Astronomers Find a Planet Like Mars in a Distant Star System

A team of astronomers has found the equivalent to planet Mars in a star system with three planets 200 light years away.  This is the planet with the lowest mass found so far around any normal star.  That’s because the methods that allow us to find the masses of planets (how much they weigh) generally work best for planets that are heavier.  Here a wonderful combination of circumstances allowed the team (including Jason Rowe of the SETI Institute) to make their record-breaking measurement.

The nameless star system is given the catalog designation Kepler 138, because the three planets, orbiting a cooler star, were first discovered by the Kepler spacecraft.  Kepler allows astronomers to find planets when they move across the face of their stars, causing a tiny eclipse (or “transit.”)  Tremendously accurate instruments aboard Kepler measured the decrease of light when each planet got in front of the star.

The three planets are each closer to their star than Mercury is to our Sun.  The planet that resembles Mars, closest of the three, takes only 10.3 days to go around.  In other words, a year on that planet is 10 Earth days.  Think how often you’d have to celebrate annual events, like your birthday!

The present team of astronomers (which also includes members from NASA’s Ames Research Center and Penn State) followed the three planets’ transits over time and noticed that they did not occur at the same time each orbit, because the gravity of the other two close planets was tugging on each one.  By measuring the size of the tugs, the astronomers could derive the gravity (mass) of each planet, something that is otherwise very hard to do.

Now here is the clever part.  When we watch a planet go in front of its star, that allows to measure how big the planet is (its diameter).  Bigger planets block more light.   So for these three planets, we now had the mass (from the tugs) and the size (from the transits).  Most of the time, when they find planets around other stars, astronomers only have one OR the other. 

Since we have both in this case, that allows us to calculate the density of each world. If a planet is dense, it is likely to made mostly of rock, like Earth is.  If a planet is not so dense, it combines rock with ice or even perhaps melted ice, such as the liquids we find inside Jupiter and the other partly liquid planets in our solar system.

This is what allowed the team to say with some certainty that the inner planet in the Kepler 138 system is about the size of Mars and about the same composition as Mars.  The planet is roughly 10% the mass of Earth and half the size of Earth, just like Mars is.   This is the smallest world for which we have both size and mass.

Almost 2000 planets are now known around other stars, a remarkable number, given that the first one was discovered just 20 years ago.  What amazes us is the variety of planets out there.   There are huge planets, bigger than Jupiter, but orbiting very close to their stars.  There are planets we are calling super-Earths, that are intermediate in size between Earth and Neptune.  And now we know that there are smaller, solid worlds like Mars.  Some smaller worlds are really close to their stars, like the one around Kepler 138, but others are much further out, like Mars is in our solar system.   Nature likes diversity in astronomical settings, much as she likes it for people.

(For a nice "infographic" about the Kepler 138 system compared with our solar system, see:

http://www.space.com/29690-small-alien-planet-kepler-138b-facts-infographic.html  )

Stars That Eat Planets for Lunch

In the last couple of weeks, astronomers have announced the discovery of two stars that show evidence of having eaten some of their own Earth-like planets. Today came the announcement of the discovery of a star with planets that is growing larger and larger and is going to be eating its inner planets in the next couple of hundred million years.

These kinds of star "cannibal activity" are not that rare and should not cause us undue distress. It's business as usual in the Milky Way Galaxy, but what's new is that we are getting direct evidence for activities that earlier we only predicted from theory.

The two stars that already finished their meal a long time ago are known by their catalog numbers, HD20781 and HD20782. These stars belong to the same star system, and formed from the same original cloud of cosmic raw materials. This is the first "binary star" system where astronomers have discovered planets around each star. One has two Neptune-sized planets close by, the other has a Jupiter-sized planet whose orbit is not a nice circle but a stretched oval-shape.

Astronomers took a look at what the two stars are made of -- something we can learn by looking at the details in the colors of light from them. Every different element leaves its own "fingerprints" in the colors of light. Because the two stars formed at the same time, from the same "cosmic womb," we have more information about what they were like at the beginning.

