Can You Observe a Hot Jupiter Transit From Your Backyard? WASP-121b Explained
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Artist's impression of WASP-121b — the ultra-hot Jupiter exoplanet 858 light-years away in Puppis, whose permanent dayside is heated to 2,500°C by its F6V host star. WASP-121b's atmospheric asymmetry was revealed by the James Webb Space Telescope in June 2026, showing dramatically different chemistry between its morning and evening terminators

Observer Guide · JWST Discovery Explained

Can You Observe a Hot Jupiter Transit From Your Backyard? WASP-121b Explained

James Webb just revealed WASP-121b's morning and evening atmospheres are wildly different. But what almost no coverage mentions: amateur astronomers with a 6-inch telescope and a DSLR can actually detect this hot Jupiter's transit from their own backyard. Here is exactly how.

Host starWASP-121
Magnitude10.4
Transit period1.27 days
Transit depth~1.5%
By Elena Reyes Published: Updated: Editorial Standards
Elena Reyes — Senior Science Editor

Elena Reyes

Senior Science Editor

Covers NASA missions, space science discoveries, and astronomical events for Telescope Advisor. Translates complex astrophysical research into practical insights for backyard observers. Based in the San Francisco Bay Area.

Quick Answer: Can You Actually Detect WASP-121b's Transit?

Yes — and this is one of the most underreported stories in amateur astronomy right now. Webb's NIRSpec and NIRISS instruments have just revealed that WASP-121b's morning and evening atmospheres are dramatically different from each other — a first-of-its-kind atmospheric asymmetry detection published in Nature Astronomy in June 2026. Every major outlet covered the discovery. Almost none of them mentioned that amateur astronomers with a 6–8 inch telescope and a DSLR can detect the same planet's transit from their own backyard.

Here is why WASP-121b is one of the best exoplanet transit targets for backyard astronomers: the host star WASP-121 sits at magnitude 10.4 in the constellation Puppis — reachable with any 70mm or larger telescope. The transit depth is approximately 1.5%, which is large enough to measure with careful differential photometry. The orbital period is just 1.27 days, meaning transits occur nearly every night. Each transit lasts roughly two hours. With a tracking mount, a camera, and free analysis software, you can produce a genuine exoplanet light curve from your driveway.

Scientific figure from the June 2026 JWST study of WASP-121b showing the atmospheric asymmetry between the morning and evening terminators — CO absorption is stronger at the hotter evening side while water vapour is more depleted there due to thermal dissociation
WASP-121b Terminator Asymmetry — JWST NIRSpec & NIRISS Study (Nature Astronomy, June 2026) — This MPIA figure from the June 2026 study shows the asymmetric atmospheric chemistry detected across WASP-121b's morning and evening terminators. The evening side is hotter and shows stronger CO absorption; the morning side retains more water. The same planet — host star magnitude 10.4, transit depth ~1.5%, period 1.27 days — is accessible to amateur photometry with a 6-inch or larger telescope. Credit: T. Mikal-Evans et al. / MPIA / Nature Astronomy (2026).


What Is WASP-121b?

WASP-121b is an ultra-hot Jupiter — a class of gas giant that orbits so close to its host star that its atmosphere is pushed to extremes beyond anything in our own solar system. It was discovered in 2015 by the Wide Angle Search for Planets (WASP) survey and immediately attracted intense interest because of its extreme physical properties.

The planet sits approximately 858 light-years from Earth in the southern constellation Puppis. It orbits its host star at a distance of just 0.025 astronomical units — roughly 1/40th of the distance between Earth and the Sun, and far inside the orbit of Mercury around our own star. At that distance, the orbital period is a mere 1.27 days. The planet is tidally locked, meaning one hemisphere permanently faces the star and one permanently faces away — a configuration that creates some of the most extreme atmospheric dynamics known in any planetary system.

Dayside Temperature

~2,770 K

The permanent dayside faces the star. At these temperatures, iron and titanium vaporise in the atmosphere.

