Focusing On ‘Second-Earth’ Candidates In The Kepler Catalog

The ongoing hunt for exoplanets has yielded some very interesting returns in recent years. All told, the Kepler mission has discovered more than 4000 candidates since it began its mission in March of 2009. Amidst the many “Super-Jupiters” and assorted gas giants (which account for the majority of Kepler’s discoveries) astronomers have been particularly interested in those exoplanets which resemble Earth.

And now, an international team of scientists has finished perusing the Kepler catalog in an effort to determine just how many of these planets are in fact “Earth-like”. Their study, titled “A Catalog of Kepler Habitable Zone Exoplanet Candidates” (which will be published soon in the Astrophysical Journal), explains how the team discovered 216 planets that are both terrestrial and located within their parent star’s “habitable zone” (HZ).

The international team was made up of researchers from NASA, San Francisco State University, Arizona State University, Caltech, University of Hawaii-Manoa, the University of Bordeaux, Cornell University and the Harvard-Smithsonian Center for Astrophysics. Having spent the past three years looking over the more than 4000 entries, they have determined that 20 of the candidates are most like Earth (i.e. likely habitable).

This figure shows the habitable zone for stars of different temperatures, as well as the location of terrestrial size planetary candidates and confirmed Kepler planets described in new research from SF State astronomer Stephen Kane. Some of the Solar System terrestrial planets are also shown for comparison. Credit: Chester Harman Read more at: http://phys.org/news/2016-08-team-second-earth-candidates.html#jCp
Figure showing the habitable zone for different types of stars, as well as the location of terrestrial size Kepler candidates. Credit: Chester Harman

As Stephen Kane, an associate professor of physics and astronomy at San Fransisco University and lead author of the study, explained in a recent statement:

“This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars. That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable.”

In addition to isolating 216 terrestrial planets from the Kepler catalog, they also devised a system of four categories to determine which of these were most like Earth. These included “Recent Venus”, where conditions are like that of Venus (i.e. extremely hot); “Runaway Greenhouse”, where planets are undergoing serious heating; “Maximum Greenhouse”, where planets are within their star’s HZ; and “Recent Mars”, where conditions approximate those of Mars.

From this, they determined that of the Kepler candidates, 20 had radii less than twice that of Earth (i.e. on the smaller end of the Super-Earth category) and existed within their star’s HZ. In other words, of all the planets discovered in our local Universe, they were able to isolate those where liquid water can exist on the surface, and the gravity would likely be comparable to Earth’s and not crushing!

Earlier today, NASA announced that Kepler had confirmed the existence of 1,284 new exoplanets, the most announced at any given time. Credit: NASA
Earlier today, NASA announced that Kepler had confirmed the existence of 1,284 new exoplanets, the most announced at any given time. Credit: NASA

This is certainly exciting news, since one of the most important aspects of exoplanet hunting has been finding worlds that could support life. Naturally, it might sound a bit anthropocentric or naive to assume that planets which have similar conditions to our own would be the most likely places for it to emerge. But this is what is known as the “low-hanging fruit” approach, where scientists seek out conditions which they know can lead to life.

“There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them,” said Kane. “This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth.”

Professor Kane is renowned for being one of the world’s leading “planet-hunters”. In addition to discovering several hundred exoplanets (using data obtained by the Kepler mission) he is also a contributor to two upcoming satellite missions – the NASA Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Characterizing ExOPLanet Satellite (CHEOPS).

These next-generation exoplanet hunters will pick up where Kepler left off, and are likely to benefit greatly from this recent study.

Further Reading: arXiv

New Finds From Kepler: 8 New Worlds Discovered in the Habitable Zone

A fascinating set of finds was announced today at the 225th meeting of the American Astronomical Society (AAS), currently underway this week in Seattle, Washington. A team of astronomers announced the discovery of eight new planets potentially orbiting their host stars in their respective habitable zones. Also dubbed the ‘Goldilocks Zone,’ this is the distance where — like the tempting fairytale porridge — it’s not too hot, and not too cold, but juuusst right for liquid water to exist.

And chasing the water is the name of the game when it comes to hunting for life on other worlds. Two of the discoveries announced, Kepler-438b and Kepler-442b, are especially intriguing, as they are the most comparable to the Earth size-wise of any exoplanets yet discovered.

“Most of these planets have a good chance of being rocky, like Earth,” said Guillermo Torres in a recent press release. Guillermo is the lead author in the study for the Harvard-Smithsonian Center for Astrophysics (CfA).

