Academic journal article The Science Teacher

Take off with NASA's Kepler Mission! the Search for Other "Earths"

Academic journal article The Science Teacher

Take off with NASA's Kepler Mission! the Search for Other "Earths"

Article excerpt


Humans have long wondered about life in the universe. Are we alone? Is Earth unique? What is it that makes our planet a habitable one, and are there others like Earth? NASA's Kepler Mission seeks the answers to these questions. Kepler is a space-based, specially designed 0.95 m aperture telescope. Launching in 2009, Kepler is NASA's first mission capable of finding Earth-size and smaller planets in the habitable zone of other stars in our galaxy. For the first time in human history, we will know if there are planets capable of supporting life beyond our solar system.

The Kepler Mission, NASA's 10th Discovery mission, provides an opportunity for interdisciplinary science teaching, as it combines Earth and space science with life science. Teachers and students can bring this mission down to Earth by using the accompanying poster, partaking in classroom activities, and visiting the mission's website (see "On the web").

Planets vary widely

The search for extrasolar planets begins with some fundamental questions about both planets and life. All planets are different and some are more habitable than others. Certain features make for a more livable planet. We are not talking about whether there are burger joints, televisions, iPods, coffee shops, or other modern conveniences but about the basic requirements for life.

Two fundamental characteristics for a habitable planet stand out. The first is surface temperature: If a planet is too close to its star, liquid water cannot exist on its surface. If it is too far from its star, any surface water could only be present as a frozen solid (Metz 2006). (Note: Water could exist as a deep ocean beneath an ice-covered surface.) The planet's orbital path around the star determines whether it is in the habitable zone--the life zone--where water is liquid on its surface.


The second characteristic is the planet's mass or size: If its mass is too small, there is not enough surface gravity to hold on to an atmosphere, as is the case on the Moon, Mercury, and Mars. On the other hand, if the mass of a planet reaches about 10 times Earth's mass, there is sufficient surface gravity to hold onto the most abundant and lightest elements in the universe: hydrogen and helium. Planets with this kind of mass can grow into gas giants, such as Jupiter, Saturn, Uranus, or Neptune, which are incapable of supporting life as we know it.

As in Goldilocks, there is a "just-right" orbit around a star that we call the habitable zone, an area in which the conditions are just right for supporting life. There is also a "just-right" size or mass, about 0.5-10 times the mass of Earth. Based on these criteria, scientists designed the Kepler Mission to detect Earth-size planets in the habitable zone of other stars. (Note: Earth's atmosphere also has other life-sustaining features. It transports water, protects us from solar ultraviolet and cosmic rays, contains the oxygen we breathe, and acts like a blanket to keep us warm.)

Hunting for Earth-size planets

We cannot send a spacecraft from star to star to find another Earth. The stars are too numerous and too far away--going to a nearby star would take hundreds of thousands of years. We cannot photograph a distant Earth-size planet with even the most powerful new Earth--and space-based telescopes. The problem is that the light from a distant star is billions of times brighter than the tiny amount of light reflected from a nearby planet, and because the planet is very close to the star, the light from the star obscures the light of the nearby planet. Thus, discovering other "Earths" requires a different approach.

To counter this problem, Kepler looks for the tiny change in the brightness of a star that occurs when a planet crosses in front of it. When the orbit of the planet is aligned along our line of sight to the star from the Kepler telescope, we can see these crossings, called transits--the transits allow us to confirm the existence of a planet and tell us more about the characteristics of that planet. …

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