## Diary - October 2015

#### October 1, 2015

This clay tablet, discovered in 2011 AD, adds to our knowledge of the Epic of Gilgamesh, a Babylonian story that goes back to 2100 BC.

How much of the past is truly lost? How much can we still hope to recover? I often wonder about that... and I'm delighted when people find things like this, or the new poems by Sappho that had been buried in an ancient Egyptian garbage dump.

After the US-led invasion of Iraq and the dramatic looting of Iraqi and other museums, a museum in Sulaymaniyah, in the Kurdish part of Iraq, did something bold and controversial. They started paying smugglers for old artifacts that would otherwise be sold outside Iraq. They didn't ask any embarrassing questions. They just bought the stuff that looked good!

In 2011, a smuggler showed them a collection of clay tablets. It had about 80 tablets of different shapes and sizes. They were still covered with mud. Some were completely fine, while others were broken. Nobody knows where they came from, but they may have been illegally dug up near the city of Babel.

While the smuggler was negotiating with the museum, the museum got Professor Farouk Al-Rawi of the School of Oriental and African Studies in London to quickly look through the tablets. When he saw this one and skimmed the cuneiform inscriptions on it, he got excited. He told the museum to buy it from the smuggler. "Just give him what he wants, I will tell you later on." The final price was $800. When Professor Al-Rawi carefully cleaned the tablet, he realized that yes, it was one of the tablets of the Epic of Gilgamesh! It's a copy of Tablet V, one of the 12 tablets of the so-called Standard Akkadian version of the epic. This version goes back to about 1200 BC. There's also an older version, the Old Babylonian one, but we have less than half of that. So, what's new about this tablet? I'm not very familiar with the Epic of Gilgamesh — I wasted too much of my youth studying math — but there's a longer description of the Cedar Forest. For example, it says Gilgamesh and his pal Enkidu saw monkeys in that forest. This was not mentioned in other versions of the Epic. Even better, in this version Humbaba is not an ogre: he's a foreign ruler entertained with exotic music at court, like a Babylonian king would be. So: a tiny snippet of the past, which could have been lost forever, has made it to the present. And now Hazha Jalal, a woman who works at the Sulaymaniyah Museum, can say: The tablet dates back to the Neo-Bablyonian period. It is a part of tablet V of the Epic. It was acquired by the Museum in the year 2011 and Dr. Farouk Al-Raw transliterated it. We are honored to house this tablet and anyone can visit the Museum during its opening hours from 8:30 AM to 2:00 PM. The entry is free for you and your guests. Thank you. For more on the new tablet, including more pictures and a video, try this story, which is where I got the picture: As you'll see, I paraphrased parts of what he wrote! For a summary of the Epic of Gilgamesh and the 12 tablets of the Standard Akkadian version, go here: For the newly discovered Sappho poems among the Oxyrhynchus Papyri, try this: #### October 4, 2015 Have astronomers found a TŻO? When a big star runs out of fuel, its core can collapse and form a dense ball of neutronium just 25 kilometers across, called a neutron star. But what happens when a neutron star hits an ordinary star? Kip Thorne is a physicist who helped write the most famous book on general relativity. Now he's helping run the LIGO project for detecting gravitational waves. Anna .ytkow is an astronomer at Cambridge who is looking for objects in the Kuiper Belt, outside the orbit of Pluto. But back in 1977, they teamed up and asked this question... and answered it! The answer is: the neutron star could fall to the center of the other star and stay there! The result is called a Thorne-Żytkow object, or TŻO. When this happens, the neutron star will suck in gas from the ordinary star. It will get extremely hot, with temperatures over a billion degrees Celsius. The heat comes from two things: energy released when infalling gas hits the neutron star, and nuclear fusion after the gas hits. If all this happens inside a red giant — a huge, puffed-up star — the inside of