Are there more trees on Earth than stars in the Milky Way?

Earth hosts about 3.04 trillion trees; the Milky Way contains somewhere between 100 and 400 billion stars. The trees win — by roughly a factor of ten.

This feels backwards. A galaxy is unimaginably vast; a forest is just... a forest. Yet the claim is genuine, and both numbers come from real, checkable estimation methods. Nobody has counted every tree, and nobody has counted every star — in both cases scientists counted a sample and scaled up. In this paper we look at where each number comes from, and do the scaling-up ourselves.

Counting the stars

We cannot count the Milky Way's stars one by one: we sit inside the disc, and dust hides most of it from view. Instead, astronomers weigh the galaxy — the orbital speeds of stars reveal the mass holding them in orbit — and then divide the mass in stars by the mass of an average star:

$$\text{Number of stars} \approx \frac{\text{total stellar mass of galaxy}}{\text{mass of an average star}}$$

Both quantities are uncertain. The dividing step is especially slippery because most stars are not like the Sun: the galaxy is dominated by dim red dwarfs with perhaps a quarter of the Sun's mass, too faint to see individually across the disc. Depending on the assumptions, the answer lands between $10^{11}$ and $4\times10^{11}$ — which is why NASA quotes the famously wide range of 100 to 400 billion stars [2].

Counting the trees

The tree count is more recent, and the method is beautifully down-to-earth. In 2015, Thomas Crowther and colleagues published the first rigorous global tree census in Nature [1], built from 429,775 ground-truthed measurements: real plots of land, all over the world, where someone counted actual trees (defined as woody stems at least 10 cm across at chest height).

Each plot gives a density. Suppose a surveyed plot measures $20 \text{ m} \times 20 \text{ m} = 400 \text{ m}^2$ and contains $30$ trees. Since $1 \text{ km}^2 = 10^{6} \text{ m}^2$, that plot's density is:

$$\frac{30 \text{ trees}}{400 \text{ m}^2} \times 10^{6} \text{ m}^2/\text{km}^2 = 75{,}000 \text{ trees per km}^2$$

Averaging thousands of such plots within each biome — rainforest, taiga, savanna — and using satellite maps for each biome's area, the count becomes a sum of multiplications:

$$\text{Total trees} \approx \sum_{\text{biomes}} (\text{average density}) \times (\text{area})$$

As a toy example, a biome with our density above and an area of $10$ million km² would contribute $75{,}000 \times 10^{7} = 7.5\times10^{11}$ trees on its own. Do this properly across every biome and the total comes to $3.04\times10^{12}$ — about 422 trees per person at the paper's 2015 population figure, or about $375$ each for today's 8.1 billion people.

Trees by a factor of ten

Putting the two counts on the same scale makes the result vivid — on an order-of-magnitude ladder, where each step is a factor of ten, the trees stand a full rung above the stars:

Even in the least favourable matchup — the top-end galaxy of $4\times10^{11}$ stars —

$$\frac{3.04\times10^{12}}{4\times10^{11}} = 7.6,$$

the trees win nearly eight-fold, and against the low-end estimate they win thirty-fold. Against a middling $3\times10^{11}$ stars the ratio is almost exactly ten. To feel the size of the winning number: counting one tree per second, day and night, would take about $96{,}000$ years. (If trillions and billions blur together, we untangle them in another article.)

Yes — there really are more trees on Earth than stars in the Milky Way, by around a factor of ten, and the comparison survives even the most generous star count. What we should hold loosely is not the verdict but the precision: the star count spans a factor of four, and the tree count depends on where you draw the line for "a tree". The deeper lesson is the method — you do not need to count three trillion of anything; you need a good sample, a density, and an honest multiplication. The same paper carries a sting in its tail: it estimates we remove about $15$ billion trees per year, and that the global tree count has fallen by roughly $46\%$ since the dawn of human civilisation. The trees are winning the race against the stars, but not against us.

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References:

[1] Crowther, T. W. et al. "Mapping tree density at a global scale." Nature 525, 201–205 (2015).

[2] NASA Goddard Space Flight Center, Imagine the Universe! — "The Milky Way Galaxy": imagine.gsfc.nasa.gov

Note: Both figures are estimates with substantial uncertainty; the Crowther count itself carries a confidence interval of roughly ±0.1 trillion, and star-count estimates depend heavily on the assumed abundance of faint red dwarfs.