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Astronomers discover Earth's 'bigger, older cousin'

Scott Sutherland
Meteorologist/Science Writer

Saturday, July 25, 2015, 12:09 PM - NASA's Kepler Space Telescope has found a planet that appears to be a bigger, older cousin of Earth. Watch live on Saturday as astronomers discuss the incredible discovery.

On Thursday, July 23, NASA hosted a live teleconference to announce a new discovery by their Kepler Space Telescope mission: a distant extra-solar planet (or "exoplanet"), named Kepler 452b, that is so like our own world that they have called it "Earth's bigger, older cousin."

At 8 p.m. Eastern Time, Saturday, July 25, the Slooh Community Observatory is hosting a live show, streamed over the web, to lead what is sure to be a fascinating discussion about this incredible find.

Slooh astronomer Will Gater will be featuring live views of this planet's parent star, Kepler-452, and will be joined by senior SETI astronomer Dr. Seth Shostak and Kepler data analysis lead Dr. Jon Jenkins to talk about Kepler 452b, what this discovery means for us and to answer the public's questions.

HAVE QUESTIONS ABOUT EARTH'S BIGGER, OLDER COUSIN? Post your queries via Twitter using the hashtag #SloohEarth2

"The discovery of Kepler-452b is another exciting step forward in our hunt for a planet that’s just like our own," Slooh astronomer Will Gater said in a press release. "With the help of exoplanet experts, we’ll be tackling some of the intriguing questions about this alien world live on the show. How did astronomers find it? How much do we know about it? And what do we need to learn about an exoplanet's structure and composition before we can say that we’ve actually found another Earth? Tune in to find out!"

Most Earth-like planet orbiting a Sun-like star

"On the 20th anniversary year of the discovery that proved other suns host planets, the Kepler exoplanet explorer has discovered a planet and star which most closely resemble the Earth and our Sun," John Grunsfeld, associate administrator of NASA’s Science Mission Directorate, said during the briefing. "This exciting result brings us one step closer to finding an Earth 2.0."

Kepler 452b is referred to as "Earth's bigger, older cousin" because it is a super-Earth - roughly 1.6 Earth radii and about 5 times Earth's mass - and it orbits a Sun-like star around 1,400 light years away, which is estimated at 6 billion years old (1.5 billion years older than our Sun). The star, Kepler-452, is a yellow G-type star which is not only older, but also slightly larger and brighter than our Sun.

Since Kepler 452b circles its parent star at a distance just slightly farther away than Earth orbits the Sun, the exoplanet receiving about the same amount of solar energy as our homeworld.

The Kepler-452 system compared with the Kepler-186 system and our own solar system. Kepler-186 is "a miniature solar system" that can fit inside the orbit of Mercury, and with a habitable zone smaller and closer in to the star. The habitable zone of star Kepler-452 is slightly larger than our Sun's habitable zone, due to Kepler-452 being older, bigger and brighter. Kepler-452b orbits at a distance of 1.05 AU - 5 per cent farther than Earth orbits the Sun - and goes around its star once every 385 days. Credits: NASA/JPL-CalTech/R. Hurt

Kepler 452b's size means there's a good chance it is a rocky world, and its distance from its star means it is a potentially-habitable exoplanet. It may not be exactly Earth-like, but it is one of the most Earth-like exoplanet discovered so far by Kepler, and is the most Earth-like planet discovered orbiting a Sun-like star.

"We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment," said Jon Jenkins, the Kepler data analysis lead at NASA's Ames Research Center who led the team. "It's awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star; longer than Earth. That’s substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet."

Prior to this discovery, NASA's best candidate for an Earth-like world found by the Kepler Space Telescope was likely Kepler 186f - about 40 per cent larger than Earth, and orbiting in the habitable zone of a tiny red dwarf star. If inhabited, life would likely be very different on Kepler-186f, due to the different amount of energy put out by its star and the different wavelengths of light that the star's energy is focused in.

With Kepler 452b, it may be larger and more massive, likely having twice Earth's gravity on its surface, but the type of star it orbits. and the distance at which it orbits, means that the planet receives about the same amount of solar radiation and at roughly the same distribution of wavelengths as we see here on Earth.

Along with this announcement, the team also added over 500 new candidates to their database, bringing their total numbers to 4,661 exoplanet candidates and 1,030 confirmed worlds - 12 of which are small habitable zone confirmed worlds.

How similar to Earth?

According to the Habitable Exoplanet Catalogue, at the University of Puerto Rico at Arecibo, so far, the most "Earth-like" exoplanet, based on its size, mass, distance from its star, amount of energy received, etc, is Kepler-438b - a planet 10 per cent larger than Earth and around 30 per cent more massive, that orbits a small orange dwarf star, 473 light years away from us. According to the catalogue's "Earth Similarity Index", which compares exoplanets to Earth's characteristics (with Earth ranking as a 1), Kepler-438b comes in at a value of 0.88.

According to that index, a truly Earth-like world would rank as at least a 0.9 on the ESI. We haven't located such a world, yet.

Exoplanets ranked by Earth Similarity Index (ESI). Credit: Planetary Habitability Laboratory, UPR Arecibo

Based on the latest update, Kepler 452b comes in at just behind Kepler 62e, with an ESI of 0.83. It has the advantages of the similar star and similar orbital distance, however, its size keeps it out of the top ten percent.

This does not mean that Kepler 452b is less habitable than Earth, though. It only means that the planet is just 83 per cent similar to Earth.

Although it doesn't rank first, or even enter the top ten percent of habitable worlds (based on ESI), the planet does manage to earn one distinction on the ESI list:

Based on what we know now, which admittedly is not all that much beyond its size and orbit, it is possible that this exoplanet could turn out to be habitable. In fact, it could even be a "super-habitable" world.

According to Prof. Abel Méndez, director of the Planetary Habitability Laboratory, at the University of Puerto Rico at Arecibo, if Kepler 452b's atmosphere has similar density and composition to Earth's atmosphere, it could have the right environment for life as we know it.

Based on its size - around 20,000 km in diameter, compared to 12,742 km for Earth - Kepler 452b would have over two and a half times the amount of surface area as Earth. Thus, if it was a rocky world with continents and oceans, and the right conditions for life to develop, and life actually did develop there, Kepler 452b would offer two and a half times more environment for that life to inhabit - making it "super-habitable."

However, the flipside of that presents a much darker scenario. The exoplanet's estimated mass - which is based on observations of other planets around the same size - makes it more likely to have an atmosphere thicker than Earth's. Even receiving the same amount of solar radiation, a thicker atmosphere would retain more heat, possibly rendering the planet hostile to all life but the most extreme of extremophiles.

Sources: Slooh | NASA | UPR Arecibo

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