that star should get a lot hotter than usual. So, weird processes should create elements that you don't usually see in such a star. And now astronomers have found a red supergiant with a lot more rubidium, strontium, yttrium, zirconium, molybdenum and lithium than usual. We know this from its spectral lines. So, they may have found a TŻO! Anna Żytkow was pleased, saying "I am extremely happy that observational confirmation of our theoretical prediction has started to emerge". The candidate TŻO is called HV 2112. It's in the the Small Magellanic Cloud, a dwarf galaxy orbiting ours, about 200,000 light-years away. The astronomer Nidia Morell found the weird elements in this star while conducting a survey of red supergiants last year. At the time she said: I don't know what it is, but I know that I like it! Read all about it here: Puzzle 1: How do you pronounce a Z with a dot on it? Clue: Anna Żytkow is Polish. Puzzle 2: What could happen if a neutron star falls to the bottom of a white dwarf, if their total mass is big enough? Puzzle 3: What could happen if a neutron star falls to the bottom of a white dwarf if their total mass is not so big? Puzzle 4: Suppose you have a neutron star inside a red supergiant. What eventually happens to it? Puzzle 5: Suppose two red supergiants containing neutron stars collide. What happens then? For some answers to these puzzles, and a lot of good conversation about astrophysics, read the comments to my G+ post. For more, read this: or Wikipedia: #### October 7, 2015 Yes, you've heard there's liquid water on Mars. But have you actually seen it? Now you have. This gif shows what's probably salty water flowing in Newton Crater on Mars. The dark stripes are between 1/2 and 5 meters wide. Stripes like this appear on steep slopes at several locations in the southern hemisphere of Mars. They show up in the spring and summer, when the temperature can rise above the freezing point. They go away when it gets colder. The photos here go from the early spring of one Mars year to mid-summer of the next year. They were taken by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. That stands for High Resolution Imaging Science Experiment. These images are not new! Here's a paper about them, written in 2011: So, what made NASA announce now that there is liquid water on Mars? Could it have anything to do with the new movie, The Martian? I don't know. Some puzzles about the movie: Puzzle 1: why is there no communication apparatus in the living habitat where the Matt Damon character winds up living? Does the book explain why? Puzzle 2: what would it actually feel like to be in a dust storm on Mars? Puzzle 3: could you really take off in a rocket on Mars with just a tarp on top? For some answers and discussion, see the comments on my G+ post. For more about these flows on Mars, see: #### October 8, 2015 A cat can love a rodent... ...if the rodent is bigger. Puzzle 1: What is this thing? Puzzle 2: Where do they usually live? Puzzle 3: Why have they been seen in the wild in Florida? For answers, see the comments on my G+ post. I don't know the original source of this picture. #### October 9, 2015 This is a picture of Super-Kamiokande, one of the neutrino detectors that won this year's physics Nobel prize. It's a tank buried 1 kilometer deep in a mine in Japan. The tank holds 50,000 tons of ultra-pure water, surrounded by 11,146 machines that can detect tiny flashes of light. When a neutrino zipping through space happens to hit a water molecule, it makes a flash of light — and Super-Kamiokande records it. Here you see some people on a raft working on the detectors. The winners of the Nobel prize were Takaaki Kajita and Arthur B. McDonald, who worked at another neutrino detector in Canada. But these big experiments involve huge teams of people! These teams, and their machines, deserve a Nobel prize because they proved something we'd begun to suspect much earlier. There are 3 different kinds of neutrinos: electron, muon and tau neutrinos. Nuclear fusion in the Sun makes electron neutrinos... but we saw only about 1/3 as many as expected. This made physicists suspect that electron neutrinos were turning into the other 2 kinds of neutrinos as they went from the Sun to Earth. But proving this was very hard. And it's