The new world completes one orbit of its star every 704 days
NASA's Kepler Space Telescope has spotted a distant world with the longest year of any planet in the mission's inventory. The exoplanet completes one circuit of its parent star every 704 days. Most of the 1,800 or so confirmed worlds beyond our Solar System orbit much closer to their stars than this one. Close-orbiting, large planets are easier to detect.
The study is to be published in the Astrophysical Journal. The planet, named Kepler-421-b orbits an orange star that is cooler and dimmer than our own Sun. It orbits this host star at a distance of 177 million km (110 million miles).
That's further out than the Earth orbits from the Sun (our planet's average distance is 150 million km) but less than Mars' distance of 228 million km. By comparison, the Red Planet takes some 780 days to complete one orbit of our host star.
With the information from Kepler, the scientists were able to calculate that the average temperature of the Uranus-sized exoplanet is around -93C (-135F). It orbits a star around 1,040 light-years away,
"Finding Kepler-421b was a stroke of luck," said the study's lead author David Kipping, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.
"The farther a planet is from its star, the less likely it is to transit the star from Earth's point of view. It has to line up just right."
The Kepler telescope was uniquely suited to make this discovery: The spacecraft stared at the same patch of sky for four years, watching for stars that dim as planets cross in front of them. This technique for discovering exoplanets is known as the transit method. Despite this, the space observatory only detected two transits of Kepler-421b due to the planet's long orbital period. The distant world's orbit places it outside the "snow line" - the line in space that divides rocky planets like Earth from the gas giants like Jupiter. Beyond this line, water condenses into ice grains that stick together to build gas planets.
"The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance," Dr Kipping said.
Since gas giants can be found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exoplanets migrate inward early in their history. Kepler-421b demonstrates that this planetary migration isn't necessary; the planet could have formed exactly where it currently is.
The ability to discover planets this distance from their host stars is crucial for understanding the diversity of planetary systems beyond our own.
Kepler space telescope mission
- Launched in 2009, the Kepler space telescope set out to find Earth-like worlds orbiting distant stars in the Constellation Lyra
- It used the so-called transit technique - looking for the periodic dips in light as exoplanets pass in front of their host stars
- Last year, astronomers used Kepler's data to estimate that one in five stars like the Sun hosts an Earth-sized world
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, also known as the Goldilocks zone, is the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces. The bounds of the CHZ are calculated using the known requirements of Earth's biosphere, its position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to life as it exists on Earth, the nature of the CHZ and the objects within is believed to be instrumental in determining the scope and distribution of earth-like extraterrestrial life and intelligence.
Since the concept of the CHZ was first presented in 1953, numerous planets have now been discovered in the CHZ. Most such planets, being super-Earths or gas giants, are more massive than Earth, because such planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. The CHZ is also of particular interest to the emerging field of natural satellite habitability, because planetary-mass moons in the CHZ might outnumber planets.
In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now believed to occur outside the circumstellar habitable zone. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by other non-atmospheric means, the basic conditions for water-dependent life may be found even in interstellar space, on rogue planets or their moons. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.
Dedicated to my NASA Junior Scientists...Jonny and Chris