Nightside Temperature

~1,000 K

The permanent nightside is dramatically cooler — though still hotter than most stars' surfaces at smaller wavelengths.

Physical Size

1.87 RJ

WASP-121b is 87% larger than Jupiter by radius — "inflated" by intense stellar irradiation heating the atmosphere from outside.

The planet's large radius is directly relevant to amateur observers: a bigger planet blocks more starlight during transit. WASP-121b's inflated size is one of the reasons its transit depth reaches 1.5% — a signal large enough to be detected by careful backyard photometrists.

What JWST Found: Asymmetric Atmospheres at the Terminator

The new research, published in Nature Astronomy in June 2026, used Webb's NIRSpec and NIRISS instruments to observe WASP-121b across two separate transits. By analysing the transmission spectrum of the planet's atmosphere as starlight filtered through it at ingress and egress — the leading and trailing edges of the planet as it crosses the star — the team could isolate signals from different parts of the atmosphere for the first time.

What they found was striking. The two terminators — the boundary zones between the day and night sides — are not the same. The evening terminator (the trailing edge, where dayside gas flows onto the nightside) is significantly hotter and shows stronger carbon monoxide (CO) absorption features. The morning terminator (the leading edge, where cooler nightside gas crosses into the dayside) has more water vapour. This asymmetry reflects the powerful atmospheric circulation patterns driven by the extreme temperature gradient between the two hemispheres.

JWST Instruments Used in This Research

NIRSpec (Near-Infrared Spectrograph)

Covers 0.6–5.3 µm. Used to detect CO absorption features and measure temperature profiles across the terminators. The prism/clear disperser mode delivers simultaneous wide-wavelength coverage at moderate resolution.

NIRISS (Near-Infrared Imager and Slitless Spectrograph)

Covers 0.6–2.8 µm. Used for SOSS (Single Object Slitless Spectroscopy) mode to measure water absorption features on the morning terminator and cross-validate the NIRSpec results.

This is the first time atmospheric asymmetry between the morning and evening terminators has been directly resolved in an exoplanet's transmission spectrum. For climate scientists and planetary modellers, the discovery validates complex general circulation models (GCMs) that predict exactly this kind of asymmetry in tidally locked ultra-hot Jupiters — but until now, the models could not be tested observationally. Webb has made that test possible.

The findings have implications well beyond WASP-121b. Atmospheric asymmetry of this kind is expected to influence whether a tidally locked planet can maintain a stable, habitable climate on its nightside — a key question for future studies of potentially habitable worlds around M-dwarf stars.

The Host Star WASP-121: Magnitude, Position, and Finder Details

Before you can detect WASP-121b's transit, you need to locate its host star. WASP-121 is an F6V main-sequence star — slightly hotter, slightly larger, and slightly more luminous than our own Sun. Here are the key details you need for your planning.

Property Value Notes for Observers
Right Ascension 07h 10m 24s Enter directly into GoTo mount or planetarium software
Declination -09° 23′ 44″ Slightly south of the celestial equator — visible from both hemispheres
Constellation Puppis Southern constellation, best viewed from Jan–March in northern hemisphere
Visual Magnitude 10.4 (V band) Reachable with any 70mm or larger telescope under suburban skies
Spectral Type F6V Slightly hotter than the Sun; appears white to slightly blue-white
Best observation season Winter / Early Spring Puppis is high in the south from the northern hemisphere in Jan–Mar

Finding WASP-121 in the field. At magnitude 10.4, WASP-121 is not visible to the naked eye, but it is well within the reach of any small telescope. In a GoTo or computerised mount, you can simply enter the RA and Dec directly, or search by the star's catalogue designation (TYC 5381-1997-1 or 2MASS J07102384-0923457). In a planetarium application such as Stellarium or Cartes du Ciel, you can search by the name "WASP-121." Once located, you will see a faint but steady 10th-magnitude point in the field. The surrounding field contains several comparison stars of similar brightness that you will use for differential photometry — software such as AstroImageJ will guide you through selecting appropriate comparison stars automatically.

