Sounds of Science – January 30, 2017

We can now watch alien worlds orbiting a distant star

Reference story: Washington Post, January 27, 2017

Note: There is considerable Canadian interest in this result. Beyond being scientifically extraordinary, it provides a natural opening to discuss the search for life on other planets in a broader context.

The star HR 8799 is about 129 light years away in Pegasus and is bright enough to be seen with the naked eye on a clear night. Scientists first announced three planetary companions in 2008, with a fourth added later. What made the system especially important was that it became one of the earliest examples of exoplanets being directly imaged rather than only inferred indirectly. After years of observations from the W. M. Keck Observatory, Jason Wang compiled images into a short animation showing all four planets moving in orbit.

Suggested questions and responses

1) We’ve heard a lot about planets around other stars recently. The Kepler satellite released news about a thousand new planets a couple of years ago. Why is this result so amazing?

The Kepler satellite detects planets by what we call the transit method: we literally see the planet go in front of the star and block out some of the light. This particular system was the first one, in 2009, where astronomers were imaging the actual light reflected or emitted by the planets themselves, which is an incredibly hard thing to do. It also carries a very important Canadian connection: one of the researchers, Christian Marois, is at NRC Herzberg in Victoria.

2) What makes it so difficult? We can see planets like Venus and Jupiter in our own sky with the naked eye. Surely telescopes should be able to see planets around other stars?

That is true as long as the Sun is not in the sky. We cannot see Jupiter or Venus during the day because the Sun is so incredibly bright. If you look at a star that is very far away, then the planet and star sit extremely close together on the sky. So you have something that is perhaps ten billion times brighter than the planet, and the two are right next to one another, almost at the limit of what the atmosphere will allow us to resolve. Without very clever ways of removing the starlight, the planets are simply lost in the glare.

3) So how do they get rid of the glare?

This is a great story because it really is a Canadian breakthrough. The problem is that light from the star is randomly scattered by our atmosphere and also within the telescope. The relative positions of the planet and star, however, stay fixed on the sky. Astronomers can use a clever subtraction process, basically rotating and comparing the image, that removes the random speckles while leaving behind the light from the planet. It is an elegant idea, and it works very well.

4) Tell us a bit more about this planetary system. Is it one that might harbour an Earth-like planet, and even life maybe?

If you really want to know about life on other planets, that is where the story becomes less exciting. The direct-imaging method can still only see planets that are roughly at Jupiter’s orbital distance and farther out. This particular planetary system looks something like the outer gas giants of our Solar System — Jupiter, Saturn, Uranus, and Neptune — but scaled up. In fact, the planets are thought to be very large, perhaps five or more times Jupiter’s mass. Their orbital periods range from about 45 to 450 years, so nobody should wait around for a movie of a full revolution. We also do not know what may lie closer to the star. HR 8799 itself is still very young, perhaps 30 to 60 million years old, compared with our Sun at 4.5 billion years.

5) So when are we going to get a picture of another Earth?

We are already detecting planets similar to Earth in mass and orbital distance, but getting an actual picture is much, much harder. It is hard enough to image the surface of stars, and stars are thousands of times larger than planets. In principle people have worked out what would be required: essentially a fleet of telescopes flying several kilometres apart in space and combining their light. That technique is called interferometry. For at least the next couple of decades, though, it is simply too expensive to do at the level needed.

6) That’s a bit disappointing. Are we going to have to wait decades to know if there’s life on other planets?

As long as you do not insist on a picture, the situation is better. Aside from radio SETI searches, which are very difficult and about which I am personally not especially optimistic, there is hope of detecting what are called biomarkers in the atmospheres of planets around other stars. If a planet passes in front of its star, some of the starlight filters through the atmosphere. Atoms and molecules absorb and emit at characteristic wavelengths, so in principle we can infer the composition of that atmosphere. It is very hard, but there is real hope that the next generation of giant ground-based telescopes will be able to do this. I am cautiously optimistic that we may see evidence for life in my lifetime, though not necessarily intelligent life.

7) So we’ve got to wait, what, a decade or so for that?

There is at least an outside possibility that the James Webb Space Telescope, then scheduled for launch in 2018, might contribute to this kind of science, but it does not quite have the ideal technology to do it well. For the really powerful version of this work we are going to have to wait for the next generation of giant 30-metre-class optical telescopes.

Hydrogen squeezed into a metal, possibly solid

Reference story: New York Times, January 26, 2017

A team at Harvard reported that hydrogen, squeezed between diamond anvils at extraordinary pressure, had been transformed into a metallic form believed to exist inside giant planets such as Jupiter. The announcement drew immediate attention because metallic hydrogen has long been regarded as one of the great targets of high-pressure physics. At the same time, the claim was met by strong skepticism from several researchers in the field, who questioned both the interpretation and the evidence.

Suggested questions and responses

1) So how exactly do you get a metal from a gas?

You can think of this as a two-step process. In a gas, atoms have quite a bit of energy and move around a lot. First you cool them, which is why gases typically become liquids. To go further and make a metal — in this case a solid metallic state, which is what makes the claim so special — you have to keep squeezing the atoms closer and closer until they can readily share electrons and settle into a packed structure. To do that you need extraordinarily high pressure, and the device used is called a diamond anvil. These researchers were claiming pressures of more than five million atmospheres.

2) You say “claiming” — why is that?

There was quite a lot of skepticism about the result from other researchers. I read the paper as a non-specialist, and even there a few things raised my eyebrows. For example, some passages leaned heavily on phrases like “through our extensive experience” rather than clearly tying points back to the literature, and there was also language such as “after some more screw turns,” which is unusual phrasing in a high-profile scientific paper. I am certainly not an expert in this branch of physics, so I would defer to the specialists, many of whom were openly questioning the work.

3) What are they concerned about?

As you might guess, it is complicated. There are concerns about whether the sample could have been contaminated, for example by aluminium from parts of the apparatus. There were also broader concerns about whether the optical signatures were enough to justify the interpretation. At the same time, some experts did speak positively about the work. Ultimately the issue comes down to whether other laboratories can independently confirm the result.

4) Why are people so interested in metallic hydrogen?

Part of it is genuinely romantic. About 80 years ago theorists predicted that hydrogen should be able to form a metallic state under high pressure, and confirming that in the lab has become something like the holy grail of high-pressure physics. Even earlier claims of liquid metallic hydrogen have remained disputed. If a stable or metastable metallic form really exists, it could be extraordinary: some theorists have suggested it might be a room-temperature superconductor, which would revolutionize electrical systems, and others have speculated about its value as an extremely energy-dense rocket fuel. The metastability question is crucial, though, and nobody knew the answer.

5) So when are we going to know?

Not immediately. My guess at the time was that it would take around a year or so for other groups to see whether they could reproduce the result. This was never going to be settled by a quick social-media reaction. It needed replication.

Zinger: #tuques4compute and Canadian supercomputing