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What Can We Actually Find on an Exoplanet? Part 4: Looking For Us
(This is Part 4 of a series on what we can actually find on an exoplanet. Read Part 1, Part 2, and Part 3 first.)
All of that from a single dot. One HWO observation of an exoplanet can reveal what’s in its atmosphere, sketch a rough map of its surface, show where its oceans and continents lie, and even pick up signs of photosynthesis and how life responds to the changing seasons.
And we can go bigger. That same dot can tell you whether there’s not just life, but intelligent life.
If a planet has life, it might also have a civilization. And civilization runs on industry, and industry leaves atmospheric signatures of its own: not just biology, but technology. These are called technosignatures, and here’s the slightly embarrassing part. Some technosignatures are actually easier to detect than biosignatures. In other words, aliens might notice our factories before they notice our forests.
Life can be ambiguous. Oxygen and methane have those dull non-living ways of sneaking into an atmosphere, which is why we have to be so strict about biosignatures. Technology, on the other hand, makes molecules that geology simply cannot fake. Chlorofluorocarbons, the CFCs that punched a hole in Earth’s ozone layer, have no natural source whatsoever. They exist because we invented them in the 1930s and pumped them into the sky for fifty years. Spot a CFC in an exoplanet’s spectrum and there’s essentially no innocent explanation. It’s not “maybe life.” It’s “somebody over there is running a chemical plant.”
The same logic covers sulfur hexafluoride, used in high-voltage electrical gear; nitrogen trifluoride, used in making semiconductors; and the perfluorocarbons like CF4 and C2F6, inert gases released as industrial byproducts. None of them occur naturally in any meaningful amount. All of them are extraordinarily stable, lingering in an atmosphere for thousands of years once released. And all of them absorb infrared light in narrow, distinctive bands. They’re basically neon signs, if neon signs were invisible except at very specific wavelengths and stayed lit for ten thousand years.
The pandemic handed us an unexpected proof of concept. During the 2020 lockdowns, satellites watched atmospheric nitrogen dioxide, the sharp brown haze over city skylines that comes from combustion, crash over major cities as cars stopped driving and factories went quiet. From orbit, you could literally watch the economy shut down. That’s how tightly nitrogen dioxide tracks industry. An alien astronomer with the right instrument could plot our GDP.
In fact, these pollution signatures beat radio. The old SETI romantics assumed our television and radio broadcasts would bathe the galaxy, but in reality they fade into the noise within a few tens of light-years. Planetary radar, the focused high-power beams we bounce off asteroids, can reach hundreds of light-years, but only if it happens to be aimed right at the listening aliens. So aliens mostly can’t hear us. But they sure can smell us. For a while.
In 1987 the world signed the Montreal Protocol to phase out CFCs, because they were destroying the ozone layer. It worked. CFC concentrations peaked in the 1990s and have been falling ever since. Give it another century or two and they’ll be all but gone. Which means the window during which Earth’s most distinctive technosignature burned brightest runs from roughly 1960 to 2050. A single century, out of four and a half billion years of planetary history. An alien astronomer watching Earth across that whole span would catch a one-frame flash of CFCs and nothing before or after.
The same story is coming for combustion and its nitrogen dioxide. If our civilization moves off fossil fuels this century, that signature will crash too. Peak visibility, from an alien’s point of view, is right now. The uncomfortable implication is that if civilizations tend to outlast their pollution phase, because they clean themselves up, then the window when they’re detectable by their pollution is short. We might be far more likely to catch a planet that’s briefly dirty than one that’s settled into a long, clean maturity.
But even if we never catch a grubby civilization on an ordinary Tuesday afternoon, we can still read a world’s continents, its oceans, its photosynthesis, and its atmosphere.
All from a single pale blue dot.
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Bass, Raman spar over policing, homelessness in mayoral debate

Los Angeles Mayor Karen Bass and City Councilmember Nithya Raman slammed each other Wednesday over their handling of homelessness, public safety and many other issues during the first face-to-face debate of the Nov. 3 election campaign.
During a series of sharp-edged exchanges before the Sherman Oaks Homeowners Assn., Bass said Raman struggled to work collaboratively with her colleagues, treating city business as a series of academic exercises. She also accused Raman of missing thousands of votes.
Raman, in turn, chided Bass over her track record on homelessness, saying she achieved big reductions in her Hollywood Hills district even as the numbers worsened citywide.
“Guess what that takes? Work,” she said. “That takes action. That takes leadership. That takes management. That takes data. That takes oversight. That’s what’s missing in City Hall right now.”
Bass said her Inside Safe program, which has moved more than 6,200 homeless people indoors, is one of the reasons Raman saw success in her district. The mayor also criticized Raman’s leadership on the council’s homelessness committee, saying its meetings were repeatedly canceled — including on Wednesday, after Raman failed to show up.
“You have lots of plans and lots of ideas, but have done very little,” Bass said.
