Exoplanet Transit Depth Calculator

What exoplanet transit depth tells you about planet size

Exoplanet transit depth is the tiny dimming you see when a planet crosses in front of its star. The number is useful because it translates a light-curve dip into a simple size comparison: how much of the star's disk the planet covers. If the planet is larger relative to the star, the block is larger and the dip is deeper. If the star is larger, the same planet covers a smaller fraction of the visible surface and the dip is shallower. That is why transit depth is one of the quickest ways to build intuition about detectability and planet size before you start fitting a full light curve.

This calculator is a first-pass exoplanet transit-depth tool. You enter the star radius and planet radius, and it returns the expected fractional dimming for the basic geometric model. That makes it handy for classroom demonstrations, target screening, and quick comparisons between stars. The point is not to replace a full photometric analysis; the point is to show the scaling immediately. A planet that looks modest around a Sun-like star can produce a much stronger signal around a smaller dwarf star, and this page makes that difference easy to see.

What the star and planet radius inputs mean

In this calculator, the star-radius field is the size of the host star in solar radii. A value of 1 means a Sun-sized star, while values below 1 describe smaller stars such as many cool dwarfs. Larger values represent inflated main-sequence stars or evolved stars. This input matters because it sets the background disk the planet is dimming: a larger star spreads the same planetary silhouette over a wider area, which lowers the depth.

The planet-radius field is the size of the planet in Earth radii. A value of 1 is Earth-sized, around 2 is a super-Earth scale, around 4 is a mini-Neptune, and about 11.2 is Jupiter-sized. Transit depth depends on radius rather than mass, so this calculator only cares about how wide the planet appears from our line of sight. The most common mistake is to enter a diameter or to mix Earth radii and Jupiter radii without converting, and because the result is squared, that error can change the depth dramatically.

The area-ratio formula behind exoplanet transit depth

The area-ratio formula for exoplanet transits comes from a simple idea: the planet blocks a patch of the stellar disk, and the blocked fraction is the area of the planet's silhouette divided by the area of the star. Because disk area scales with radius squared, the depth is approximately the square of the radius ratio. In this model, the only thing that matters is how large the planet looks compared with the star.

The JavaScript on this page converts the star radius from solar radii into meters and the planet radius from Earth radii into meters before taking the ratio. That conversion is important because the numerator and denominator must be in the same unit. The script uses 1 solar radius = 6.9634 × 10⁸ meters and 1 Earth radius = 6.371 × 10⁶ meters. Once the units match, they cancel, leaving a dimensionless fraction that can be shown as a percentage. If you see a value such as 0.0335%, that means the star is predicted to dim by 0.0335 percent during the transit in this simplified model.

There is no separate weighting step in this calculator, because transit depth is governed by one geometric relationship rather than a bundle of unrelated inputs. More detailed astronomy tools do add terms for limb darkening, dilution, or observing noise, but those effects belong to full light-curve models rather than to this quick estimate. For a fast check, the squared radius ratio is the key quantity.

That is why the direction of the result is so intuitive: double the planet radius and the depth becomes four times larger. Double the star radius and the depth becomes four times smaller. The formula is simple, but the scaling is powerful.

Worked example for a Sun-like star and a 2 R⊕ planet

For a Sun-like star with a planet radius of 2 Earth radii, the calculator follows the same unit conversion and ratio used above. The planet radius is 2 × 6.371 × 10⁶ m = 1.2742 × 10⁷ m. The star radius is 6.9634 × 10⁸ m. Their ratio is about 0.0183, and squaring that gives about 3.35 × 10^-4. As a percentage, the predicted transit depth is about 0.0335%.

That is a shallow dip, but it is still a meaningful signal. It corresponds to roughly 335 parts per million, which is the scale of precision photometry used in exoplanet work. The example shows why a modest planet can be difficult to see around a Sun-sized star. If you keep the planet at 2 Earth radii but shrink the star to 0.5 solar radii, the depth becomes four times larger, so the same planet is far easier to detect around the smaller star.

How to interpret the exoplanet transit-depth result

To interpret an exoplanet transit-depth result, read the percentage first and the scientific notation second. The percentage gives you a direct sense of how much the star fades, while the fractional notation is handy when the dip is tiny, which is common for Earth-sized planets around Sun-like stars. Very small values can still be scientifically important if the photometry is precise enough.

A quick mental scale helps. Values below about 0.01% are subtle; values around 0.1% are noticeably deeper; and values near 1% are large, classic transit signals often associated with big planets or small host stars. The observing challenge still depends on noise, cadence, stellar activity, and telescope quality, but the depth alone tells you whether the signal is likely to be delicate or obvious.

If the output surprises you, check two things: whether you entered radii rather than diameters, and whether the trend makes sense. A larger planet must produce a deeper transit, while a larger star must produce a shallower one. If your intuition says the opposite, there is probably a unit mix-up or a radius/diameter mix-up in the inputs.

Common exoplanet transit scenarios at a glance

These benchmark exoplanet cases make the calculator easier to read at a glance. They are not special cases in the code; they are familiar transit configurations that show how much the depth changes when you move from Earth-sized planets to gas giants or from Sun-like stars to smaller dwarfs.

When you compare targets, the table helps you translate a decimal into a physical situation. A small percentage is not automatically unimportant; it may simply mean the planet is modest in size or the star is large. Likewise, a larger percentage may come from a very large planet, a very small star, or both.

Assumptions and limits of the simple transit-depth model

This calculator deliberately uses the simplest exoplanet transit model, which makes it fast and easy to understand but not a full light-curve fit. Real stars are not uniformly bright disks. Limb darkening makes the edges dimmer than the center, grazing transits change the overlap geometry, and contaminating light from nearby stars can dilute the apparent depth.

• Limb darkening: the stellar surface is brighter near the center than at the edge, so a detailed model adjusts the depth and shape.
• Geometry: this page assumes a straightforward projected overlap; grazing transits need more careful modeling.
• Contaminating light: background stars or unresolved companions make the observed dip look shallower than the true planet-to-star ratio.
• Starspots and activity: active stars can add brightness variations that distort the transit signal.
• Input uncertainty: errors in the stellar radius feed directly into any planet-size interpretation built on the result.

Even with those limits, the area-ratio estimate remains extremely useful. It captures the dominant geometry and gives the right scaling behavior, which is why it shows up so often in introductory exoplanet work. If you later fit real data, you can use this page as the quick baseline that tells you what to expect before the more detailed astrophysical corrections are added.

Using the transit-depth result in practice

In practice, a transit-depth estimate helps with target selection, quick consistency checks, and observing-planning decisions. You might compare two stars to see which host gives the stronger signal for the same planet, or compare a measured dip with a published stellar radius to see whether the reported planet size is plausible. Because the relationship runs both ways, you can also think in reverse: if you know the depth and trust the star radius, the planet radius scales with the square root of the depth.

That is also why the optional mini-game below is tied so closely to the calculator. It trains the same habit of mind: look at the star, look at the planet, and ask how the squared size ratio changes the dip. Once that intuition clicks, the result box stops feeling like an abstract decimal and starts reading like a physical measurement of how much starlight the planet blocks.