This also doubles the count of suspected terrestrial exo-worlds — planets with less than twice the diameter of the Earth — inferred to orbit in the habitable zone of their host stars.

Fans on exoplanet science will remember the announcement of the first prospective Earth-like world orbiting in the habitable zone of its host star, Kepler-186f announced just last year.

The Kepler Space Telescope looks for planets used a technique known as the transit method. If a planet is orbiting its host star along our line of sight, a small but measurable dip in the star’s brightness occurs. This has advantages over the radial velocity technique because it allows researchers to pin down the hidden planet’s orbit and size much more precisely. The transit method is biased, however, to planets close in to its host which happen to lie along our solar system-bound line of sight. Kepler may miss most exo-worlds inclined out of its view, but it overcomes this by staring at thousands of stars.

Kepler launch
The launch of Kepler from the Cape in 2009. Credit: NASA/Kim Shiflett.

Launched in 2009, Kepler has wrapped up its primary phase of starring at a patch of sky along the plane of the Milky Way in the directions of the constellations of Cygnus, Lyra and Hercules, and is now in its extended K2 mission using the solar wind pressure as a 3rd ‘reaction wheel’ to carry out targeted searches along the ecliptic plane.

Both newly discovered worlds highlighted in today’s announcement orbit distant red dwarf stars. Kepler-438 b is estimated to be 12% larger in diameter than the Earth, and Kepler-442 b is estimated by the team to be 33% larger. These worlds have a 70% and 60% chance of being rocky, respectively. For comparison, Ice giant planet Uranus is 4 times the diameter of the Earth, and over 14 times more massive.

A comparison of the new exoplanet finds between Earth and Jupiter. Credit: NASA/Kepler.
A comparison of the new exoplanet finds between Earth and Jupiter. Credit: NASA/Kepler.

“We don’t know for sure whether any of the planets in our sample are truly habitable,” Said CfA co-researcher in the study David Kipping. All we can say is that they’re promising candidates.”

The idea of habitable worlds around red dwarf stars is a tantalizing one. These stars are fainter and cooler than our Sun, and 7.5% to 50% as massive. They also have two primary factors going for them: they’re the most common type of stars in the universe, and they have life spans measured in trillions of years, much longer than the current age of the universe. If life could go from muck to making microwave dinners here on Earth in just a few billion years, it’s had lots longer to do the same on worlds orbiting red dwarf stars.

There is, however, one catch: the habitable zone surrounding a red dwarf is much closer in to its host star, and any would-be planet is subject to frequent surface-sterilizing flares. Perhaps a world with a synchronous rotation might be spared this fate and feature a habitable hemisphere well inside the snow line permanently turned away from its host.

The team made these discoveries by sifting though Kepler’s preliminary finds that are termed KOI’s, or Kepler Objects of Interest. Though these potential discoveries were far too small to pin down their masses using the traditional method, the team employed a program named BLENDER to statically validate the finds. BLENDER has been employed before in concert with backup observations for extremely tiny exoplanet discoveries. Torres and Francois Fressin developed the BLENDER program, and it is currently run on the massive Pleiades supercomputer at NASA Ames.

It was also noted in today’s press conference that two KOIs awaiting validation — 5737.01 and 2194.03 — may also prove to be terrestrial worlds  orbiting Sun-like stars that are possibly similar in size to the Earth.

The proposed target regions for the Kepler K2 mission. Credit: NASA/Kepler.
The proposed target regions for the Kepler K2 mission. Credit: NASA/Kepler.

But don’t plan on building an interstellar ark and heading off to these newly found worlds just yet. Kepler-438b sits 470 light years from Earth, and Kepler-442b is even farther away at 1,100 light years. And we’ll also add our usual caveat and caution that, from a distance, the planet Venus in our own solar system might look like a tempting vacation spot. (Spoiler alert: it’s not).

Still, these discoveries are fascinating finds and add to the growing menagerie of exoplanet systems. These will also serve as great follow up targets for TESS, Gaia and LSST survey, all set to add to our exoplanet knowledge in the coming decade.

The LSST mirror in the Tuscon Mirror Lab. (Photo by author).
The LSST mirror in the Tuscon Mirror Lab. (Photo by author).

And to think, I remember growing up as a child of the 1970s reading that exoplanet detections were soooo difficult that they might never occur in our lifetime… now, fast-forward to 2015, and we’re beginning to classify and characterize other brave new solar systems in the modern Age of Exoplanet Science.

-Looking to observe red dwarf stars with your backyard scope? Check out our handy list.