Data Dive 4.4

Are planets like Earth rare?

We have found thousands of planets and none like Earth. What does that tell us?

You will need paper and a pencil, and a browser for step 3

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Step 1 of 5 · Your guess

What do you think?

Astronomers have confirmed 5,981 planets around other stars. None of them are like Earth: about Earth's weight, taking about a year to go around its star.

Someone says that settles it. Planets like Earth must be rare.

Write it down before you lookDoes finding no planets like Earth mean they are rare? Write yes, no, or not sure, and why.
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Step 2 of 5 · Look

Every planet found so far

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 mass0.11101001,0000.11101001,00010,000100,0001,000,00010,000,000100,000,000

Confirmed planets around other stars. Earth is marked in yellow.

How to read it. Each dot is one planet. How far right it sits is how long its year is. How high it sits is how heavy it is compared with Earth. Each gridline is ten times the one before, so tiny and huge planets fit on one chart.

Find out
  1. Find Earth. What is near it?
  2. Is the empty space around Earth a fact about planets, or about something else?
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Step 3 of 5 · Try it

Now you try it, on the same data

This opens the pool, the site's chart tool. The words in bold below are buttons and menus you will see on the screen.

Do these five things

Open the pool on this file: data.sfhsbta.com/thepool?file=void_exoplanet_mass_period.csv&mark=point

  1. Under across, pick orbital_period_days, how many days a planet's year lasts. Under up, pick mass_earths. Look at it. Then set scale to log and look again.
  2. Find where Earth would be: 365 days across, 1 up. What is the nearest dot?
  3. Set keep to to 1, so only planets as light as Earth or lighter are left. How many are left? Does any have a year longer than 100 days?
  4. Clear keep to. Under colour by, pick discovery_method, the way each planet was found. Which method finds planets with long years? Which finds the light ones?
  5. Write one sentence you would add to the chart so nobody reads it as “we are alone.”
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Step 4 of 5 · Say it

Your answer, in one sentence

I think planets like Earth <are rare, are not rare, or we cannot tell yet> because <what the chart shows> and I checked it by <what you did to the chart>.

The last part matters most. It shows you did not just look at the chart, you tested it.

Here is one exampleI think we cannot tell yet, because the ways we find planets miss small planets with long years, which is exactly where Earth sits. I checked by coloring by discovery method, and almost every light planet was found by a method that needs a short year.
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Step 5 of 5 · What you found

Check your numbers

0
planets lighter than Earth with a year longer than 100 days
1.67%
planets of 10 Earth masses or less with a year of 100 days or more
The big idea: empty can mean we cannot seeAn empty space on a chart does not always mean nothing is there. It can mean our tools cannot see there. Planet hunters mostly find big planets or planets with short years. Planets like Earth sit right where those tools struggle.
One more thing to think aboutSomeone uses this chart to say we are alone in the universe. What one sentence on the chart would stop that?
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After the dive

The charts you used today

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.

library · ladder · dives · the pool · this dive’s worksheet

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