As you may recall, I’ve already written somewhat extensively about exoplanets and super-Earths. In what turned into an unexpectedly long article, I explained what qualifies a cosmic body as an exoplanet and the distinctions between exoplanets and super-Earths. I’m sure we’re all pros on this stuff, but I’ll review the actual definitions of these terms in a hot second.
According to a report released a couple days ago, there have been some more exoplanets discovered, all of which orbit in the ‘Goldilocks zones’ of their host stars. And cooler than that, two of those eight exoplanets are thought to be more similar to our home planet of Earth than any others that have been discovered to date.
Compared to the other story I covered, there’s not quite as much to tell you about the discovery, but I’ll report on everything that they’ve told us so far and maybe explain the significance of these findings.
As I’ve explained before, an exoplanet is any solid body that has a stable orbit around a star that is not our own [tweet]. If it orbits the same sun that we do, it’s simply called a planet and no an exo-planet. There isn’t really any strict criteria beyond that. And although we haven’t actually found and classified all of them yet, experts say that just in our own Milky Way galaxy there could be as many as 400 billion exoplanets, or perhaps even more.
A super-Earth is a planet with a mass larger than that of our Earth, but smaller than gas giants like Uranus (15 Earth masses) and Neptune (17 Earth masses). As you know, there are no super-Earths in our own solar system; therefore, all super-Earths are exoplanets, but not all exoplanets are super-Earths.
So what’s so special or exciting about exoplanet and super-Earth discoveries if they’re so common? The buzz surrounding exoplanets is due to NASA’s Kepler mission in which they’re looking for other potentially habitable planets similar to Earth. In other words, scientists are looking for planets in which all the crucial variables align in a way that makes it possible, or even likely, for life to exist on that planet completely independent of life here on Earth.
And that brings me to another important component or consideration in the search for exoplanets: the exoplanet’s orbit. In order to be a candidate for habitability, the exoplanet or super-Earth must orbit within its host star’s Goldilocks zone, or its habitable zone. In short, that’s the spatial and temporal ring around a star in which an orbiting planet’s surface wouldn’t be too hot or too cold to sustain liquid water, which is seen as the most basic necessity and precursor to life.
That’s simple enough, right?
On Tuesday, astronomers working at the Harvard-Smithsonian Center for Astrophysics announced some exciting new discoveries in a press conference at a meeting of the American Astronomical Society. They discovered eight exoplanets — that’s in addition to over 4,000 candidates that NASA is already aware of — that orbit within the Goldilocks zones of their host stars, which has them excited for a number of reasons.
For one thing, these eight newly discovered exoplanets double the number of small planets (considered to be less than twice the Earth’s diameter) that orbit within the habitable zones of their stars where they could sustain water in liquid form. Additionally, two of the eight exoplanets are more similar to Earth for a number or reasons than any other exoplanets that have been discovered to date.
The most Earth-like exoplanets yet discovered
The naming conventions for extra-solar planets, or planets that orbit stars other than our own sun, are kind of lax and vary quite a bit, but they usually involve a name or noun, a number (usually sequential and related to the order of discovery), and a lowercase letter to denote that planet’s position in its planetary system — like Earth would be given a ‘c’ because it’s the third planet in orbit around the sun. The two planets that caught the eyes of the study authors have the very unexciting names of Kepler-438b and Kepler-442b.
Both the planets orbit a red dwarf star, though not the same one. Since the red dwarf stars are significantly smaller and cooler than our own sun, that means the Goldilocks zone around each of those stars is a much more narrow space that also requires the planets’ orbits to be rather close to the red dwarf stars. In fact, Kepler-438b can complete a full orbit of its star in 34 Earth days, while Kepler-442b completes its orbit in 112 Earth days.
As for size, Kepler-438b (470 light-years away) is about 12% larger than Earth with about a 70% chance of being rocky, or having a surface that is a combination of both land and water like we have here on Earth. Kepler-442b (1,100 light-years away) is about a third larger than Earth and has a 60% chance of being rocky as well. However, Kepler-438b gets 40% more light from its star than Earth gets from the sun; to put that into perspective, Venus gets twice the amount of solar radiation as we do. With some additional considerations, that puts Kepler-438b’s chance of being within its star’s habitable zone at 70%. Kepler-422b, by contrast, gets two-thirds of the light that we get here on Earth, and astronomers feel that Kepler-422b has a 97% chance of being in its star’s habitable zone.
Before this study, the exoplanet though to be the closest to being an “Earth analogue” was Kepler-186f, which is 10% larger than Earth and receives about a third of the light Earth receives. Kepler-62f was another candidate being about 40% larger than Earth and receiving 41% as much light.
The exoplanets studied by NASA’s Kepler mission are too small to confirm planets and their habitability with mass alone. Instead, the team uses a sophisticated and complex program called BLENDER that uses a variety of measurements and inferences to determine statistical probabilities. This is the program that was used to validate some of Kepler’s most popular and exciting discoveries to date, such as the first Earth-sized planet, the first sun-sized star, and the first exoplanet smaller than Mercury.
The Harvard-Smithsonian Center for Astrophysics (CfA), located in Cambridge, MA, is a joint venture between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. Scientists at the Center for Astrophysics are organized into six different divisions that study the origin, evolution and ultimate fate of the universe.
The paper that reported these findings is available here and has been accepted for publication in The Astrophysical Journal.What do you think about the Kepler mission and the search for super-Earth exoplanets? Is it a waste of time and money, or might we eventually find life elsewhere in the universe? Comment below.
NASA’s Kepler Website
Kepler’s 1000th Exoplanet Discovery, More Small Worlds in Habitable Zones | NASA
Exoplanet Data Explorer
The Search for Another Earth | NASA PlanetQuest
Kepler Finds Most Earth-Like Planets Yet | Fox News
Eight New Exoplanets in ‘Goldilocks Zone’ Discovered by Kepler | Huffington Post
Newfound Exoplanets Are Most Earth-Like Yet | Scientific American
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