A Star With SEVEN Earth-like Planets

An international team of astronomers today announced that they have found a faint cool star that is surrounded by a system of seven planets, each of which resemble the Earth in size. Three of the planets orbits in what we call the "habitable zone" where water can be liquid and temperatures might be right for life.

The star, located about 40 light years away, is so faint and cool, it doesn't have a name like bright stars do. It's referred to by the name of the telescope that discovered it and given a number (TRAPPIST 1). Each of the planets is then given a letter from b to h. (See the diagram above.)

Note that the planets are all very close to their dim star, taking from 1.5 days to about 20 days to orbit it. (By comparison, the closest planet to the Sun, Mercury, takes 88 days to orbit.) Planets e, f, and g are the ones where the combination of a cool little star and close-by planet work out to make the temperatures potentially reasonable for life. This star enters the record books as the one with the largest number of Earth-like planets, and the largest number of candidate planets in the habitable zone.

Astronomers caution that the kind of star these planets live around (called an "ultra-cool red dwarf") tends to have a lot of "activity" on its surface when they are young. Great flares of energy and particles are given off in this kind of activity, which might flood the nearby planets with high-energy radiation. That might not be so healthy for the formation of life there until the star settles down to a more stable adult existence.

On the other hand, such low-mass stars (this one contains only 8% of the "stuff" our Sun has) tend to live much much longer than a star like the Sun, so there may eventually be a much longer opportunity for the planets to evolve their surfaces and atmospheres and give birth to life.

Another complication for planets so very close to their star is that their motion probably resembles that of our Moon in a crucial way. The Moon (and these planets) take the same time to orbit as to spin, which means they keep the same face toward the object they go around. So one side of each planet always faces their star and the other side is always in darkness. There is no day and night cycle on these worlds -- you either live on the star-facing side and have perpetual day or you live on the other side and have perpetual night. Only a significant atmosphere might make such a world more bearable and astronomers are using a variety of telescopes to probe whether these planets are surrounded by an air layer and how much and what kind of air they have.

Just to put the discovery in context, astronomers now know over 3,000 planets orbiting other stars, ranging from balls of gas and liquid much bigger than Jupiter, down to rocky balls smaller than Venus. Experts now estimate that perhaps half of the stars in our Milky Way Galaxy may possess planets, and many stars will have more than one planet, just like TRAPPIST 1 and the Sun do. The universe seems rich with planets of all kinds, making it more likely than ever that we are not the only form of semi-intelligent life in the cosmos.

Below is a little poster NASA created to show how the other planets in this system would look from the surface of one of them.

First "Earth Cousin" Planet Found

Astronomers working with the Kepler telescope, led by Elisa Quintara of the SETI Institute, have announced the discovery of the first planet orbiting another star that meets the two characteristics we have been particularly waiting for.  The planet is Earth-sized AND it orbits its star in the zone where water is likely to be liquid (called the “habitable zone.”)

The planet and star have no name, but only a catalog number – Kepler 186.  Located in the constellation of Virgo, about 500 light years away, the star is “red dwarf” – smaller and cooler than the Sun.   So a planet has to be closer to it to have the right temperature for liquid water.  But every star has its own habitable zone and Kepler 186 is no exception.

We have actually found five planets around Kepler 186 so far, but the other four planets are very close to the star and much too hot for life as we know it.  Called Kepler 186f, the newly found planet takes about 130 days to go around its star, and its distance is in the zone where water could be liquid (if the planet has a significant atmosphere.)

Astronomers actually know of some 1,800 planets around other stars so far, orbiting at a wide range of distances and showing a wide range of sizes.  In the past, we have found a number of planets that were the same size as Earth but all of these were too hot – orbiting too close to their stars.  We have also found a number of planets that were in the habitable zone of their stars, but these were bigger than Earth.  Most likely they were so big they would look more like Neptune or Jupiter, made mostly of gas and liquid (or at least having a huge shell of gas and liquid before you could touch solid ground.)

