Detecting exoplanets

(Image credit: Martin Vargic)

Take a look at the telescopes below.

Which one would you go for?

Celestron AstroMaster 70AZ — around $100. The classic starter: a small lens-based telescope on a tripod that sets up in ten minutes. It will show you the craters of the Moon, Jupiter's four biggest moons, and Saturn's rings as a tiny but unmistakable "hat." Cheaper telescopes exist, but below this price the views are usually a disappointment.

Sky-Watcher Heritage 130P — around $200. The one that astronomy forums recommend to almost every beginner. It's a "tabletop" telescope — no tripod, you stand it on a table — with a 130mm mirror, which gathers far more light than the budget option. The rule of the hobby is simple: the wider the mirror, the more you see, and this gives you the most mirror per dollar on the market.

Sky-Watcher 8" Classic Dobsonian — around $450. A big, simple tube on a base that swivels — no electronics at all. With a 200mm mirror, this is the first level where galaxies and nebulae stop being faint smudges and start showing real structure. The trade-off: it's the size of a water heater.

ZWO Seestar S50 — around $500. A completely different animal: a "smart telescope." You don't look through it at all — it's a robotic camera that finds objects itself and stacks images on your phone, so after twenty minutes you have a colour photo of a nebula. Purists hate it; beginners love it.

Celestron NexStar 8SE — around $1,700. The famous orange tube. A computerised telescope with the same 200mm of light-gathering power as the Dobsonian, but it finds and tracks thousands of objects at the push of a button, and it's compact enough to carry in one hand. This is the "grows with you for twenty years" purchase.

A brief history of telescopes

In 1609, Galileo pointed a homemade telescope at the night sky. It was about as powerful as a cheap pair of modern binoculars, but it was enough to change everything: he saw mountains on the Moon and four moons circling Jupiter — proof that not everything orbits the Earth.

For the next three centuries, the story was simple: build bigger mirrors, on higher mountains. By 1949, the giant Hale Telescope in California, with a mirror five metres wide, could photograph galaxies billions of light-years away. But every telescope on Earth shares one enemy: the atmosphere. The same moving air that makes stars twinkle also blurs their images, like looking at a coin at the bottom of a swimming pool.

The solution was radical: put the telescope above the atmosphere. In 1990, NASA launched the Hubble Space Telescope. It got off to an embarrassing start — its mirror had been polished to the wrong shape, and astronauts had to fly up and fit it with "glasses" — but once repaired, it became probably the most productive scientific instrument in history. Hubble measured the age of the universe (13.8 billion years) and photographed galaxies whose light left them when the universe was young.

Its successor, the James Webb Space Telescope, launched in 2021 with a gold-coated mirror six and a half metres wide. Webb sees infrared light — heat — which lets it look through clouds of dust and, crucially for our lesson, analyse the atmospheres of planets around other stars.

And the story isn't finished. In Chile, engineers are building the Extremely Large Telescope, with a mirror 39 metres across — wider than a basketball court. One of its first jobs will be to study planets like GJ 887 d.

So how do telescopes work, exactly?

Let's check out this video that is made for kids.

It's a little child-ish in places, but it really breaks things down in a clear and easy-to-understand way.

Note: many language students find it super helpful to watch content designed for kids

In the last class, we came across this wobble method, and also learned a fancy term: radial velocity.

I gave my very limited explanation of how this might work.

Now try to explain back to me, in simple terms, how this might work.

The Wobble method

Here is the problem: a star is billions of times brighter than its planets. Looking for a planet next to a star is like looking for a moth flying around a lighthouse — from another country. So astronomers usually don't look for the planet at all. They look at what the planet does to its star.

We say that planets orbit stars, but that's not quite true. In reality, a star and its planet both circle around their shared centre of gravity. Because the star is enormously heavier, it only moves a tiny bit — but it does move. Imagine a huge dog on a lead held by a small child: the dog mostly goes where it wants, but the child's pull still makes it wobble slightly. The planet is the child.

So the star wobbles — sometimes moving slightly towards us, sometimes slightly away. And here is the clever part. You know how a police siren sounds higher when the car comes towards you and lower as it drives away? Light does the same thing. When the star moves towards us, its light becomes very slightly bluer; when it moves away, very slightly redder. This is called the Doppler effect.