only possible if neutrinos have mass! You see, time doesn't pass for a massless particle, since special relativity says time slows down for you when you're moving fast, and it comes to a halt if you're moving at the speed of light. So, a massless particle can't turn into something else until it hits another particle. As early as the 1950s we knew that neutrinos were almost massless. So, we thought they were massless. But now, thanks to these experiments, we know neutrinos really do change from one kind into another. So, we know they have a tiny but nonzero mass. Here's what the Nobel prize committee says about it: The discovery that neutrinos can convert from one flavour to another and therefore have nonzero masses is a major milestone for elementary particle physics. It represents compelling experimental evidence for the incompleteness of the Standard Model as a description of nature. Although the possibility of neutrino flavour change, i.e. neutrino oscillations, had been discussed ever since neutrinos were first discovered experimentally in 1956, it was only around the turn of the millennium that two convincing discoveries validated the actual existence of neutrino oscillations: in 1998, at Neutrino '98, the largest international neutrino conference series, Takaaki Kajita of the Super-Kamiokande Collaboration presented data showing the disappearance of atmospheric muon-neutrinos, i.e. neutrinos produced when cosmic rays interact with the atmosphere, as they travel from their point of origin to the detector. And in 2001/2002, the Sudbury Neutrino Observatory (SNO) Collaboration, led by Arthur B. McDonald, published clear evidence for conversion of electron-type neutrinos from the Sun into muon- or tau-neutrinos. These discoveries are of fundamental importance and constitute a major breakthrough. I would put it this way: in the old Standard Model, neutrinos were massless. In the new improved Standard model, they have a nonzero mass. In fact, there's a whole 3 × 3 matrix of numbers, the 'neutrino mass matrix', which says what neutrinos do as they're flying through empty space. These numbers actually say how the neutrinos interact with the Higgs boson. This determines their masses, but also how the 3 kinds turn into each other. We don't know why the numbers in this matrix are what they are. We may never know. But maybe someday someone will figure it out. Physics is full of slow-burning mysteries like this. For the full story, go here: The neutrino mass matrix is also called the 'Pontecorvo–Maki–Nakagawa–Sakata matrix'. In 1962, right after the muon neutrino was discovered, Ziro Maki, Masami Nakagawa and Shoichi Sakata speculated that electron and muon neutrinos could turn into each other, and invented a 2 × 2 matrix to describe this. And even earlier, in 1956, Bruno Pontecorvo had considered the possibility that neutrinos and antineutrinos could turn into each other. If you want to actually see the numbers in this matrix, go here: #### October 11, 2015 Light hydrogen There's an element that's 1/9th as heavy as hydrogen. Apart from that, it's a lot like hydrogen. For example, its radius is almost exactly the same - just half a percent bigger. Its chemical properties are also almost the same. But there's one big difference. It's unstable. On average, it decays in just 2.2 microseconds! That sounds like a short time, but in the world of chemistry it's not. A lot of chemical reactions only take nanoseconds - that is, billionths rather than millionths of seconds. So, there's plenty of time for this light version of hydrogen to form molecules - and these days, chemists are so good that they can study what happens! For example, this image shows an atom of light hydrogen trapped in a crystal of silicon. The blob is the probability distribution of finding the atom in different locations. It's more smeared out than it would be for ordinary hydrogen. Why? Because the atom is lighter! This image is the result of a computer calculation, not experiment. But chemists also do experiments with light hydrogen. The main reason is to check that our calculations in chemistry are really working. We think we can calculate properties of atoms and molecules with great precision, using the laws of quantum mechanics. But could we be fooling