Kepler 186f (sorry planets don’t get names yet) is the first planet that is both the right size and the right distance from its star.  Today, at their national press conference, the scientists who discovered it made sure not to call it an “Earth twin,” however.  Instead, they used the term “Earth cousin” to describe their discovery.  That’s because Kepler 186 is a cool red star, about half the size and mass of the Sun.  So the light of this star will look different on the newly discovered planet – instead of the yellow sunlight we are used to, anyone standing on the surface of Kepler 186f would see reddish-orange sunlight.  The scientists speculated whether any plants on the new planet would receive enough energy to do photosynthesis, and their first conclusion was a tentative yes.

What’s especially important about this discovery is that roughly 80% of all the stars in our Galaxy are red dwarf stars.  If, as our observations are starting to show, MOST stars will have planets, then most planets are likely to orbit red dwarfs.  So places like Earth and our Sun may be the exception.  And Kepler 186f may be a mainstream kind of world in the vastness of the Milky Way.

On our diagram, above, you can see the orbit of the Kepler 186 system’s planets to the same scale as the inner planets in our own solar system.  The fuzzy green region in each case is the habitable zone of each star. You can see the Earth comfortably in the habitable zone of our Sun, and 186f in the habitable zone of 186.  The painting of 186f in this picture is just from the artist’s imagination.  We really have no idea what the planet looks like.

(By the way, for those of you who like to count, you may wonder why the fifth planet in the Kepler 186 system gets the letter f, when f is the sixth number of the alphabet.  That’s because in this pretty awkward naming system we are using, the star is called Kepler 186a, and the letters of the planets start with b.  I don’t endorse this system, folks, I just explain it.)

A Nearby Faint Star With 6 or 7 Planets

Big news today from astronomers studying "exoplanets" -- planets that orbit other stars in the sky. Groups of astronomers in Europe and America, working together, have found a faint nearby star which has 6 or maybe even 7 planets orbiting it, three of which are in the "habitable zone" -- where water can be a liquid.

The name of the star is Gliese 667C (part of a system of three stars that orbit around each other). Only the faintest of the three stars has been found to have planets, but that faint one has quite a family of them. You can see the planets and the star on our diagram.

The way astronomers name these things is by giving each object a letter in order of discovery. So in this Gliese 667 system, capital A, B, and C are the letters for the three stars. Then the planets around star C are given lower-case letters, starting with b. (Planet "h" is not fully confirmed, so it has a question mark next to it.) The green zone is where the planets are that have the right temperature for life as we know it.

The three planets in the green zone of this star are what we call "super-Earths" -- they are bigger than Earth, but smaller than Uranus and Neptune. The method we use to discover these planets only gives us an estimate of their mass, but given how crowded the habitable zone seems to be, scientists are feeling reasonably sure that these planets are no bigger than about 10 Earth masses.

This is the largest number of planets ever found in the habitable zone of another star. (Planet h, the one that is not yet confirmed, is just tantalizingly at the edge of that zone.)

All the planets and the three stars are about 22 light years away in the constellation of Scorpius. (The closest star is 4 light years away, so, in the cosmic scheme of things, Gliese 667 is one of our closest neighbors!)

What's especially interesting about this discovery is that the faint star that has all the planets is what astronomers call an M-type star -- it has only 1/3 the mass of our Sun and shines with only about 2% of our Sun's light output. So the habitable zone is much closer to the cool star than our Sun's is. The planets in the habitable zone take between 28 and 62 days to orbit the star. (Recall that the Earth takes 365 days to go around our much hotter Sun!)

The interesting part is that M type stars are much more common in the universe than stars like our Sun. So if an M type star like Gliese 667C can have 6 or 7 planets crowded around it, that means that perhaps other such M type stars also have families of planets and the number of sites we can look for life in the universe has just gone up.

For more technical details, more pictures, and even short videos, see: http://www.eso.org/public/news/eso1328/

(By the way, Gliese comes from the name Wilhelm Gliese, a German astronomer, who constructed one of the most important catalogs of nearby stars. Many faint stars near us have Gliese numbers from his catalog.)