Because Puppis lies in the southern part of the sky (declination -09 degrees), it transits relatively low from mid-northern latitudes such as the UK or central USA. Observers at latitudes south of 45 degrees north will find the target reaches a more comfortable altitude above the horizon, improving atmospheric seeing conditions and reducing atmospheric extinction — both of which matter for precision photometry.

What Is a Transit and How Is It Detected?

An exoplanet transit occurs when a planet passes in front of its host star from our line of sight, temporarily blocking a fraction of the star's light. From Earth, we cannot see the planet itself — it is far too small and too faint to image directly. What we can measure is the tiny, temporary dip in the star's brightness as the planet crosses the stellar disc.

For WASP-121b, the transit geometry works out as follows:

  • Transit depth: approximately 1.5%. This means the star dims by about 1.5% — or 15 millimagnitudes — while the planet crosses its disc. A star that was shining at magnitude 10.40 temporarily appears at roughly magnitude 10.42 during mid-transit. That is a small change, but it is measurable.
  • Transit duration: approximately 2 hours. The transit begins at "first contact" (ingress), when the leading limb of the planet first touches the stellar disc, and ends at "fourth contact" (egress), when the trailing limb clears the disc. The flat-bottomed mid-transit section lasts roughly 90 minutes.
  • Transit period: 1.27 days. WASP-121b completes one orbit every 1.27 Earth days, meaning transits are very frequent — roughly every 30 hours. In any given month, there may be 20 or more transit windows, vastly increasing your opportunities to observe one.

Differential Photometry: How to Detect a Transit

The detection technique is called differential photometry. Instead of trying to measure the absolute brightness of WASP-121 in physical units (which is extremely difficult because of atmospheric variation), you measure how its brightness changes relative to nearby comparison stars over time. If the comparison stars remain constant and your target dips by 1.5%, the dip is real and corresponds to the planet's transit.

Here is the basic workflow: you image the same field containing WASP-121 and several comparison stars continuously throughout the night, using exposures of 30–60 seconds. After the observing session, software such as AstroImageJ measures the brightness of all stars in each frame, computes flux ratios, and produces a light curve showing how the target's brightness changed over time. If a transit occurred, the light curve shows a characteristic flat-bottomed dip with smooth ingress and egress slopes.

What Equipment Do You Actually Need?

Transit photometry imposes specific requirements on your setup — different from visual observing or wide-field astrophotography. Here is an honest breakdown of what is needed at each level.

Minimum Viable Setup

This will detect WASP-121b's transit on a good night

  • +
    Telescope: 6-inch (150mm) or larger aperture on an equatorial or GoTo alt-azimuth tracking mount. The mount must track accurately enough to keep the target and comparison stars in the same pixels throughout a 2-hour session. An equatorial mount with a polar alignment is preferred; a good GoTo alt-az mount (like the NexStar SE series) with periodic error under 30 arcseconds is workable.
  • +
    Camera: A DSLR (Canon or Nikon) attached via a T-ring adaptor, or a dedicated CCD/CMOS astronomy camera. The camera does not need to be cooled, but cooling reduces thermal noise and improves repeatability.
  • +
    Software: AstroImageJ (free, open-source) for data reduction, plus Stellarium or Cartes du Ciel for planning. Transit times from the Exoplanet Transit Database (ETD) or NASA Exoplanet Archive.

Better Setup (Recommended)

Delivers more reliable, cleaner light curves with less noise

  • +
    Telescope: 8-inch (203mm) Schmidt-Cassegrain or Newtonian on a dedicated equatorial or GoTo tracking mount. The Celestron NexStar 8SE (GoTo SCT) is the most popular choice for this work.
  • +
    Camera: Dedicated astronomy camera such as ZWO ASI series (ASI183MC, ASI294MC, or the monochrome ASI183MM for maximum precision). These cameras have smaller pixels, lower read noise, and better linearity than DSLRs, all of which improve photometric accuracy.
  • +
    Autoguider: A secondary small guide scope and guide camera (e.g., ZWO ASI120MM + 50mm guide scope) running PHD2 guiding software. This dramatically improves tracking accuracy and reduces systematic noise in the light curve.
  • +
    Narrow-band or V-band filter (optional): A V-band photometric filter eliminates colour-dependent atmospheric effects and is the standard choice for exoplanet transit photometry.

