Thursday, May 23, 2013

Kepler, Exoplanets and Dark Matter in the News

NASA's Kepler mission is not doing well.  Its planet-hunting days are probably over because one of its reaction wheels failed and it cannot point accurately anymore. The research isn't finished because the data in the archive still has to be analyzed and follow-up observations from ground-based telescopes will carry on for years, but this problem is obviously disappointing news.

Here, I'll discuss the strong public interest in Kepler's planet results and the widespread media coverage that's been generated. It's like the scientists have been playing a game to one-up each other, as more and more records have been broken for the smallest planet, or the planet that's most likely to be hospitable to life, and so on. These gains occurred naturally, as the length of the mission increased and the ability to detect small planets in the habitable zone improved (later in the article I'll comment about the difficulties with the term "habitable zone").

The results released in April are a good example of this. The release was about planets in the habitable zone, where liquid water may exist. Because at least three transits are needed to identify a planet, and objects in the habitable zone of stars similar to the Sun can have periods of hundreds of days, these results could not have been obtained early in the mission.

The artist's concept depicts Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra. Kepler-62f orbits it's host star every 267 days and is roughly 40 percent larger than Earth in size. The size of Kepler-62f is known, but its mass and composition are not. However, based on previous exoplanet discoveries of similar size that are rocky, scientists are able to determine its mass by association. Caption from Kepler web-site.
Credit: NASA/Kepler Mission.

This work, led by the Kepler PI Bill Borucki, is excellent for publicity. The Science paper, titled "Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone" contains strong claims and superlatives:
"Therefore Kepler-62e and -62f are Kepler’s first HZ planets that could plausibly be composed of condensable compounds and be solid, either as a dry, rocky super-Earth or one composed of a significant amount of water (most of which would be in a solid phase due to the high internal pressure) surrounding a silicate-iron core."
and
"With radii of 1.61 and 1.41 [solar radii] respectively, Kepler-62e and -62f are the smallest transiting planets detected by the Kepler Mission that orbit within the HZ of any star other than the Sun."
With statements like this, it's easy to see that the title of the press release, "NASA'S Kepler Discovers its Smallest 'Habitable Zone' Planets to Date" is justified by the paper.

Dark Matter Hints?

Sometimes there can be a big difference between the claims in the science paper and those in the press release, leading to problematic media reports. Jumping from exoplanets to cosmology, here's a recent example concerning results from the Alpha Magnetic Spectrometer (AMS). Concerning their detection of an excess of positrons with AMS, the press release says:
"These results are consistent with the positrons originating from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations."
AMS in orbit on the Space Station photographed on July 12, 2011. Credit: NASA/AMS-02 collaboration.

Even saying they might have detected dark matter is a strong claim. So, what does the science paper say about dark matter? Explicitly, nothing. That's not completely true because reference [2] mentions "Proceedings of the Tenth Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe, Los Angeles (to be published)", but that hardly counts. The closest the text of the paper comes to mentioning dark matter is in the final sentence before the acknowledgements:
"These observations show the existence of new physical phenomena, whether from a particle physics or an astrophysical origin."
That's a vague statement, and the resulting press coverage was not terrific. Dark matter expert Katie Mack gave an excellent summary in her article "Space Station's Detector Has Not Found Dark Matter, Despite What Some Media Reports Say" at the new blog Physics Focus. One article I spotted
starts with this sentence:
"Physicists announced on Wednesday that they have discovered the most convincing evidence yet of the existence of dark matter – the particles that are thought to make up a quarter of the universe but whose presence has never been confirmed."
This sentence is problematic because much better pieces of evidence have been found for dark matter, including cosmic microwave background observations (CMB) by WMAP released in 2003, Planck CMB results released earlier this year, and observations of the Bullet Cluster with ChandraHST and other telescopes.

This composite image shows the galaxy cluster 1E 0657-56, also known as the "bullet cluster", formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra is seen as two pink clumps in the image and contains most of the "normal" matter in the two clusters. An optical image from Magellan and the Hubble Space Telescope shows galaxies in orange and white. The blue clumps show where most of the mass in the clusters is found, using a technique known as gravitational lensing. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark. This result cannot be explained by modifying the laws of gravity. Caption taken from this web-site.
Credit
X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Use of "but whose presence has never been confirmed" can also be a problem because some people might infer that it has now been confirmed.