Instruments like HARPS and ESPRESSO — the two used to confirm GJ 887 d — are spectrographs: machines that split starlight into a rainbow and measure these colour shifts with almost unbelievable precision. ESPRESSO can detect a star moving at walking speed. From the rhythm of the wobble, astronomers read the planet's year (for GJ 887 d: 51 days), and from the size of the wobble, its weight (at least six times Earth's).

There is one honest limitation: the method tells you a planet's minimum weight, not its exact weight, and nothing about what it's made of. That is why the papers say GJ 887 d "could" be rocky — nobody has seen it yet.

Discussion

Learning complex things is very much a process. But what new information have you learned from this text?

What further questions would you have for an expert here?

Have you heard of HARPS or ESPRESSO before?

Do you remember anything about spectographs or light from your school days?

So why Chile?

Both instruments that confirmed GJ 887 d sit on mountaintops in Chile's Atacama Desert — and so do many of the most powerful telescopes on Earth. That's no accident. The Atacama is close to being the perfect place on the planet for astronomy, for five reasons.

#1 It's dry - the driest non-polar desert in the world. Some weather stations there have never recorded rain. Water vapour in the air absorbs and blurs starlight, so for an astronomer, dry air is gold.

#2 It's high. The observatories sit at 2,400 to over 5,000 metres, above a good part of the atmosphere and above most clouds. The result is around 300 clear nights per year — a telescope in Northern Europe would be lucky to get 60.

#3 The air is unusually calm. The cold Pacific current just offshore creates smooth, stable layers of air flowing in from the ocean, with very little turbulence. Less turbulence means less twinkling, and less twinkling means sharper images.

#4 It's dark. The desert is vast and almost empty, far from the light pollution of big cities.

#5 Chile is in the Southern Hemisphere. The sky you can see depends on where you stand on Earth, and GJ 887 is a southern-sky star, sitting low or invisible from most of Europe and North America but riding high over Chile. The southern sky also happens to contain the bright centre of our own galaxy and our two neighbouring dwarf galaxies.

Discussion

Now, I guess it's fairly clear why Chile was selected, right?

Have you heard of this Atacama dessert before?

Some people visit this place, both for professional work and for amateur stargazing camps. Does it sound like a once-in-a-lifetime opportunity or a bit of a waste of a holiday?

Finally, let's have a bit of a recap of how, exactly, GJ 887 d was discovered

Step 1: A promising star gets chosen (before 2020). Astronomers running a project called RedDots were hunting for planets around the red dwarf stars closest to Earth. GJ 887 was one target.

Step 2: The first wobbles are measured (2020). For about three months, the team pointed HARPS at the star almost every night. Two clear, repeating wobbles appeared: planets b and c, orbiting every 9 and 22 days.

Step 3: A mystery signal appears. Hidden in the same data was a third, fainter rhythm, repeating roughly every 50 days — exactly where the habitable zone should be.

Step 4: Back for more evidence (2020–2025). Instead of announcing a planet, the team spent years collecting proof. They gathered over a hundred new measurements with HARPS, and added a dozen more from ESPRESSO — a newer, even more sensitive spectrograph.

Step 5: A lucky break. During the new observations, the quiet star woke up — its magnetic activity increased. The activity revealed the star's rotation period, about 39 days. Now the team knew exactly which rhythm belonged to the star itself.

Step 6: Separating star from planets. With the rotation known, the team built a statistical model (a technique called a Gaussian process) to subtract the star's own noise from the data

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Step 7: Confirmation — plus a surprise. The 50-day signal survived every test: GJ 887 d is real — a super-Earth at least six times Earth's mass, orbiting every 51 days in the habitable zone, receiving about 80% of the energy Earth gets.

Step 8: Publication (March 2026). The results were checked by other scientists and published in the journal Astronomy & Astrophysics — six years after the mystery signal first appeared.

Now imagine you were having a casual chat with someone and they made a comment like, "How on Earth can scientists tell if there are exoplanets or not if they can't see them?"

How confident would you be in giving a meaningful response? 😊