Key Software Tools (All Free)

AstroImageJ

The standard tool for amateur exoplanet transit photometry. Handles calibration frames, aperture photometry, differential flux, and light curve plotting. Free download from the University of Louisville.

ETD / Exoplanet Watch

The Exoplanet Transit Database (Czech Astronomical Society) and NASA's Exoplanet Watch citizen science program both provide predicted transit windows and accept data submissions from amateur observers.

Muniwin / Astro Planner

Muniwin (free) is an alternative to AstroImageJ for differential photometry on Windows. Astro Planner helps with session planning, field charts, and comparison star selection.

Step-by-Step: How to Observe a WASP-121b Transit

Follow these steps in order. Preparation is the critical phase — a transit window lasts only two hours and you cannot make up for poor planning once it has begun.

  1. 1

    Find the next transit window

    Go to the Exoplanet Transit Database (var2.astro.cz/ETD) or NASA's Exoplanet Watch (exoplanetwatch.org) and look up WASP-121b. Both provide predicted transit mid-times in UTC for the coming weeks, along with altitude plots for your location. Because the period is 1.27 days, there are frequent opportunities. Select a window when the star is above 30 degrees altitude for the full transit duration and the sky is forecast to be clear.

  2. 2

    Locate WASP-121 using a finder chart

    In Stellarium or Cartes du Ciel, search for WASP-121 at RA 07h 10m 24s, Dec -09° 23′ 44″ in Puppis. Print or save a finder chart showing the surrounding field to 0.5-degree radius, noting the positions of 3–5 comparison stars of similar brightness (magnitude 9.5–11.5). Avoid comparison stars that are known variables.

  3. 3

    Set up 30–60 minutes before ingress

    Set up your telescope and camera well before first contact. Polar-align your mount carefully — poor polar alignment causes field rotation, which shifts comparison stars in and out of your aperture and introduces systematic noise. Focus precisely using a bright star nearby. If using a GoTo mount, complete your star alignment and slew to WASP-121 to confirm it is centred.

  4. 4

    Take calibration frames

    Before or after your observing session, take a set of dark frames (same exposure time as your science images, with the lens cap on), flat frames (image of a uniformly lit surface), and bias frames (zero-second exposures). These calibration frames are used by AstroImageJ to remove camera artefacts from your science images, significantly improving your photometric precision.

  5. 5

    Shoot continuous exposures throughout the transit

    Set your camera to continuous capture with 30–60 second exposures. Choose an ISO / gain setting that keeps WASP-121 well below saturation (aim for 50–60% of the camera's full well capacity). Start capturing at least 30 minutes before predicted ingress to establish a pre-transit baseline, and continue at least 30 minutes after predicted egress for a post-transit baseline. Do not change any camera settings mid-session.

  6. 6

    Reduce your data in AstroImageJ

    Load your science images into AstroImageJ. Apply calibration frames (darks, flats, biases). Use the multi-aperture photometry tool to select WASP-121 as the target and your comparison stars as references. Run the automated photometry on all frames. AstroImageJ produces a light curve plot — you should see a flat baseline, a smooth dip of approximately 1.5% during transit, and a return to the baseline level. A clean ingress and egress are the clearest signs of a genuine detection.

  7. 7

    Submit your light curve to Exoplanet Watch or ETD

    NASA's Exoplanet Watch and the Czech ETD both accept amateur light curve submissions. Your observation joins a global database that astronomers use to refine transit timing and detect transit timing variations (TTVs) — gravitational perturbations from additional planets in the system. Your data has genuine scientific value.

Why Hot Jupiters Are the Best Amateur Transit Targets

Not all exoplanet transits are observable from the backyard. The reason hot Jupiters dominate amateur exoplanet photometry comes down to three physical properties that make them uniquely tractable.