ourselves? As a check, it's great to see what happens when you replace hydrogen with light hydrogen. You should be wondering if I'm making this up. You probably never heard of light hydrogen in school! It's a real thing, but it's usually called by a different name. You make it by shooting a beam of protons at a chunk of stuff like beryllium. If the protons have enough energy, this produces a bunch of short-lived particles called pions. Pions come in three kinds: positively charged, negatively charged and neutral. A positively charged pion quickly decays into another positively charged particle called an antimuon. This lasts much longer: it has a half-life of 2.2 microseconds. An antimuon is about 1/9 as heavy as a proton, but they're both positively charged. A lone proton in ordinary matter will often grab an electron and form an atom of hydrogen. An antimuon does the same thing! The result is an exotic atom called muonium. It's just like hydrogen, except it has an electron orbiting an antimuon instead of a proton. Since most of the mass of hydrogen comes from the proton, muonium is about 1/9th as heavy as ordinary hydrogen. But since the size and chemical properties of hydrogen depend mostly on the mass of the electron, muonium acts chemically like hydrogen. Puzzle 1: The term 'muonium' is unfortunate, because it was chosen before people had developed a systematic naming scheme for exotic elements. It should really be called 'muium'. There's a different exotic element that really deserves the name muonium... and people call it true muonium. What is true muonium? Hint: if you remember my August 28th diary entry on protonium, that should help! True muonium is like protonium in some ways, which make them count as 'oniums'. Puzzle 2: What are some other oniums? Puzzle 3: How does the radius of true muonium compare to the radius of hydrogen? True muonium has not yet been made! However, we know enough about physics to know how big it will be. There's actually a whole textbook on muonium chemistry! You can read the beginning here: • D. C. Walker, Muon and Muonium Chemistry, Cambridge U. Press, Cambridge, 2009. First chapter available here. You can see answers to some of my questions in the comments to my G+ post. The picture above comes from the website: and appears in this paper: • Takashi Miyake, Tadashi Ogitsu and Shinji Tsuneyuki, Quantum distributions of muon in muonium state in crystalline silicon, Phys. Rev. Lett. 81 (1998), 1873–1876. #### October 15, 2015 The magazine New Scientist recently announced an unbelievable discovery: Even the craziest planets concocted by theorists still tend to trace conventionally near-circular orbits in a flat plane. Not so the corkscrew planet. Mind-bendingly, these worlds could exist in a sort of orbital limbo, spiralling about an axis between two stars in a binary system, pulled hither and thither by their competing gravities. But when you read something unbelievable, maybe you shouldn't believe it. When the planet gets close to one star, what force would push it back towards the other star? So, when the science fiction writer Greg Egan read about these corkscrew orbits, he decided to see for himself if they really existed. They were supposedly discovered in a paper called 'Stable Conic-Helical Orbits of Planets around Binary Stars', published in The Astrophysical Journal. He got the paper and read it. He discovered that the New Scientist story was exaggerated. The orbits discussed in the paper look nothing like the picture here! They involve a planet that is much closer to one star than another. But even so, Egan was unable to verify that the orbits worked as claimed in The Astrophysical Journal. Indeed, he wound up convinced that this paper was mistaken. For details, go here: He presents both a mathematical argument that these orbits are impossible, and some computer calculations. You can download his Mathematica notebooks and check these calculations yourself! Unfortunately the New Scientist article is not free - this magazine is owned by Elsevier. The paper in The Astrophysical Journal is also not free - you have to pay$9 to read it. I feel like saying something about how open-access science works better than pay-to-play science. But mainly I'd like some physicists to check Egan's work.

