Last week was a good one for exoplanet enthusiasts, with yet more news relating to how other worlds are now being found by the thousands, and that there may be many habitable planets out there. Now there’s already another discovery being announced of three more planets almost the same size as Earth, all orbiting a nearby star.
Even after problems threatened to end the Kepler space telescope’s mission for good last year, the planet-hunting observatory has continued to help astronomers discover thousands of exoplanets orbiting other stars, including ones that are potentially habitable. As reported yesterday at the American Astronomical Society (AAS) meeting, Kepler has now confirmed just over 1,000 exoplanets, with thousands more awaiting confirmation. A growing number are also potentially habitable, at least by Earthly standards.
When it comes to exoplanets, the most exciting for many people are, of course, the ones which may be the most Earth-like, since these are regarded as the most likely to possibly support some form of life. Now, two new findings announced today will help astronomers to find these worlds and narrow down the best places to search for evidence of life in other solar systems.
With so many exoplanets now being discovered on a regular basis by astronomers, the focus has turned to what number of them might be habitable for some kind of life. For life as we know it at least, that depends on a number of factors, including being in the “habitable zone” of stars, where liquid water could exist on the surfaces of smaller, rocky planets like Earth. It has been thought that planets with extreme axial tilts, even horizontal to the plane of their orbits, would be less likely to host life. But now a new study suggests that they could still be quite habitable, if they are covered by oceans.
The Kepler space telescope has found its first new exoplanet, a “super-Earth,” of its secondary mission phase. The discovery adds to a current tally of 996 confirmed exoplanets and 4,183 planetary candidates already found by the revolutionary planet-hunting telescope.
Some of the most interesting exoplanets discovered so far are the “super-Earths,” rocky worlds which are significantly larger and more massive than Earth but still smaller than the ice giants such as Uranus or Neptune. Since they are not present in our own Solar System, their existence elsewhere can provide valuable information about planetary formation processes around other stars. One of the most significant aspects of this is the detection of water vapor in the atmospheres of these or other exoplanets, as this can help astronomers determine which super-Earths, or other exoplanets, may also have liquid water and be potentially habitable.
As it turns out, with current technology, it is easier to find water vapor in the atmospheres of hot super-Earths than it is in cooler ones. On hot super-Earths, water is more likely to be in the form of vapor in the atmosphere rather than liquid on the surface of the planet. By studying these hot worlds, however, scientists can find clues as to how much liquid water might be possible on cooler super-Earths. The super-Earths already found so far seem to come in a rather wide variety, from planets cloaked in dense, steamy atmospheres to rocky worlds covered completely by a global ocean.
As told to Astrobiology Magazine, “We are interested in knowing how much water there is on super-Earths regardless of the phase – liquid or vapor,” said Nikku Madhusudhan of the University of Cambridge and lead author of the new paper in a special exoplanet edition of the International Journal of Astrobiology.
The big question, of course, is how the discovery of water, in any form, might relate to a planet’s possible habitability. Exoplanets with liquid water would be the most exciting—any place on Earth where there is water, there is life. Of course, it is still unknown whether that holds true for other alien worlds as well, but it increases the chances significantly according to our current understanding of how life originated and evolved here.
It is, however, also important to know what kind of atmospheres these mega-Earths have. Studying the mass and radius of a planet can help astronomers determine the type of atmosphere it has. In our own Solar System, rocky planets can have very different atmospheres, ranging from Earth’s water-rich atmosphere to Venus’ extremely hot atmosphere full of carbon dioxide and sulphuric acid to the incredibly thin atmosphere of Mars.
So how can astronomers figure what kind of atmosphere a super-Earth exoplanet has, and how potentially habitable it may be? The mass and radius of a planet can provide the first clues. For example, a planet with a larger-than-expected radius for a world with lots of water, probably has an atmosphere with a composition lighter than water vapor. Temperature is also a key factor. Venus, for example, lies at the edge of the habitable zone in our Solar System, but its searing temperatures under a thick, choking carbon dioxide atmosphere make life there extremely unlikely.
Hot super-Earths are also easier to study because of how the high temperatures affect cloud formation. At very high temperatures, clouds would tend to form out of iron or silicates instead of water. These clouds would likely stay closer to the ground, meaning that more of the spectra of the atmosphere could pass through, making it easier to analyze. Of course, the closer a super-Earth is to our Solar System, the easier it may be to study its composition including its atmosphere.
As of now, there are just two super-Earths which are considered ideal for observation of their atmospheres: 55 Cancri e and HD 97658 b. Both are hot super-Earths, orbiting very close to their stars. 55 Cancri e has a temperature of over 3,100 ˚F (1,700 ˚C) and HD 97658 b has a temperature of about 1,100 ˚F (630 ˚C).
As Madhusudhan noted, “Clearly, 55 Cancri e is the ideal candidate at the moment.”
