Planet Discovered Around the Nearest Star

Astronomers at the European Southern Observatory are announcing today that they have found an Earth-sized planet orbiting the nearest star to the Sun, Proxima Centauri.  The star is only 4.2 light years away (which is roughly 25 thousand billion miles – that’s close for astronomers!)

The newly found planet takes 11 days to orbit its reddish star (compare that to Mercury, the closest planet in OUR solar system, which takes 88 days to go around the Sun.)   You might think that a planet that takes only a week and a half to orbit its star must be outrageously hot, but the difference is that Proxima Centauri is a much fainter and cooler star than the Sun.  So the planet could be in the star's habitable zone (liquid water may possibly exist on its surface.)

We found this planet, as we have many others, not by getting a picture of it, but by measuring the pull of the planet on the star.  As the planet circles the star, from one side of it to the other, the planet’s gravity, small as it is, makes the star wiggle just a little bit.  And it’s this wiggling of the star that we can detect from Earth and use it to estimate the mass (gravity) of the planet.

This method can only tell us the minimum mass that the planet must have, and that turns out to be 1.3 times the mass of the Earth. (The wiggle, by the way, only changes the motion of the star by about 3 miles per hour, toward us and then away from us.  That’s a tiny change, but modern instruments are able to make out changes in speed that small.)

A planet with that low a mass is most likely a solid rocky planet, like ours.  Being so much in the gravity embrace of the star, the planet may have one side permanently pointed toward it, much like our Moon has one side permanently pointed toward the Earth as it goes around us. The Proxima Centauri planet would then also rotate in 11 days, making its day and its year the same length!   That’s weird to think about.

The fastest planetary probe we have built so far would take something like 80,000 years to get to Proxima Centauri.   It’s hard to get graduate students to volunteer for that kind of trip!  But, as you may have heard, Yuri Milner, the Russian-American billionaire physicst, has, earlier this year, pledged a hundred million dollars for a research effort to develop a mini-probe, smaller than the smallest cell phone, yet with working instruments, that could ride powerful laser beams and get to the nearest star system in only 20 to 25 years.

Clearly, Project Breakthrough Starshot, as it’s called, will now focus on Proxima Centauri, the nearest of the three stars in the closest star system.  Bets have been on one of the other two stars (called Alpha Centauri, after the brightest in the pair) to have planets, but the first discovery belongs to Proxima.  What an exciting time to be alive.

Above is an artist's conception of what Proxima would look like from the planet's vantage point, and below is a diagram with key characteristics of the Proxima system compared to our own.

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.)