The discovery of three new super-Earths by scientists with NASA's Kepler mission is helping to reveal the bounty of extrasolar planets across the Milky Way.
Now another team is set to build a new orbiting planet hunter that, during a two-year mission, will search for other worlds closer to our sun's neighborhood.
The Transiting Exoplanet Survey Satellite (TESS), which NASA approved under its Explorer program earlier this month, will be the first orbiting observatory to hunt for planets all around the sky.
NASA and the project's scientists, led by George Ricker of the Kavli Institute for Astrophysics and Space Science at the Massachusetts Institute of Technology in Cambridge, Mass., aim to launch the observatory in April 2017.
In essence, TESS hopes to build on Kepler's pioneering role as an extrasolar-planet census taker and bring that nose count closer to home, where existing and future ground- and space-based telescopes can study in detail the planetary systems TESS uncovers.
Such studies "will allow us to really begin to figure out what their atmospheres are made of, what the temperature is like – actually characterize those planets," says Doug Hudgins, project scientist for Kepler and TESS at NASA headquarters in Washington.
This kind of analysis not only will help astronomers uncover the range of solar system configurations and test ideas on how planets form and evolve. Such up-close looks also could provide evidence for life on any of the newly discovered worlds.
Among the signs researchers might look for: the presence of ozone – a molecule made of three oxygen atoms – and nitrogen oxides in a planet's atmosphere. On Earth, fossil evidence indicates that the first algae capable of photosynthesis, which produces oxygen, emerged some 3.5 billion years ago. On Earth, bacteria also produce copious amounts of nitrogen oxides.
The hunt for signs of life elsewhere in the galaxy is one of the drivers behind Kepler, and the inspiration for TESS. But Kepler isn't looking directly for life. Instead, it is looking for Earth-mass planets orbiting at Earth-like distances around sunlike stars in order to provide an estimate of how common such planets are.
On Thursday, Kepler's science team announced the discovery of three super-Earths in or on the boundary of their stars' habitable zones. The habitable zone is a region around the star far enough away so that a planet doesn't overheat, but close enough so it doesn't freeze either. In principle, a planet orbiting in its star's habitable zone should be able to host liquid water in stable quantities on its surface. Liquid water is seen as essential for organic life.
But the nearest of these new systems is 1,200 light-years away. Although the team speculates that one of the two super-Earths there is a water world and the other likely has a rocky surface, and while both fall into their star's habitable zone, they are too far away and their star is too dim to study with anything more than computer models.
TESS's targets should fall well within the gaze of a new generation of ground- and space-based telescopes. But those telescopes need to know where to look, Dr. Hudgins notes. And that's where TESS comes in.
Like Kepler, TESS is designed to detect planets as they pass in front of their host stars, dimming the starlight by a tiny fraction. If one could look back at the sun from beyond the solar system and watch for the wink an orbiting Earth would impart, the light would vary by just 0.000085 percent, a no-see-um in human terms.
TESS is slated to carry an array of four exquisitely sensitive telescopes that can detect variations of starlight as small as 0.000040 percent. That sensitivity not only brings Earth-mass planets at Earth-like distances orbiting sunlike stars well within TESS's ability to detect it. It also boosts the chance the observatory will detect small planets around smaller, dimmer stars – including red dwarfs. These are estimated to be the most numerous type of star in the galaxy.
Over TESS's two-year mission it will be tough for the observatory to spot planets that take 365 days or more to orbit their star, because TESS will not stare at the same patch of sky continuously, as does Kepler.
But Kepler has shown that virtually every star has at least one planet, and where there's one, often there are many, Hudgins notes. So for stars about the size of the sun or somewhat larger, TESS may only find the tip of an extrasolar planetary iceberg. It will be up to ground- and space-based telescopes to monitor these systems over longer periods of time to detect additional planets.
But for smaller, cooler stars, especially red dwarfs, habitable zones are closer to the star, where it doesn't take as long for a planet to complete an orbit.
In all, TESS could find several hundred Earth-mass planets, MIT's Dr. Ricker has estimated.
The mission will be able to take advantage of two research tools Kepler has pioneered for overcoming the limitations inherent in using the transit method of detecting planets, as the orbital winking is known.
The Kepler team has discovered, for instance, that it can detect slight variations in the timing of a planet's orbit when a system hosts more than one planet. The subtle change in timing results from the gravitational interaction among planets in a multiplanet system.
If Kepler can spot at least one of the additional actors, the data and a detailed knowledge of the host star allow the team to estimate the masses of the two planets. Armed with the planets’ sizes from the transit method and masses from timing the orbits, researchers can estimate the planets' densities. And knowing the orbital period, they can estimate how far the planets are from the star. Armed with their knowledge of how hot the star is, they can figure out whether any of the observed planets orbit within a star's habitable zone.
The Kepler team also has evolved a method for ruling out to a high level of confidence so-called false detections of planets – winks in a star's light that could come from variations in its own brightness or be influenced by another, dimmer star close by.
These approaches have allowed the Kepler team to use models to get a rough estimate of what the planets it discovers would be like without having to wait for additional observations from other teams. Such observations are typically too difficult to make because the stars in Kepler's field of view are so distant and dim.
"If you build really good tools, there's all kinds of smart, creative people that are going to figure out innovative ways to do things that you never imagined you could do with that instrument before you flew it," Hudgins says.
TESS's discoveries will have the benefit of closer, more detailed scrutiny by others. But Kepler's techniques will allow the TESS team to make initial inferences about the planets they discover, inferences that should help set priorities for follow-up measurements from other observatories, where telescope time is a precious, limited commodity.
Although TESS is four years away from its currently planned launch, the two missions send what Hudgins sees as an exciting message.
"We live in a galaxy that has an incredible number of planets in it. Virtually every star in our galaxy likely has planets around it. What TESS is going to do is show us the planets that are in our own backyard," he says.