Bass and Raman were the top vote-getters in the June 2 primary, with the mayor winning 34% and the council member securing 29%. Reality television star Spencer Pratt failed to make the runoff, coming in third.
The most recent campaign finance reports showed Bass with a big financial lead, taking in more than $510,000. Raman raised just over $170,000 in the same reporting period.
Wednesday’s debate was acrimonious almost from the beginning, with the two interrupting each other throughout the night. At one point, Raman told her opponent: “You’ve got to let me answer, Mayor Bass.”
A few minutes later, Bass shot back: “Wait a minute now, it’s my turn.”
Raman hit Bass early on in the debate, saying the mayor negotiated an expensive package of police raises that the city couldn’t afford. That, in turn, forced city leaders to cut basic services, such as street resurfacing and streetlight repairs, she said.
Even after those raises went into effect, the LAPD has lost officers and has been struggling to fill its Police Academy classes, Raman said.
“Our city’s hiring and personnel programs, which she’s promised to change, have never been changed,” Raman said.
Bass said the LAPD pay raises were needed to keep more officers from leaving for other law enforcement agencies. She argued that Raman had “a lot of nerve” talking about the police hiring after voting against the hiring of 170 officers in January.
“I think that she’s irresponsible in what she talks about, because she talks out of both sides of her mouth,” Bass said.
The two candidates spent much of the night skewering each other over their respective track records.
Bass said Raman had been unresponsive to her constituents, forcing the mayor’s office to step in and work with them when Raman’s office wouldn’t. District residents reached out to the mayor’s office over homelessness in the Sepulveda Basin and problems facing local businesses, Bass said.
“[Your constituents] were very concerned when you decided to run for mayor,” Bass told Raman. “Because their concern was, if you treated them this way … then how on earth were you going to manage the city?”
Raman quickly shot back, saying she hears criticism of Bass from residents across the city who have had to contend with diminished services and deteriorating streets and sidewalks.
“Two-thirds of primary voters said no to this incumbent mayor because they see the evidence in front of their eyes,” Raman said.
Bass touted her support from a wide array of labor unions, business organizations and even the three other council members who were elected with major support from the Democratic Socialists of America.
Raman said council members picked Bass over her because they were asked to do so early on in the campaign. Raman entered the campaign in February, on the last day to file.
Bass and Raman were political allies at one point, with Raman even endorsing the mayor before launching a campaign to unseat her.
Bass, a veteran Democrat who served 12 years in Congress, has attempted to torpedo Raman’s image as a foe of the status quo, pointing out she’s been on the council for nearly six years. She also criticized Raman’s tenure as the chair of the council’s homelessness committee.
Raman, first elected to council in 2020, countered that she is just one out of 15 council members and lacks the power wielded by Bass. She contends that Bass failed to act with urgency on the city’s most pressing problems, including housing production and homelessness.
Last week, Council President Marqueece Harris-Dawson announced he is removing Raman from her post as homelessness chair, effective next month.
Raman responded by suggesting she was being punished for running against Bass, a Harris-Dawson ally, and for calling for a city controller audit of the mayor’s work on homelessness.
Harris-Dawson denied that the move was punishment and said Raman will remain on the five-member homelessness committee, just not as chair.
News
Mamdani Sues City Council Over $10,000 Bonuses to Teachers’ Aides
New York City says the Council’s legislation violates collective bargaining law. The dispute could set up the first high-profile battle between the mayor and a public sector union.
News
What Can We Actually Find on an Exoplanet? Part 3: Reading the Face of a Planet
(This is Part 3 of a series on what we can actually find on an exoplanet. Read Part 1 and Part 2 first.)
Oh, it’s enough. It’s never enough, of course, but it’s enough to justify building this monstrous planet-hunting telescope, because a single pixel gives you far more than you’d think. That dot has a color, maybe even a pale blue one. It has a spectrum. And it changes brightness as the planet spins on its axis and swings through its orbit.
The HWO is our first purpose-built biosignature machine. If life is common, meaning present on some meaningful fraction of habitable-zone rocky planets, the HWO will find it. And if it doesn’t, that tells us life is much rarer than we hoped. Either answer is worth having.
It helps that, to gather enough light, the HWO will have to stare at each planet for days to weeks. Every planet becomes its own project: months of preparation and planning, then weeks of observation, then a lifetime of analysis. That’s why studying just 25 worlds will take a decade. But because we watch each planet for so long, by the time we’ve built up a decent signal it has a lot to tell us. A lot more than atmospheres.
Back in 1990, Carl Sagan and colleagues talked NASA into pointing the Galileo spacecraft, then on its way to Jupiter, back at Earth for a look. The resulting 1993 paper, “A Search for Life on Earth from the Galileo Spacecraft,” is a classic. They deliberately analyzed the data as though they had no idea what planet they were looking at, treating Earth as an unknown target. They found oxygen and methane together, that classic cocktail in disequilibrium; an unexpectedly sharp jump in reflectivity at red wavelengths (hold that thought, it’s going to be fun in a minute); and a spray of narrow-band radio signals that no natural process could explain. That last one was us, blasting away across the radio dial.