According to Trey Mack, of Vanderbilt University, a graduate student who did the detailed analysis, both stars show evidence of elements in their atmospheres that were most likely not there when the two stars formed. Instead they are tell-tale signs of the stars having eaten planets made of rock. One star seems to have eaten the equivalent of 20 Earths, while the other consumed "only" ten Earths earlier in its history.

Astronomers have thought for a while that big "bully" planets like Jupiter and Neptune can, under the right circumstances, move inward and force smaller planets (like our Earth) to approach their stars until they are consumed. We are fortunate that this did not happen in the case of our solar system, and the Earth has a stable, undisturbed orbit which has allowed this blog's readers to evolve here. But it's getting clearer and clearer that not all star system will have the same history as ours did.

The other work deals with an act of cannibalism that happens late in the life of a star. All stars, at some point, have a "life-crisis" when their first fuel for making energy is all used up and the star has to adjust by briefly swelling up into a red-colored giant. The news is that astronomers have discovered that one of the stars around which the Kepler space mission has found three planets is on its way toward this period of swelling up. The star is known as Kepler 56, and the way in which it has started to swell up is telling us that it will eventually swallow its two inner planets, leaving only its outermost planet as survivor.

The devastation will not occur until more than a hundred million years have gone by, so it's not an immediate tragedy. We have seen many stars that swell up like this, but none of them have been known to possess planets. Kepler 56's worlds are therefore the first planets known to be orbiting other stars whose doom we can now predict. (By the way, in case you are worried, our own Sun will also have such a crisis and will swell up. But this is not happening for another five or six billion years, so I am not ready yet to include this eventuality on my home insurance policy.)

The image, by the way, is an artist's impression of an Earth-like planet being torn apart and then "eaten" by a Sun-like star.

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.)

Earth-like Planet Discovered in a Ridiculous Orbit

 Astronomers from two continents made a startling announcement last week. They had found a world similar in size and composition to the Earth that orbited its star in only eight and a half hours.... Just think about that for a minute. Our planet takes 365 and a quarter days to complete its orbit. The innermost planet in our system, Mercury, takes 88 days to circle the Sun. The new planet, designated Kepler 78b, takes only about a third of one of our days to orbit its star. In other words, a year on Kepler 78b is only 8.5 hours long -- a two-Earth-year-old toddler on this alien world would already be 2,063 years old in local time!

The planet is a bit larger in size than our Earth, but made of dense rock like our inner planets (and not gas and liquid, like our outer planets.) Since it circles so close to its star, it must be torridly hot, so we imagine its surface is molten rock and not solid like our own crust.  Some are calling it a lava planet.

How can astronomers know so much about a distant world like Kepler 78b? As its name implies, the planet was discovered around a faint star in the constellation of Orion by the Kepler telescope in space. Kepler's camera measures the size of a planet when the planet is seen going across the face of its star and diminishing the star's light briefly. But that can only tell us how big the planet is across, and not what kind of material it's made of.

But once Kepler found the planet, astronomers in the U.S. and Europe used giant telescopes on the ground to find the tiny wiggle the pull of the planet causes in the motion of its star. This "wiggle method" tells us how much pull (gravity or mass) the planet has.

When astronomer combine the size of the planet from Kepler and the mass of the planet from the wiggle method, they can calculate the planet's "density" (mass per unit volume). In this case, all the measurements made it clear this was a dense world, made of rock, just like our Earth.

The mystery is: how did an Earth get SO outrageously close to its star. If it was falling in, what made it stop? We know it couldn't have been born so close to the star, because the star was larger when it was young, and the planet would have been inside the star, where no planet can exist. Kepler 78b is part of a group of strange planets Kepler has been discovering -- all of them too close to their stars for their own good and for our peace of mind.

A Gorgeous New Hubble Image and News from Kepler

Scientists working with the Hubble Space Telescope have just released a magnificent new image of one my favorite astronomical objects -- the Horsehead Nebula, a great cloud of "cosmic dirt" in the constellation of Orion. What makes this image a little different from usual is that we are not seeing the tower of dust with visible light, but with heat-rays (what scientists call the "infra-red.") 