1. Large Transit Depth

Transit depth scales with the square of the planet-to-star radius ratio. Hot Jupiters are 10–15% the diameter of their host stars, producing transit depths of 0.5–3%. Earth-sized planets produce depths of 0.008% — roughly 200 times smaller. Only space-based precision photometry can detect Earth-sized worlds. Hot Jupiters are within reach of DSLRs and small CCD cameras.

2. Short Orbital Periods

WASP-121b transits every 1.27 days. Compare this to Earth's 365-day orbital period (from Earth's perspective, a transit of an Earth-analogue would be observable once per year at most). Short periods mean many transit opportunities per year, reducing the impact of bad weather and increasing your chances of catching a clear window.

3. Bright Host Stars

Hot Jupiters are more likely to have been discovered around bright, relatively nearby stars because the WASP survey targeted bright stars for precisely this reason. WASP-121 at magnitude 10.4 is accessible to any 70mm or larger telescope. Brighter stars provide more photons per second, which reduces shot noise and improves the signal-to-noise ratio of your photometry.

Other well-observed hot Jupiter systems accessible to amateurs include WASP-50b (host star magnitude 11.6, transit depth 1.1%), HAT-P-7b (host star magnitude 10.5, transit depth 0.7%), and WASP-76b (host star magnitude 9.5, transit depth 1.8%). WASP-121b sits at the easier end of this range, making it an excellent first target for newcomers to exoplanet transit photometry.



Best Equipment for Exoplanet Transit Observation

These are the three telescope setups we recommend for WASP-121b transit work, based on their tracking accuracy, aperture, and mount GoTo capability. All three have been used successfully by amateur exoplanet observers worldwide.

Editor's Pick — Best for Exoplanet Transit Observation
Celestron NexStar 8SE 8-inch computerised Schmidt-Cassegrain telescope on its single-arm GoTo mount

Celestron NexStar 8SE — 8-inch GoTo SCT, the standard for amateur transit work

203mm aperture 2032mm focal length GoTo alt-az mount f/10 SCT

The NexStar 8SE is the most widely used telescope for amateur exoplanet transit photometry, and for good reason. The 8-inch aperture provides enough light-gathering power to generate a high signal-to-noise ratio on a magnitude 10.4 star in short (60-second) exposures. The GoTo computer automatically slews to your target, and the motorised alt-az mount tracks it continuously for the full duration of the transit without requiring polar alignment. The f/10 focal ratio produces a narrow, high-magnification field that keeps WASP-121 and several comparison stars on the camera sensor simultaneously — exactly what differential photometry requires.

Why GoTo matters for transit work: A transit lasts two hours. You need the telescope to track the same field continuously and accurately throughout that window. Manual Dobsonians require constant intervention. GoTo mounts with motorised tracking handle this automatically, letting you focus on monitoring the data as it comes in.

Why we picked it: Aperture, GoTo accuracy, widespread community support, and a huge aftermarket of accessories. More exoplanet transit light curves have been produced with the NexStar 8SE than any other single telescope model at the amateur level.

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Celestron NexStar 6SE 6-inch Schmidt-Cassegrain telescope

Celestron NexStar 6SE — 6-inch GoTo SCT, excellent entry point for transit work

150mm aperture 1500mm focal length GoTo alt-az mount f/10 SCT

The NexStar 6SE delivers the same GoTo tracking capability as the 8SE in a lighter, more portable package, at a meaningfully lower price. For WASP-121b specifically, a 6-inch aperture is sufficient to achieve the photometric precision needed to detect a 1.5% transit depth — provided you use good calibration frames and select appropriate comparison stars. The f/10 focal ratio is the same as the 8SE, so the image scale and field of view characteristics are identical; you just collect slightly fewer photons per second. On bright nights or from darker sites where longer exposures are possible without saturation, the difference between the 6SE and 8SE narrows considerably.

Best for: Observers who want a capable transit-capable setup at a lower cost, or who need a more portable system for dark site travel.