The picture here is from a blog article about the incredible corkscrew orbits:

It was drawn by The Digital Welshman.

#### October 17, 2015

Alien megastructures?

The Planet Hunters project shows that ordinary citizens can do good science. For 4 years, a telescope orbiting the Earth scanned a patch of sky, looking for signs of planets. Now the data is being analyzed by computers, but also by a team of volunteers — you can join them if you want! They've found several planets. But that's not what everyone is talking about.

Recently some of these volunteers found a star called KIC 8462852 that's being called "the most interesting star in the galaxy".

It's a star that keeps suddenly getting dimmer. It can lose up to 22% of its brightnesss. It stays dim for between 5 and 80 days and then bounces back. It does this in a very irregular, unpredictable way. You have to really look at the graphs to see how wacky it looks. There are some repeating patterns that last for a while — but then they go away.

It can't be a planet coming in front of the star because it's too irregular. It's probably not the star itself getting dim, because it's an F-type star, not very different from our Sun, and stars like this don't seem to flicker. It could be clumps of dust that appear and then go away - maybe formed by asteroid collisions? It could be swarms of comets knocked into the star by a neighboring star. It could be something else.

It could be large structures built by an extraterrestrial civilization.

We don't know. We should find out! We should watch this star more carefully! Right now no telescopes are studying this star: the Kepler project is done.

The day before, Joel Aschenbach, a science reporter for The Washington Post, wrote a column titled:

The day before that, the astronomer Phil Plait wrote a blog article called:

The day before that, Atlantic magazine started the public furor with an article that quoted Jason Wright, an astronomer at Penn State. He said:

"Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build."

He must not mean that literally. If aliens should always be the very last hypothesis you consider, you'll never get around to considering them — since you can always make up other hypotheses to explain anything you see. God, for example.

But Wright can't mean that — because he has written a series of 4 very interesting papers on the search for extraterrestrial civilizations, including one on this star.

What do I think is making KIC 8462852 suddenly get dimmer?

First of all, I think something absolutely fascinating is going on, and we should study it more. Get some telescopes on it!

Second, I think it can be incredibly powerful and liberating, when you don't know something, to say "I don't know".

People seem embarrassed to do this. People want to act like they know what's going on. But we don't need to say it's "improbable" that we're seeing alien megastructures — and we certainly shouldn't say it's "probable". It's best to admit our ignorance and get on with the business of learning more.

To learn more, I really recommend looking at the graphs in the actual paper:

The analysis here is also good:

The Atlantic article is very fun to read:

I also recommend reading Phil Plait's article:
Here is the Planet Hunters blog:
The image above is by SPAR, available at DeviantArt.

#### October 18, 2015

378 light years away, in the constellation of Andromeda, there's a planet over 4 times as massive as Jupiter. It's orbiting a star somewhat bigger than our Sun. What makes it special is that it's orbiting very close: 1/15th the distance from Mercury to our Sun! It's called WASP-33b.

It's so close to its star that its surface temperature is about 3,200 °C. There are other Jupiter-like planets close to stars, called hot Jupiters. But this one is the hottest planet we've ever seen!

It's so hot that its atmosphere contains vaporized titanium dioxide. That's a white compound used in paint and sunscreen. Sunscreen wouldn't work very well on this planet.

And it's so close to its star that it orbits once every 1.22 days — that is, Earth days.

It's even so close that its orbit should be quite different from an ellipse! In our solar system, Mercury's orbit precesses very slightly due to the oblateness of the Sun and one of the effects of general relativity. Both these roughly add an inverse cube force to the usual inverse square force of Newtonian gravity. That makes the orbit of Mercury precess. But they are very tiny effects.

For WASP-33b these effects are much bigger: for example, its precession due to the oblateness of the star it's orbiting should be 9 billion times more than the corresponding effect for Mercury. We might even see another effect due to general relativity: frame-dragging, where a spinning object (the star) pulls spacetime along with it. But to see it, we'd need to carefully study WASP-33b for a long time — more than 10 years:

The star WASP-33b orbits is called WASP-33 or HD 15082. It's a Delta Scuti variable, a kind of star whose brightness oscillates faster than once a day.

#### October 19, 2015

When you drive past a farm with plants in a rectangular grid, you'll see flickering lines as they momentarily line up in various ways. Here you can see that in 3 dimensions.

If you were standing in a rotating space filled with dots, one dot at each point with integer coordinates, this is what you'd see.

It's all about number theory. Suppose you have a farm with one plant at each point $$(x,y)$$ where $$x$$ and $$y$$ are integers. Then you'll clearly see lines of plants with slopes $$y/x$$ when $$y$$ and $$x$$ are small integers. So, slopes like 0/1, 1/1, 1/2, 2/3 and so on. There will also be lines where $$y$$ and $$x$$ are large integers, but these will be harder to see.

The same sort of thing happens in 3 dimensions. See all the ways the dots line up?

But notice, you're not shooting past these dots like driving past a farm. The dots in front are moving left. The dots in back are moving right!

So, I think these dots are actually rotating around a vertical axis. If take a dot that's not too close, and not too far, and follow it with your eye, you can see it go round and round! It's a bit hard to do, but it's fun to try.

