Imagine trying to spot a moth flying around the rim of a searchlight. If the light is a few feet from you, there’s a chance you would catch the occasional flicker of motion, but the moth would be largely hidden by the glare.
Now imagine the spotlight shines as bright as the sun and is several light-years away. Chances are all signs of that moth are obliterated.
Such is the challenge placed at the feet of those searching for habitable Earthlike worlds orbiting sunlike stars.
Prowling the poster hall here at the winter meeting of the American Astronomical Society in Long Beach, I came across a group of people who hope to overcome the obstacles by sending a giant gold daisy into space.
So far there’s more than 300 confirmed exoplanets—worlds orbiting other stars—on the books. Most have been found by detecting gravitational hints of their presence, while a few have even been directly imaged.
But the vast majority of exoplanets we find are pretty darn huge, many times the size of Jupiter. That’s because the behemoths either tug hard enough on their host stars to be readily apparent or sufficiently dim them at regular intervals when they pass between their star and Earth.
Finding an exoplanet the size of Earth has been a real challenge, especially if it needs to be close enough to a bright, warm star that liquid water could exist on its surface.
Several missions are now in the works to send out probes that are designed to look for these so-called other Earths, including one from NASA called Kepler, due to launch this year, that will search for planets that pass in front of their host stars.
The block of posters I stopped to examine is for a proposed mission, the New Worlds Observer, that was recently awarded $3 billion NASA dollars to support its development study.
The mission aims to block out a distant star’s glare using a device called a starshade that would lie about 80,000 miles (128,000 kilometers) in front of a space telescope twice the size of Hubble.
Like an outfielder shielding his or her eyes to catch an oncoming baseball, the telescope gets most of the light from the target star blocked out so it can see the much fainter glow of orbiting Earths.
The idea is an adaptation of a standard coronagraph—the special disk that allows probes examining our sun to shut out the main glow and see structures such as loops and flares within the corona.
But a straight-up round disk doesn’t work as well when the objects you’re trying to see are so far away that they simply vanish in the hazy ring of light around a partially blocked star. To see anything on the scale of an Earthlike planet, you have to dim the light to next to nothing.
That’s where the flower petals come in. The specific, mathematically defined shape of these petals controls diffusion so that the low amount of light that does leak out from behind the shade illuminates rocky worlds without overpowering them.
A simulated view of what New Worlds might see; the pale blue dot (upper left) is an Earthlike planet
I won’t go into the exact equations, but the tech-minded among you can read more about the petals’ math here.
Several of the posters here at AAS are about what the research team hopes to do if the New Worlds mission goes forth to explore the cosmos.
By far the coolest goal in my book is the simulation that suggests the mission will not only see other Earths, but will be able to tell us their rotation rates, rough land masses, and seasonal changes.
That’s right: We’ll be able to map habitable alien planets.
The idea is that once we’ve found a world that looks about right, we can watch its changes in brightness over time to tell, for example, whether we’re seeing bright desert or dark loamy soil, snow-capped peaks or the wide expanse of oceans.
We’ll then be able to watch for changes in brightness within specific regions that tell us whether a low-lying plain is covered with snow vs. grass on regular intervals, i.e, whether it’s winter or spring and how long it takes for the seasons to turn.
Obviously at such great distances our maps of the land forms on other Earths won’t be very precise—think maps of the New World from the 1700s.
The simulation I saw on one poster showed what our Earth would look like using this technique, and it’s basically two blobs of green over a blue ocean that—if you squint and stand to the left—sorta look like the continents we know and love.
But hey, after the building excitement over finding other habitable worlds, if they do finally find one and manage to make even the most basic of maps, I’ll take it!