Humans may or may not be in a unique position in the universe, but we now know Earth is definitely not.
Notre Dame astrophysicist Justin Crepp was part of the team that discovered the first Earth-sized terrestrial planets in a habitable zone. His team published their findings in the journal, Science, last week.
Crepp said the discovery adds five new planets to a confirmed list of 62 systems that contain terrestrial planets. What is most notable is the unprecedented similarity to Earth in terms of size, with two of the planets having radii just 1.41 and 1.61 times greater than the Earth's, he said.
Crepp said the planets are similar to Earth in several ways and more similar than other known planets.
"The reason it's special is that they're pretty much the closest thing that we have found that resembles Earth," Crepp said. "In terms of their size, and their orbital period they resemble the Earth and satisfy some of the conditions that we think are necessary for life to form in the first place-one of them being having the right temperature."
Crepp said he undertook this latest endeavor as a member of a team of astrophysicists using NASA's Kepler Space Telescope to view the transits of exoplanets to discover Earth-like entities orbiting solar-type stars. Transmits are the orbits directly in front of the star respective to the viewer.
Crepp said the telescope focuses on a particular section of the nights sky and measures very slight changes in the brightness of stars.
"It all starts with Kepler staring continuously at a hundred-and-fifty thousand stars, and there's this patch of sky that it looks at with an unblinking eye," Crepp said. "It's monitoring the brightness of stars and whenever a planet passes in front of the star, it casts a shadow or blocks some of that light, and Kepler is sensitive enough to actually measure that."
Crepp said he follows up with a ground-based optics approach to further characterize the system once notified by a sophisticated algorithm that combs through the Kepler data to determine planetary candidates.
"I use a large telescope from the ground and my role in this particular Kepler discovery was to follow up those objects of interest with imaging," Crepp said. "From the ground I can actually see if there are any sources of contamination that might trick us into thinking we're looking at planets, but we're actually just looking at a vanilla binary."
He said one of the most important aspects of this step in the process is ruling out all probable alternatives that may result in the signal being a false positive, such as when the system actually has two stars - a binary - that occult each other. This binary problem occurs with 50% of all stars, Crepp said.
Despite the recent discovery, Crepp said he believes that they've reached the limits of what they can currently characterize in the Kepler 62 system.
Crepp said the next step, beyond continuing to look through the NASA database of roughly 3,000 Keplar Objects of Interest (KOIs) for other Earth-like planets, is to start looking for such planets closer to home.
Crepp said he is working to develop an instrument he calls the infrared Large Binocular Telescope Exoplanet Recovery, or iLOCATER, which will use a fundamentally different detection technique to search for Earth-like planets roughly 10 lightyears away - two orders of magnitude closer than the current sampling performed by the Kepler project.
"My instrument uses a different technique; it doesn't use the transit technique. It uses the Doppler radial velocity technique where you look at a star's 'wobble' in-and-out of the sky," he said. "If a planet is orbiting a star it will orbit its center of mass and so if you just isolate what the star is doing, it's being tugged on by the planet ever so subtlety, a few meters per a second as it turns out - about as fast as you would run. It's impressive that you can actually measure that for a star."
Crepp said he will have the iLOCATER making measurements in two years, which will then allow researchers to make the next step in characterizing the atmospheres and composition of nearby Earth-like planets that are discovered.
Crepp said the discovery is an important step in human understanding of the universe, but it prompts more questions than it answers.
"It's still a small step in the grand scheme of things but it's an essential, really important step," he said. "You answer one question and it brings up others."