I think a substantial amount of the responsibility for articles like this lies with the AMS publicity effort and the large disparity between the paper and the press release. Use of the term "consistent with" in the release is especially problematic, because the scientific use of this term (not inconsistent with) differs from the use that most people assume (agrees with). To use an extreme example, one could also say that the observations are consistent with invisible fairy dust or alien exhaust fumes. It's a term that's best avoided.

Astrophysicist and writer Ethan Siegel gave a detailed explanation of the AMS result and a forceful critique of the publicity effort, arguing that the press release and press conference were misleading and even deceitful.

This is not easy work to explain. The results from the various attempts to detect dark matter directly are very complicated and often seemingly contradictory, as Katie Mack points out in another excellent blog post. With their large workloads, science writers need all the help they can, especially the ones who don't specialize in astronomy.

The Three S's

The contrast with Kepler research is stark. Kepler has three great strengths regarding publicity: simplicity, success and sexiness. The way Kepler works - finding transits - is simple and easy to understand. Clearly, Kepler been very successful at finding planets, or more specifically planet candidates. Finally, the search for planets is, in my opinion, sexy science, in part because of the connection to finding life. So, Kepler has some clear advantages over dark matter detection work.

These light curves of Kepler's first five planet discoveries show not only drop in star brightness as the planet transits the star, but an indication of the planet's inclination--how far from the center the planet is passing across the star. Caption taken from this web-siteCredit: NASA/Kepler Mission

Although astronomy publicity is renowned for beautiful images, Kepler hasn't had them and hasn't needed them. However, it has inspired some outstanding animations, visualizations and illustrations. Examples are these videos from the Kepler team available here and here and this graphic from the New York Times.

But, there have been some challenges. Kepler has been so popular with the media that it has led to discussion about whether there has been too much exoplanet news. Here's a very interesting blog post by John Rennie titled "Exoplanets bore me (and what that means for science news)", with some particularly good discussion between Rennie and astrophysicist August Muench.

There are also challenges involved with reasonable use of "habitable zone" and "Earth-like planets". I've already used "habitable zone" freely in this article but the concept involves many subtle details. An audience member during "The Great Exoplanet Debate" mentioned that it would be great if astronomers could keep discovering habitable planets and MIT astrophysicist Sara Seager interrupted to say:
"Hold on. Let me just interrupt. There's a correction involved here. That is: people keep claiming the first habitable planet, and as far as exoplanet astronomers go, there's no agreement that there's any habitable planets."
If the experts can't agree on whether there are any habitable planets then you know it's a term to approach with caution. Earlier, Seager advocated use of "potentially habitable" to make it clear that they are making educated guesses.

Astrophysicist John Johnson describes the challenge nicely by explaining that its almost impossible to know if a planet is truly habitable. This is because "we don't even know the conditions for habitability on our own planet!". He then gives a long list of factors or questions that may or may not have been significant for the development of life on Earth, following a discussion with fellow exoplanet expert Jason Wright.

As astrophysicist and writer Matthew Francis explains in a blog post:
"Habitability is a complex and fascinating notion, and of course until/unless we discover life on another world, we can’t be absolutely certain what conditions are truly “just right”."
Then there's the issue of "Earth-size" vs "Earth-like" planets. As Seager explains,
"Those are two very different concepts. And its almost impossible to communicate that. Even professionals slip up. Earth-like means it’s like Earth, with oceans and land and trees and everything great. Earth-size could be anything. It could be hotter than anything that you could imagine and be Earth-size."
Science writer Lee Billings has also written about the challenges of defining Earth-like planets, and astrophysicist and writer Caleb Scharf has asked whether we should expect other Earth-like planets at all.

Kepler hasn't been the only observatory to make major contributions in exoplanet research. The early work was dominated by radial velocity studies - the "wobble" method - and more recently there have been notable observations using this technique. However, Kepler has inspired much of the debate and discussion described above. This discussion will continue as new results are pulled out of the Kepler archive and astronomers keep using ground-based facilities to search for exoplanets. The next dedicated effort from NASA will be the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) should also make big advances.

Sizes of planet candidates found with Kepler. The percentages in yellow show the changes in the numbers of planet candidates in different categories, when comparing the January 2013 and February 2012 catalogs.
Credit: NASA/Kepler mission

The exoplanet field has been active for less than 20 years, but has expanded enormously in that period, especially in the Kepler era. Astronomers have been surprised by what they've found many times. The detection of planets around pulsars was a surprise as was the early detection of hot Jupiters. More recently the detection of large numbers of "super Earths", with masses sizes between Earth and Neptune, has been surprising, along with the large diversity in planet characteristics. It will be fascinating to see what else can be found with Kepler and with future observations.

Exoplanet Publicity in the Future

It's difficult to predict where exoplanet science will go in the future, but I'm confident that public interest will increase. Although some writers might feel that planet news reached saturation levels, I think there's room for growth. As an informal demonstration, I've played around with Google Trends showing how terms used in Google web searches have changed with time. The numbers here should be treated with caution, as there can be multiple uses of the same search term, and I haven't spent a long time experimenting with this tool.

The first plot here shows the recent increase in searches containing "exoplanet" and "habitable zone", compared with a flat curve for "black hole galaxy". (I include "galaxy" to place limits on the results for "black hole". I found that other variations are also flat, but with different normalizations. I also excluded terms for "exoplanet" because of searches unrelated to planets).

In the next plot I kept the same search term for "exoplanet" but searched for "black hole" by itself, without "galaxy". I replaced "habitable zone" with "new planet" and I added two other search terms, the fictitious planet "Nibiru" and "Pluto planet". I restricted this search just to the US, so that the results for "Nibiru" are not exaggerated because the term is used in different languages. You can see the blue line for "exoplanet" now almost disappears because it is so small. Part of the issue is that the Kepler results are still relatively new and the terminology may not have sunk in.

What are all the peaks? The peak for "Pluto planet" occurred in August 2006 when the IAU voted to reclassify Pluto into a dwarf planet and the peak for Nibiru occurred at the end of 2012 because it was identified as a potential culprit for the end of the world. The peak for "black hole" occurred in September 2008 when the Large Hadron Collider (LHC) turned on and some people were concerned that a black hole would be created and destroy the Earth. One conclusion is that fanciful threats to the Earth and votes that cause changes in textbooks generate a lot of interest. (*)

The peaks in "new planet" correspond to announcements for the discovery of the dwarf planet Sedna in March 2004, Xena and other dwarf planets in July 2005, the exoplanet Gliese 581c in April 2007, the exoplanet Gliese 581g in October 2010, and the exoplanet Kepler-22b in December 2011.

These peaks are reasonably large. For example, they are comparable to the size of peaks for "climate change" and, for the last couple of years, to "global warming", a term that's declining in use. In the plot shown here they are the only strong peaks corresponding to new scientific results, showing the high public interest in planets. It's notable that all five of these popular stories correspond either to new objects in our solar system or the discovery of new exoplanets that may harbor life. A discovery like the possible planet around Alpha Centauri B - announced in October 2012 - received less attention, perhaps because that exoplanet is much too close to its star to be habitable.

To place these results in perspective, I replaced "pluto planet" with "beer" and this time the other search terms are so small they almost disappear. I'm not sure interest in exoplanets will ever consistently rival interest in beer, but as I said earlier, there's room for growth. Interest in planets is high when discoveries are made, but it's not obvious that this level is sustained.

There are many exciting fields in astronomy, including black holes, supernovas and cosmology, but exoplanet work has the potential to capture public interest in an unprecedented way. Astronomers are already starting to predict that biosignatures may be detected in exoplanets not too many years into the future, and such a discovery would surely surpass the interest shown in far-fetched speculation about mini-black holes in the LHC. If we find life outside the Earth it would change our view of life on Earth for ever.



(*) One interesting aspect of the results is that the terms for "exoplanet" and "habitable zone" have high search rates in just a few populous states like California and New York. For the other more popular terms there are high search rates across almost all of the US.

3 comments:

  1. We don't know if Mars or Venus* is habitable, and they are the closest bodies to us. I suspect that it will take a long time before we know nearly as much about any exoplanet, and so the argument about habitability will continue.

    * Yes, the surface is dead. 40 km up, however, is another matter and there certainly might be extromophiles there.

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  2. Great post. Interesting that the results for "exoplanet" seem to trace centers of exoplanet/astronomy research…well, I'm not sure if I should find that interesting or blindingly obvious, but I'm intrigued by how much sway geography holds!

    1. Maryland
    2. Massachusetts
    3. Arizona
    4. Colorado
    5. California
    6. Washington

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    Replies
    1. Thanks for commenting, Mark. Yes, that trend is interesting, & I wouldn't have guessed that it would be so constrained by geography. It's fun to play around with Google Trends, but I'd like to know more about it before using it too much. It feels too much like a black box, & I'm not used to that.

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