It is in such clouds of dust and gas that new stars and planets are being regularly born. Because dust can block regular light, infrared images like this allow us to peer deeper into these regions of star birth. This particular image is about 2.5 light years across (where each light year is about 6 thousand billion miles) -- so we are seeing a good-sized pillar of cosmic "raw material" here. (Yet the Horsehead is just a part of a much larger complex of gas and dust called the Orion Molecular Cloud, which is roughly 1500 light years away from us.)

You can see two recently born stars at the top ridge of dust in the Horsehead in this image, confirming that star birth is happening in this dusty clump. Note that the colors we see on this picture are not real (since these are rays our eyes are not sensitive to.) The colors were picked by Hubble scientists to give a sense of the dustiness of the Horsehead.

You can contrast this infrared picture with a visible-light Hubble image taken with the Hubble in 2000-2001:
http://hubblesite.org/newscenter/archive/releases/2001/12/image/a/

and with an image of a larger region around it taken with the Canada-France-Hawaii Telescope on the ground at:http://www.nasa.gov/multimedia/imagegallery/image_feature_89.html

Aren't they gorgeous images?

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In other news you may have read that the Kepler mission, photographing 150,000 stars regularly in its search for planets orbiting other stars, has found three more planets that are just a little larger than Earth and orbiting in the "habitable zone" of their stars -- where water could be warm enough to be liquid. 

For the full story, see: http://www.kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=243

That page gives you access to the quick info, the paintings of what the planets might like, animation, etc. To get the story in a more organized way, scroll down toward the bottom and click on the link to the full NASA news release.

The gist of the discovery is that we are finding more and more planets that are roughly earth-like -- perhaps a bit bigger, not always around the same kind of star as our Sun -- but Earth-like in their temperatures and other conditions. The Kepler team said that the current discovery is just an appetizer. Many more such planets may be among the 2740 candidate planets Kepler found that they are still examining and not yet ready to confirm.

Discovery of the Smallest Planet Yet

Scientists with the Kepler Telescope in space announced the discovery today of the smallest planet yet found around another star, with the catalog number Kepler 37b. The new planet is smaller than Mercury, and only about 1/3 the size of Earth. That our technology is now good enough to detect a planet this small, when it is 210 light years away, is a tribute to the engineers and scientists involved with Kepler.

The new planet, one of three orbiting the nameless star we call Kepler 37, takes only 13 days to go around its star. (Mercury, the closest planet to the Sun, takes 88 days to orbit.) So conditions on the newly found planet are scorchingly hot. In one of NASA's news stories, the authors comment that a penny, laid out in its intense starlight, would soon melt. 

It was only in the last year or so that we were able to find Earth-size planets orbiting other stars, and now here we are finding much smaller worlds. You can compare the planets in the Kepler 37 system with those in our own solar system in the NASA image above.

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By the way, several readers asked me to comment on the chunk of rock that exploded over the Ural Mountain region of Russia over the weekend. As best we can tell, there was no connection between this cosmic visitor and the one I previewed in my previous post. It's just a coincidence that one came close by the Earth and the other fell toward Earth in the same 24-hour period. The smaller Russian chunk exploded high up, making a spectacular fireball in the sky. It appears that only small fragments made it to the ground. The injuries that were widely reported in the media came from the shock wave that spread out from the explosion in the atmosphere. This blast wave shattered windows, just at the moment when people were attracted by the fireball's intense light to look outside.

The two visitors are vivid reminders that we live in a system that has quite a bit of "cosmic debris" among the planets. We (and the Moon, and all other objects in the solar system) are subject to being hit by some of that debris at any time. Astronomers are working hard to catalog all the larger (and most dangerous) chunks that cross the Earth's orbit around the Sun. But the smaller chunks are so hard to see and so numerous, it's not currently possible to get to know them and predict their collisions with our planet.