That kicked off a whole tradition of using Earth as a test case, either by repurposing distant probes or by taking nearby ones and deliberately degrading their data, to see what Earth would look like to alien astronomers of various skill levels. And the theorists haven’t been sitting idle either. Bored theorists get creative, and they’ve dreamed up all sorts of ways to wring every last drop of information out of a bare pixel.
If you’ve ever seen sunlight bouncing off a lake at sunset, you’ve seen glint. It’s the mirror-like flash off a smooth, flat surface, technically called specular reflection, as opposed to the diffuse, matte scattering off dirt or sand or grass, which throws light in every direction. Water does the mirror trick. Land does not. Which means an ocean-bearing planet, seen from the right angle, should sparkle in a way a dry rocky planet simply can’t.
The right angle turns out to be crescent phase, when we’re seeing the planet mostly from its night side with just a thin sunlit sliver showing, the way we see a crescent Moon. In that geometry, sunlight skips off the ocean at a steep grazing angle and heads more or less straight back at us, and the whole crescent lights up like a mirror. Simulations show that ocean glint can nearly double Earth’s total brightness at crescent phase. That’s the difference between a wet world and a dry one, sitting right there in how the planet’s brightness changes as it goes about its business.
Better still, the glint isn’t steady as the planet turns. When a continent rotates into view it interrupts the mirror bounce and the glint dims; when ocean swings back around, it brightens again. The planet appears to blink. And that blinking is a fingerprint of a partly oceanic surface that you can’t fake with clouds or ice caps alone.
Push it further. If you can watch an exoplanet through one full rotation, one of its days, however long that lasts, and track how its total brightness changes as different parts of the surface turn into view, you can mathematically invert those brightness swings to reconstruct a rough map of what lies at which longitudes. Bright deserts, dark forests, blue oceans, white ice caps: each reflects differently, and each rotates into view at a different moment. Measure the brightness changes precisely enough and you can back out where each kind of surface sits on the globe. From brightness alone. Without ever resolving the planet as anything more than a single point.
Stretch the same technique across months and multiple orbital phases, so the tilt of the planet’s axis brings different latitudes into view at different times, and you can start recovering crude two-dimensional maps, latitude and longitude both. This too has been done for Earth, using data from a let’s-pretend-this-is-a-distant-alien-telescope satellite, and the recovered maps show recognizable continents. From a single pixel.
So we can pull out oceans, continents, shapes. And vegetation. Chlorophyll, the molecule that makes plants green, absorbs red and blue light greedily (that’s the light it eats) and reflects green (which is why leaves look green: the green bounces back to your eye). But something remarkable happens just past the red end of the visible range, in the near-infrared. Plant leaves suddenly become extremely reflective. Not for photosynthesis, since near-infrared photons don’t carry enough energy to power the chemistry anyway, but as protection, so leaves don’t soak up too much heat and cook themselves. The switch from absorbing red to reflecting near-infrared happens across a very narrow band of wavelengths, right around 700 nanometers.
Plants go from absorbing most of the red light that hits them to reflecting most of the near-infrared, a jump of a factor of five or more, across a stretch of the spectrum narrower than the width of a laser pointer. This feature is called the vegetation red edge, and it is genuinely bizarre. Nothing else in nature, no rock, no mineral, no ocean, no cloud, no atmospheric molecule, produces a step this sharp at this wavelength.
Which means that if we ever spotted a red-edge-like feature in the reflected light of an exoplanet, a sudden sharp jump in reflectivity at a specific wavelength, we’d have compelling evidence for surface photosynthesis. Forests. Or the alien equivalent. The catch, and it’s a real one, is that even on Earth the red edge is only a few percent of the total reflected light, because plants cover only about 60 percent of the land, land covers only about 30 percent of the surface, and clouds blanket about half of everything at any given moment. The signal gets badly diluted. But it’s still there, and the HWO would be sensitive enough to catch it on a nearby world if plant-like life is abundant.
The really fun part arrives when you add seasons. On a planet with axial tilt like Earth’s, covered in deciduous forests, the strength of the red edge should rise and fall over the year, growing as leaves come out in spring and summer, weakening in fall and winter. A world whose red edge swelled and shrank in step with its orbit would be a world whose surface was thriving in summer and hunkering down in winter. That would be one of the most stunning biosignatures imaginable: not a static chemical fingerprint, but a dynamic one, keyed to the planet’s seasons.
Of course, alien plants don’t have to be green. Plenty of photosynthesis on Earth is done by red algae, so we don’t have to fixate on this exact wavelength. But if we see sharp features in the spectrum shifting as the planet moves through its seasons, that is big.
In Part 4, we go looking for something even bolder than life in that single dot: the smokestacks of an alien civilization.
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