I found this image here:

#### October 21, 2015

It's impossible to accurately draw a proton, but this picture is a good try. It's like a bag of virtual particles! There are lots of quark-antiquark pairs (the red and green balls) and gluons (the springs).

A gluon can split into a quark and an antiquark. A quark and antiquark can meet and become a gluon.

Other stuff can happen too, which is not shown here. A quark or antiquark can absorb or emit a gluon. A gluon can split into 2 gluons. Conversely, 2 gluons can collide and become one! Also, 2 gluons can collide and become 2 other gluons... or 3 can become 1... or 1 can become 3.

All these things are happening all the time inside a proton, in a kind of quantum blur. And since every possible process is literally happening all the time, nothing is actually changing!

That's a bit weird, I admit.

It makes no sense to ask how many virtual quarks, antiquarks and gluons are in a proton. There's not a fixed number of them waiting to be counted. They are virtual, not real. That means if you start probing the proton, you'll find them: they will become real, created by whatever form of energy you used to look for them. But what you see depends to some extent on how you look.

This is why it's impossible to accurately draw a proton. It's also damned near impossible to describe it accurately in plain English. That's why physics uses math.

Anyway, if you look really carefuly at this picture you'll see 3 quarks — green balls — that aren't next to antiquarks. Can you spot them?

Indeed: mixed in among all the virtual particles, a proton has 3 real quarks in it.

But physicists have long believed it's possible to have a particle that's made only of virtual quarks and gluons, with nothing else. This is called a glueball.

People have looked for gluballs and found some candidates. One is about 2900 times as heavy as an electron, another is about 3300 times as heavy, and there are others. For comparison, the proton is 1836 times as heavy as an electron.

The big problem is that there are particles called mesons that are made of a quark and antiquark along with virtual stuff. It's hard to distinguish between a meson and a glueball!

Worse still, just as in quantum mechanics you can have a cat that's a superposition of live and dead, you can have a particle that's a superposition of a glueball and a meson!

Roughly, the idea is this. A particle is a meson if when you probe it, not zapping it with too much energy, you usually see a quark and an antiquark. It's a glueball if you usually see just gluons. But in fact, there's always a chance you can see either.

In short, the difference between a glueball and a meson is just slightly more precise than the difference between a 'planet' and a 'dwarf planet'. You can make up rules to decide what counts as a glueball and what counts as a meson... but someone else could argue with those rules.

Recently 2 physicists did some calculations and came up with evidence that the particle 3300 times as heavy as an electron is really a glueball. So, now the newspapers are shouting Physicists found a glueball!

That's an okay newspaper headline, but the actual title of the paper is a bit more technical. It's called "Nonchiral enhancement of scalar glueball decay in the Witten-Sakai-Sugimoto model". It argues that if this particular glueball candidate is really a glueball, it should decay a lot into kaons and eta mesons, which is pretty much what we see.

By the way, this glueball candidate is called the f0(1710). This means its rest energy is 1710 MeV. In plain English, that means it has 3300 times the mass of an electron. The lighter glueball candidate I mentioned is called the f0(1500). And there are even lighter candidates, called the f0(500) and f0(980). These particles are known to exist — the problem is figuring out whether they are glueballs, mesons or superpositions.

Also by the way, there's a million dollar prize waiting for whoever who can mathematically prove that if the world had nothing in it but gluons, they would form glueballs with nonzero mass! Check it out:

These glueballs would not decay into anything else, since there would be nothing for them to decay into. At least, that's what we believe. Proving it in a rigorous way is not easy.

Here's the paper that caused the fuss:

It uses ideas from string theory.

I found this picture at the website of the particle accelerator called DESY, the Deutsches Elektronen-Synchrotron:

#### October 23, 2015

Mercury is hot. It orbits the Sun every 88 days.

But in 2012, astronomers using the telescope on NASA's Kepler satellite found a planet called KIC 12557548 that orbits its star once every 16 hours! It's so hot that rocks on the day side can melt and boil away. And it seems this planet is disintegrating.

