a light source, in this case a planet, was moving toward you, its light wavelength would shorten and look then look bluer. When it is moving away, it will look redder. Because of this we can determine the distance and duration of a planet’s orbit. This is how, besides knowing a planet’s size, scientists can determine how far it is from parent star.

When will we know more?

In general, all we can really know about the exo-planets right now are the following characteristics:

1. That the planet is there.
2. How big it is
3. How close it is to its sun
4. How fast it moves

Because of those facts, we can guess what the planet is made of and how hot or cold it is.

We will know much more within the next decade when NASA launches its Terrestrial Planet Finder project into space and when ESA launches its DARWIN project. These two projects will take the technologies which we described above and lunch them into outer space to get a closer, less noisy, view of what is out there. Eventually we will be able to use color spectrometry to determine which elements and molecules constitute the planets’ masses. We will see if there is oxygen, methane, water, etc…all clues that would indicate possibilities of life elsewhere in the galaxy.

We have a ways to go, but maybe someday will finding ourselves looking at someone who is looking back at us. Well, because of the speed of light it would mean they were looking at us about 30 years ago and we are just seeing them now.