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NASA's latest planet-hunter, the Transiting Exoplanet Survey Satellite or TESS, rocketed into space on April 18. It will be two months before the refrigerator-sized satellite maneuvers into its unusual operating orbit, a highly elliptical path inside the moon's orbit that takes TESS twice around Earth every time the moon circles Earth. TESS passes the moon at the most distant point in TESS's orbit. The two-to-one orbital times ratio makes for stability because, if TESS drifts a little off-path, the moon will pull it back into synchronization on the next pass.

TESS follows in the footsteps of NASA's famous Kepler orbiting telescope, which is running out of fuel after nine years in space. TESS will use the strategy Kepler used to confirm more than 2,600 "exoplanets." TESS will monitor the brightness of more than 200,000 "nearby" stars, looking for the tiny dips in a star's brightness that occur when an exoplanet passes directly in front of its star. The planet is said to "transit" the star. Detecting a dip is challenging because, even if a huge Jupiter-sized planet transits a distant star, the star's light dips by only 1 to 2 percent. And if a small Earth-sized planet transits its star, the starlight dips by a minuscule 1/100th of 1 percent.

Observations of transiting planets during successive orbits reveals the planet's size (determined from the amount of starlight blocked by the transiting planet) and the time required for each orbit. Further information about the planet can be determined from the way the star wobbles in response to the planet's orbital motion; one back-and-forth wobble accompanies each planetary orbit. This wobbling is detectable by other telescopes based on Earth. It's then possible, from the combined data, to determine an accurate mass (weight) and size for the planet.

Most of the stars observed by TESS will be "red dwarfs" much smaller and cooler than our sun and other sun-like stars. Red dwarfs make up the vast majority of stars, and most planets probably orbit such stars.

There's a second reason TESS will look mostly at red dwarfs. NASA is looking for life out there. For life to exist, the planet must have a "Goldilocks" temperature -- not too hot and not too cold to support life. The temperature should allow liquid water (not just ice or steam), which is probably essential for life. For the sun, this "possibly habitable zone" extends from outside the orbit of Venus to the orbit of Mars. For red dwarfs, this zone lies only a few million miles from the star, much closer than Earth's 90 million miles from the sun. Since it detects only transiting planets, TESS is far more likely to detect such a possibly habitable planet orbiting in a small tight loop around a red dwarf star, because such a planet spends a much larger fraction of its time transiting its star.

According to computational astrophysicist Elizabeth Tasker's fact-filled book "The Planet Factory," the Kepler satellite found an astonishing variety of planets of every sort imaginable within the laws of physics: worlds made of pure diamond, worlds completely covered by water, worlds covered by "oceans" of tar, worlds with two suns in their sky, planets far larger than Jupiter yet orbiting so close to their star that their "year" (their orbital time) lasts only a few days, and many planets "tidally locked in" to permanent day on one side and permanent night on the other.

Many discovered planets fall into two categories: rocky super-Earths up to 50 percent larger than Earth with a thin atmosphere, and mini-Neptunes (Neptune is our solar system's farthest planet) several times larger than Earth and made mostly of barren ice clouds.

NASA hopes TESS will find tens of thousands of new exoplanets, including hundreds of super-Earths. Although TESS cannot directly detect signs of life, it should identify hundreds of strong planetary candidates for life. This is especially exciting because TESS will look at stars as close as 10 to 50 light-years from Earth--the distance through which light travels in 10 to 50 years (our pancake-shaped Milky Way galaxy is 100,000 light years across). Such planets are so close that the James Webb space telescope, to be launched in 2020, might detect signs of life on them.

My guess: Life exists around many stars in our galaxy, and we will have convincing evidence of this by 2040. If our planet's 4 billion-year history is any guide, most life will be microbial and larger life forms will have insufficient intelligence to have developed technology.

Commentary on 05/08/2018

Print Headline: TESS searches for life signs

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