The star is a K-type main sequence star, meaning it's a bit smaller and cooler than our Sun. But the planet's distance from this star is only twice the star's diameter! So it must be very hot, probably about 2000 °C.

But why do we think this planet is falling apart? We know this planet exists only because of how it dims the star when it comes in front. But the amount of dimming varies each time it goes around! The planet blocks between 0.2% to 1.3% of the star's light. How are these changes possible?

Another clue is that the dimming is asymmetrical: the star gets dim slowly and then bright more quickly.

The best theory so far is that the planet is evaporating and falling apart, creating a cloud that changes size. If this cloud has a long tail, as shown here, it would produce asymmetrical dimming.

Scientists love puzzles like this. In 2013 two astronomers named Daniel Perez-Becker and Eugene Chiang studied this planet, and argued that it's in the final catastrophic stage of evaporating away. We know it's not very heavy, because it's not making the star wiggle detectably. Perez-Becker and Chiang believe it has lost most of its original mass, with only the inner iron core surviving.

This is their paper:

• Daniel Perez-Becker and Eugene Chiang, Catastrophic evaporation of rocky planets.

Abstract. Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. Small enough planets evaporate completely. We construct a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets, accounting for dust-gas energy exchange in the wind. Rocky planets with masses < 0.1 MEarth (less than twice the mass of Mercury) and surface temperatures > 2000 K are found to disintegrate entirely in < 10 Gyr. When our model is applied to Kepler planet candidate KIC 12557548b — which is believed to be a rocky body evaporating at a rate of dM/dt > 0.1 MEarth/Gyr — our model yields a present-day planet mass of < 0.02 MEarth or less than about twice the mass of the Moon. Mass loss rates depend so strongly on planet mass that bodies can reside on close-in orbits for Gyrs with initial masses comparable to or less than that of Mercury, before entering a final short-lived phase of catastrophic mass loss (which KIC 12557548b has entered). Because this catastrophic stage lasts only up to a few percent of the planet's life, we estimate that for every object like KIC 12557548b, there should be 10–100 close-in quiescent progenitors with sub-day periods whose hard-surface transits may be detectable by Kepler — if the progenitors are as large as their maximal, Mercury-like sizes (alternatively, the progenitors could be smaller and more numerous). According to our calculations, KIC 12557548b may have lost ~70% of its formation mass; today we may be observing its naked iron core.

This is another good paper on this planet, with lots of nice graphs:

The picture above was made by a NASA artist and appears in the Wikipedia article on this planet and its star:

#### October 25, 2015

A smaller system

Kepler-11 is a star 2000 light-years away that's very similar to our sun. It has at least 6 planets. But this solar system is small. All the planets would fit inside the orbit of Venus — and all but one fit inside the orbit of Mercury!

We used to think gas giants like Jupiter, Saturn and Neptune could only exist far from their host star. But that's when we just knew one solar system — our own. Now we know that there's a huge variety. Many have hot Jupiters or hot Neptunes — gas giants close to the star. We think they formed farther away and migrated in toward their stars when they got tired of the cold winters.

But beware: the easiest planets to detect are big ones close to the star! We're seeing the planets that are easy to see, not necessarily the 'typical' ones. There are probably lots of smaller planets we haven't seen yet.

Kepler-11 got its name because it's the 11th star where the Kepler spacecraft saw planets. Even better, they were found in 2011. Its planets have boring names: they're called b, c, d, e, f and g in order of increasing distance from their star. But they're pretty interesting. They have masses between those of Earth and Neptune. Their densities are all lower than Earth, so they're probably not rocky worlds. Planets d, e and f probably have a hydrogen atmosphere. Planets b and c seem to contain lots of ice.

Puzzle 1: how can you have a planet with lots of ice closer to a sun-like star than Venus is to the Sun?

Puzzle 2: why is there no planet a?

For answers see the comments on my G+ post. The second puzzle was posed by Thomas Lawler in the comments.

For more on Kepler-11, see: