The Crowd and the Cosmos: Adventures in the Zooniverse
Page 25
The bright oxygen line explains the distinctive colour of the
peas, and it’s a sign of rapid star formation. A quick look at the
Serendipity 205
data available for the sample the Peas Corps had assembled was
shocking. The peas were systems in which stars were forming at
a prodigious rate; despite being dwarf galaxies with only about a
tenth of the mass of the Milky Way, they match or even exceed
our own galaxy’s production of stars. These tiny systems are
the most efficient star factories in the local Universe. Located in
the backwaters of the cosmos, in the least populated parts of the
Universe, something has caused them to convert all of their gas
to stars. Understanding why this is happening, and what it tells
us about the histories of their larger counterparts, is a matter of
quite some debate, with more than fifty papers devoted to their
properties and their nature in the literature.
One exciting idea is that these are the last galaxies to undergo
the kind of star formation episode which the most massive gal-
axies might have experienced early on. Such enormous bursts of
star formation might have been responsible for what’s called
reionization—the point in the Universe’s history where neutral
gas throughout space was excited for the first time by light from
newly formed stars. The peas would then represent our only
chance to see what a galaxy undergoing such an event looks like
locally.
Unlike the Voorwerp, which to the best of our knowledge had
never been spotted before, the peas weren’t completely new to
science. I’ve found them lurking in papers going back as far as the
1950s, hidden within catalogues of systems which shine brightly
in oxygen and other atomic emission lines. But no one had
looked—not taken so much as an idle glance—at these systems,
and so no one had noticed that these things don’t seem to be nor-
mal galaxies and might therefore be worth some attention. It’s
not even true that only people—citizen scientists—could have
found them, as a careful selection of systems with particular
colours can lead to a sample consisting solely of peas and not
206 Serendipity
much else, but we needed the volunteers to point us to what was
worth selecting before such an exercise would seem worthwhile.
Even in modern astrophysics, dominated by big data and machine
learning, the critical insight that something might be worth fol-
lowing up remains a very human one.
8
IS IT ALIENS?
The discovery of wonderful, unexpected things isn’t limited to Galaxy Zoo. Volunteers on the Planet Hunters project may—
may!—have found the signature of an advanced alien civilization
orbiting a nearby star. While perhaps not the most probable
explanation for what they’ve found, the theory is plausible enough
for one of the top astronomical journals to publish a paper which
seriously suggests that what was found qualified as an ‘alien
megastructure’, words which, as it turns out, get you noticed.
The star in question was once known only as KIC8462852. KIC
stands for Kepler Input Catalogue, which tells you that this par-
ticular star lies in a particular patch of sky. Lying across the bor-
der of the constellations of Cygnus and Lyra, it was stared at by a
space telescope called Kepler, built by NASA to hunt for exo-
planets. Kepler is not a large telescope, and it didn’t have an exciting task; for three years it stared at that same patch of sky,
monitoring the brightness of 150,000 stars selected from the 1.5
million or so in the catalogue. The Kepler field covers an area
about four times the size of the full Moon. The fact that such an
apparently small field has so many stars accessible to even a small
telescope is a reminder that even smaller instruments can con-
tribute much to our exploration of the vast unknown.
208 Is It AlIens?
The spacecraft’s unblinking gaze was combined with a camera
capable of measuring the brightness of stars very precisely, mak-
ing Kepler the most powerful planet hunter yet built. The discovery of planets around other stars has been one of the great
scientific stories of the last few decades, with progress since the
first unambiguous discovery in 1995 being both dramatic and
exciting.* Kepler alone has been responsible for the discovery of thousands of likely worlds.
Most known planets, and all of the Kepler ones, are impossible to image directly. The planets are bright enough and close enough
to us to be seen, but the dazzling light from their neighbouring
stars outshines them. Attempting to pick out a planet close to a
star which is thousands of times brighter is a nearly impossible
task, like trying to spot the faint light of a glow-worm hovering
next to a stadium floodlight. Instead, planet hunters rely on a
variety of indirect methods to track down their quarry.
Kepler, for example, looks for faint blinks that represent the passage of a planet in front of its parent star. Such an alignment
causes a brief drop in the star’s brightness as seen from Earth,
but such transits are not obvious. Even a large planet close to its
star will still cover only a tiny fraction of the stellar disc, causing a dip of much less than 1 per cent in the star’s brightness. The
subtlety of the effect means that it is easier to get a reliable sighting without dealing with the distorting effect of Earth’s atmos-
phere, which is why Kepler is a telescope in space. Even in such ideal circumstances, though, the odds of seeing a transit are low.
Most possible orbits won’t happen to take a planet in front of its
* Picking 1995 as the date of the first discovery is slightly controversial; there were claimed detections of planets before then, some in systems which did indeed turn out to be real, whether or not the original claim had enough statistical weight to hold up. There’s also the strange tale of planets around pulsars, but whatever they are they’re not normal and we can ignore them for now.
Is It AlIens? 209
parent star as seen from Earth, and so projects like Kepler monitor many stars in order to increase the odds.
Once data is on hand, recording the brightness of many stars
over an extended period of time, the task becomes that of finding
any regular sequence of dips that might indicate the presence of
a planet. Looking for a regular pattern is the kind of thing com-
puters were built for and so this is an easy problem—or it would
be if the data was clean. Instead, though, there are plenty of
sources of confusion. The measurement is difficult and subtle,
and noise in the camera can easily cause random fluctuations in
signal big enough to mimic or to mask the signal. Worse, many
stars vary in brightness. The well-behaved ones do so following
regular cycles, but plenty are irregular and many have starspots,
just as the Sun has sunspots.
Studying these changes in brightness is a science in itself, and
can tell us a lot about the structure and evolution of stars, but as
far as planet hunting goes they are a nuisance. The usual remedy
is just to wait; once many separate transits have been seen at
planet’s existence, but even then things are not straightforward
and time is not always on our side. Planets in orbits close to their
stars whizz around, racking up transits, but for most transits are
separated by months or even years. Faster, better searches are
needed.
In 2010 we were approached by Debra Fischer at Yale to see if
we could help. Kevin Schawinski had moved from Oxford to a
position there, and had been singing the praises of citizen sci-
ence. Debra has a formidable record as one of the leading obser-
vers in the nascent field of exoplanet science, and explained at
our first meeting that even the Kepler team themselves were
reduced to inspecting possible planets by eye. (This is mentioned
in the Kepler papers, though you have to dig through them
210 Is It AlIens?
carefully to spot it.) Rather than rely on a single expert classify-
ing a small number of candidates that had been first filtered by
algorithm, Debra and her colleagues were convinced it made
sense to have a large number of volunteers sort through the
whole Kepler data set.
This seemed mad to me. Only a few years into my Zooniverse
adventure, I didn’t have the confidence that I do today that people
were donating their time to Galaxy Zoo solely because of a love
of science. I still suspected that whatever the surveys said, and
however grotty the images of the average galaxy were, people
were still attracted by the experience of looking at images. The
idea was that people would sort through not images, but graphs
of brightness against time that astronomers call light curves. The
proposition that people would be willing to give their spare time
to look at graphs for fun, searching for something as indistinct
and ill-defined as a dip in brightness, seemed like a stretch to say
the least. The fact that we could add simulated planets to the data
helped, as it at least gave people something to see as well as guar-
anteeing we could measure how effective the search was, but I
was worried.
As a result, I spent days wondering whether we should be
attacking this problem at all. I supposed eventually that even if
we didn’t find anything we could write a paper describing how
good the Kepler team were at spotting planets. By this point, though, you should know what happens when I think a project
will fail. Planet Hunters, the project we ended up building in
response to Debra’s challenge, was consistently our most popu-
lar project for years after it launched. The results were better than I could possibly have imagined—volunteers found nearly a hundred planet candidates that had been missed by the main search.
I’m following Kepler parlance: a planet candidate is defined as something that we estimate has a 95 per cent chance of being
Is It AlIens? 211
real. There remains a possibility that one in twenty of them will
turn out to be contaminated by other effects, most likely by the
presence of a distant pair of stars which eclipse each other and
which happen to lie behind the star we think we’re studying. The
vast majority of our planet candidates will be real, though, and
some are truly special. I have a soft spot for the world known by
the ungainly moniker Planet Hunters 2b (PH2b), a Neptune-sized
world which lies at the right distance from its Sun-like star such
that any large moons would have a lovely, temperate climate, and
would provide suitable homes for life (Figure 26).
One of the discoverers of PH2b was Roy Jackson, a 71-year-old
retired police inspector from Gateshead. Asked on local television
Figure 26 Planet Hunters 2b, a Neptune-sized world in the habitable
zone of its parent star, as it might appear from an Earth-sized moon.
212 Is It AlIens?
why he took up the unusual hobby of planet hunting, Roy’s reac-
tion was textbook British understatement. ‘There’s nothing to
watch on TV’, he said, ‘and there’s only so much gardening you
can do.’ If you want a sign of the speed of scientific progress,
I think going in less than two decades from finding the first planet
around another star to it becoming something you do from home
while bored on a rainy Sunday afternoon is a pretty good one.
Even better, in this project too the unusual and the unexpected
showed themselves. Planet Hunters 1b is a remarkable system
with a terrible name, the only planet known in a four-star sys-
tem. Two pairs of stars, each locked in a tight orbit around its
partner, circle their common centre of mass. Planet Hunters
volunteers identified a world which circles two of them. A few
such circumbinary planets are known, but this is the only one in
such a complicated system. As a result, just the fact that it exists
tells us something new about how planets form.
That brings me to KIC8462852, the most unusual star in the
Kepler database. Shortly after the space telescope began moni-
toring it, the star blinked. The drop in brightness amounted to
almost 1 per cent of the star’s normal luminosity; large for a
planet, but not completely unprecedented. A few months later, a
nearly identical dip was recorded. Three is normally enough to
announce a discovery, and so a third dip would have provided
evidence that something—either a large planet or a small and
hitherto unsuspected star—was in orbit around the primary.
Instead, nothing more was seen for over a year. The star con-
tinued to shine brightly, as if nothing had happened. Either each
of the two dips had been caused by separate objects, neither of
which had yet completed a full orbit and returned to transit
again, or the star itself was misbehaving. Among all the excite-
ment of finding real planets, no one paid much attention to a star
with a couple of glitches in its past.
Is It AlIens? 213
Then the star dimmed dramatically. For a few short hours, it
was suddenly 20 per cent fainter than before. It then returned to
its former brightness, where it remained, as if embarrassed by its
brief glitch, for more than a year. At this point all hell broke loose.
The star dimmed, recovered, and then faded again. It looked like
all was returning to normal, but then the star’s brightness
suddenly plunged again, once more fading by a factor of about
20 per cent (Figure 27).
This sort of thing kept happening, but eventually, after a few
weeks of such baffling behaviour, the star returned to normal.
Shortly after, the main Kepler mission ended. Not by design, but because the spacecraft’s reaction wheels, responsible for keeping
it pointing at the same patch of sky, had worn out. Monitoring of
the star thus stopped, but the data safely received on the ground
had brought it to the attention of Planet Hunters volunteers, who
quickly realized they were dealing with something extraordinary.
Even inexperienced volunteers who ran across KIC8462852
realized they were looking at something odd. The time taken for
whatev
er it was that was getting in the way to pass in front of the
star was longer than you’d expect from a planet, and the dip and
return weren’t nicely symmetrical in the way they would be if
1.01 Dip #1
2
3 4
5
6
789 10
1.00
0.99
zed fluxali 0.98
rm
No 0.97
0.96 0
500
1000
1500
Time (BJD-2454833)
Figure 27 The ‘light curve’ for the WTF star as seen by Kepler, showing the brightness of the object over nearly three years. The dramatic dips in the centre and near the end are almost unprecedented.
214 Is It AlIens?
caused by a nice, round, boring planet. The task of investigating
what was going on was taken up by a small group of volunteers,
led by Daryll LaCourse.
Over the course of the project, this bunch of citizen scientists
had learned to use many of the tools provided for professional
astronomers to work with data from Kepler. One of the first com-ments on KIC8462852—now renamed the WTF star*—pointed
out the similarity to a set of unusual objects called ‘heartbeat
stars’ that the group had already investigated.
These are bizarre systems in their own right, with a name
deriving from the fact that the graph showing their brightness
over time—what astronomers call a light curve—resembles the
trace of an ECG in a casualty ward. Periods of stasis are broken by
a sudden increase in brightness, then a decline, and then a return
to the long-term norm. Planet Hunters volunteers learned about
these stars when watching Jim Fuller and Dong Lai, researchers
from Caltech and members of the Kepler team, present at a conference in California. The meeting was streamed live on the inter-
net, and so the audience involved volunteers scattered around
the globe as well as the gathered community of planet-hunting
professionals. Before the end of the talk, which described the first
two heartbeat stars, the volunteers realized they’d seen similar
behaviour and started pulling a list of these oddities together.
The heartbeat stars turn out to be unusual binary stars, with a
smaller secondary star on an elliptical orbit. When the second-
ary swings close to the primary, the latter rings like a bell, and the resulting changes in brightness appear as the distinctive pulses