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Sky-Watcher Classic 200P 8-inch Dobsonian reflector telescope

Sky-Watcher Classic 200P Dobsonian (8-inch) — if you already have a motorised tracking platform

203mm aperture 1200mm focal length Push-to Dobsonian Equatorial platform compatible

A standard Dobsonian without motorised tracking is not suitable for exoplanet transit photometry — you cannot keep the target on the sensor for two hours by hand-nudging. However, if you already own or plan to purchase a motorised equatorial platform (such as a Lazy Susan-style tracking platform), the Classic 200P becomes a competitive transit scope at a significantly lower cost than a GoTo SCT. The 8-inch aperture provides excellent light-gathering, the wide 2-inch focuser accepts a wide range of camera adaptors, and the open-tube design cools quickly to ambient temperature — reducing thermal air currents that degrade image quality.

Important caveat: Only consider this option if you have or intend to acquire a motorised equatorial tracking platform. Without tracking, you cannot perform transit photometry.

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Prices and availability subject to change. All product links are affiliate links — see our editorial standards for our review process.


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Frequently Asked Questions About Observing WASP-121b

Can amateur astronomers detect exoplanet transits?

Yes. Amateur astronomers regularly detect exoplanet transits using telescopes as small as 6 inches (150mm) paired with a DSLR or dedicated astronomy camera and a motorised tracking mount. The technique is called differential photometry: you measure how a target star's brightness changes relative to nearby comparison stars over time. Hot Jupiters like WASP-121b produce transit depths of 1–3%, which are large enough to detect with amateur equipment. NASA's Exoplanet Watch citizen science program specifically trains and supports amateur observers to contribute scientifically valuable transit data.

How bright is WASP-121, the host star of WASP-121b?

WASP-121 has a visual magnitude of 10.4 in the V band. This is well below the naked-eye limit (about magnitude 6) but easily accessible with any telescope of 70mm aperture or larger. Under suburban skies with a 6-inch or 8-inch telescope, WASP-121 appears as a steady point of light with an ample signal-to-noise ratio in 30–60 second camera exposures. The star is located in the constellation Puppis at RA 07h 10m 24s, Dec -09° 23′ 44″, and is best observed from winter through early spring from northern hemisphere locations.

How often does WASP-121b transit its host star?

WASP-121b has an orbital period of 1.2749 days, meaning it completes one full orbit and produces one transit approximately every 30.6 hours. In practical terms, transits occur nearly every night. In any given month during Puppis's observable season, there may be 20 or more transit windows. This extremely high transit frequency is one of the reasons WASP-121b is one of the most observed hot Jupiter systems by amateur astronomers — bad weather or poor seeing on one night rarely means missing the system entirely for long.

What equipment do I need to observe an exoplanet transit?

The minimum setup for exoplanet transit photometry is: (1) a telescope of at least 6 inches (150mm) aperture, (2) a motorised tracking mount — a GoTo alt-az or equatorial mount is strongly preferred, (3) a DSLR camera or dedicated astronomy camera attached via a T-ring adaptor, and (4) free reduction software such as AstroImageJ. A more capable setup adds a dedicated astronomy camera (ZWO ASI series), an autoguider for improved tracking accuracy, and a photometric filter. Calibration frames (darks, flats, biases) are essential for achieving the precision needed to detect a 1.5% dip in stellar brightness.

Where can I find WASP-121b transit predictions?

Three free resources provide predicted transit windows for WASP-121b: (1) the Exoplanet Transit Database (ETD) at var2.astro.cz/ETD, maintained by the Czech Astronomical Society — search for "WASP-121b" to see upcoming transit mid-times in UTC and altitude plots for your location; (2) NASA's Exoplanet Watch citizen science program at exoplanetwatch.org, which also accepts amateur light curve submissions; and (3) the NASA Exoplanet Archive at exoplanetarchive.ipac.caltech.edu, which provides orbital ephemerides you can use to calculate transit times yourself. All three resources are free to use.