Data Dive · Rung 4 · What Isn't There · 4.4

Earth-like planets are rare

The largest question a person can ask, answered by an instrument.

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The claim on the table

“Earth-like planets are rare.”

Nearly six thousand confirmed planets. Plot every one by mass and orbital period, and look at where Earth would sit.

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The evidence

Mass against orbital period

Every confirmed exoplanet, by mass and orbital period5,981 planets, both axes logarithmic - NASA Exoplanet Archivemass, Earth masses (log scale)orbital period, days (log scale)a confirmed exoplanetEarth - 365 days, 1 Earth mass1

5,981 confirmed planets, both axes logarithmic. Earth marked in yellow.

1.66%
are both low-mass and long-period
0
are lighter than Earth beyond 100 days

Earth is one Earth mass at 365 days. The archive contains nothing like Earth.

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The verdict

It does not mean they are rare. It means we cannot see them.

A transit search needs repeated transits, so a long-period planet gives a mission two or three chances in its whole lifetime. A radial-velocity signal shrinks with mass and with period.

The empty corner is the shape of our instruments, printed onto the sky.

AskSomebody uses this chart to argue we are alone. What is the one sentence you would add to the chart to stop that reading?
The three questions
  1. Where does this data start and end, and what decided that?
  2. Does the evidence reach the claim?
  3. What would you need before you would sign it?
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After the dive

The shapes on this page, in the library’s own words

the scatter

Each dot is one thing measured twice. A tight cloud along a line means the two move together.

What it leaves outTwo things moving together is not one causing the other, and a third thing neither axis shows may be moving both.
the scatterplot matrix

Every pair of measures plotted against every other, so you can spot which ones move together.

What it leaves outPlot enough pairs and some will line up by chance alone. The more panels there are, the more likely a false one looks real.

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