At present, it is also still easier to analyze the atmospheres of larger gas giant-type exoplanets given their huge size. Smaller super-Earths are more difficult, but progress is being made in being able to study the composition of their atmospheres as well. Even from what we already know about them, super-Earths can also have a wide range of atmospheres, which can greatly affect the potential habitability of those worlds. It is also easier to detect the atmosphere of a warmer planet than a cooler one, which is why the first exoplanet atmospheres currently being examined are those of “hot-Jupiters,” massive planets orbiting close to their stars.
Today’s space and ground-based telescopes can’t yet analyze the atmospheres of super-Earth or smaller planets, but the next upcoming ones, such as the James Webb Space Telescope, should be able to, at least to a limited degree. It is still not an easy task, given that these planets are so far away and much smaller than their host stars.
Other telescopes, like the Transiting Exoplanet Survey Satellite (TESS), the Characterising ExOPlanet Satellite (CHEOPS) and the PLAnetary Transits and Oscillations of stars (PLATO) are expected to discover hundreds more super-Earths, including ones in the habitable zones of their stars.
This next generation of space- and ground-based telescopes will also be able to study their compositions and atmospheres, including looking for possible biosignatures such as methane or oxygen. Those telescopes will also be capable of discovering many more of these worlds, which data from Kepler and other current telescopes suggests number in the millions in our galaxy alone.
The study of these super-Earths is one more big step in being able to find and learn about exoplanets which are smaller and more Earth-like, the long-held holy grail of exoplanetary science. Kepler has already shown that smaller rocky worlds similar in size to Earth are common and probably number in the millions in our galaxy alone.
Now, the preliminary evidence already for water in some exoplanet atmospheres will hopefully lead astronomers to the discovery of true alien water worlds, perhaps even inhabited ones.
The abstract for the new paper is available here.
This article was first published on AmericaSpace.
The search for exoplanets is about to enter an exciting new phase, as the Transiting Exoplanet Survey Satellite (TESS) mission has now been cleared for development by NASA. TESS will greatly expand the number of stars being observed for evidence of exoplanets orbiting them, as the next step forward from the Kepler space telescope and others which have already found thousands of such worlds outside of our own Solar System.
“This is an incredibly exciting time for the search of planets outside our solar system,” said Mark Sistilli, the TESS program executive from NASA Headquarters, Washington. “We got the green light to start building what is going to be a spacecraft that could change what we think we know about exoplanets.”
TESS will scan the entire sky with four cameras in its search, in both hemispheres, rather than focus on one particular patch of sky like Kepler does. The preliminary mission will last for two years, followed by a third year where ground-based telescopes will continue to monitor the exoplanets found during the first two years by TESS. By the end of the mission, TESS should have covered about 90 percent of the sky.
According to TESS Principal Investigator George Ricker of the Massachusetts Institute of Technology, Cambridge, Mass., “During its first two years in orbit, the TESS spacecraft will concentrate its gaze on several hundred thousand specially chosen stars, looking for small dips in their light caused by orbiting planets passing between their host star and us.”
TESS is anticipated to find at least 5,000 new exoplanet candidates, from large gas giants to smaller rocky worlds like Earth. At least 50 new Earth-sized exoplanets are expected to be found, some of which would hopefully orbit in the “habitable zone” of their stars, where temperatures and other conditions could allow liquid water to exist on their surfaces. These would be the most exciting discoveries: exoplanets which are similar to Earth and may support some kind of life.
As TESS Project Manager Jeff Volosin at NASA’s Goddard Space Flight Center in Greenbelt, Md., explained, “The most exciting part of the search for planets outside our solar system is the identification of ‘earthlike’ planets with rocky surfaces and liquid water as well as temperatures and atmospheric constituents that appear hospitable to life.” He added, “Although these planets are small and harder to detect from so far away, this is exactly the type of world that the TESS mission will focus on identifying.”
The next major step for the mission is the Critical Design Review in 2015.
“After spending the past year building the team and honing the design, it is incredibly exciting to be approved to move forward toward implementing NASA’s newest exoplanet hunting mission,” Volosin said.
Also different from Kepler, TESS will be able to find exoplanets orbiting stars closer to Earth than Kepler can, within a couple hundred of light-years.
“TESS should discover thousands of new exoplanets within two hundred light years of Earth,” Ricker said. “Most of these will be orbiting bright stars, making them ideal targets for characterization observations with NASA’s James Webb Space Telescope.”
As Volosin added, “The Webb telescope and other teams will focus on understanding the atmospheres and surfaces of these distant worlds, and someday, hopefully identify the first signs of life outside of our Solar System.”
Discovering exoplanets closer to home will be important in the search for life elsewhere. With these exoplanets, it will be easier for other coming missions, such as the James Webb Space Telescope, to determine their characteristics, such as density and atmospheric conditions, which could help scientists determine which exoplanets may have a habitable environment.
“This unique new data will comprise a treasure trove for astronomers throughout the world for many decades to come,” Ricker said. Volosin sums the mission up nicely: “I’m still hopeful that in my lifetime, we will discover the existence of life outside of our solar system and I’m excited to be part of a NASA mission that serves as a key stepping stone in that search.”
More information about TESS is available here.
This article was first published on AmericaSpace.