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Kepler discoveries.

A bar graph of Kepler Discoveries. The vertical axis is labeled “Fraction Observed”, from 0 to .3, and the horizontal axis is labeled “Planet Size (Earth = 1)” from 0.5 – 0.7 to 16 – 23. A bar labeled “55” is above 0.5 – 0.7 Planet Size and approximately 0.03 Fraction Observed. A bar labeled “165” is above 0.7 – 1 Planet Size and approximately 0.07 Fraction Observed. A bar labeled “381” is above 1 – 1.4 Planet Size and approximately 0.165 Fraction Observed. A bar labeled “520” is above 1.4 – 2 Planet Size and approximately 0.23 Fraction Observed. A bar labeled “567” is above 2 – 2.8 Planet Size and approximately 0.26 Fraction Observed. A bar labeled “268” is above 2.8 – 4 Planet Size and approximately 0.12 Fraction Observed. A bar labeled “94” is above 4 – 5.7 Planet Size and approximately 0.04 Fraction Observed. A bar labeled “54” is above 5.7 – 8 Planet Size and approximately 0.025 Fraction Observed. A bar labeled “53” is above 8 – 11 Planet Size and approximately 0.025 Fraction Observed. A bar labeled “39” is above 11 – 16 Planet Size and approximately 0.02 Fraction Observed. A bar labeled “17” is above 16 – 23 Planet Size and approximately 0.01 Fraction Observed. At the top of the graph planets in our solar system are shown above their representative size as labeled on the x-axis. A gap between 1.4 – 2 and 2 – 2.8 is labeled “Sizes not seen in our solar system”.
This bar graph shows the number of planets of each size range found among the first 2213 Kepler planet discoveries. Sizes range from half the size of Earth to 20 times that of Earth. On the vertical axis, you can see the fraction that each size range makes up of the total. Note that planets that are between 1.4 and 4 times the size of Earth make up the largest fractions, yet this size range is not represented among the planets in our solar system. (credit: modification of work by NASA/Kepler mission)

What a remarkable discovery it is that the most common types of planets in the Galaxy are completely absent from our solar system and were unknown until Kepler’s survey. However, recall that really small planets were difficult for the Kepler instruments to find. So, to estimate the frequency of Earth-size exoplanets, we need to correct for this sampling bias. The result is the corrected size distribution shown in [link] . Notice that in this graph, we have also taken the step of showing not the number of Kepler detections but the average number of planets per star for solar-type stars (spectral types F, G, and K).

Size distribution of planets for stars similar to the sun.

A bar graph of Size Distribution of Planets for Stars Similar to the Sun. The vertical axis is labeled “Average Number of Planets per Star”, from 0 to .3, and the horizontal axis is labeled “Planet Size (Earth = 1)” from 1 – 1.4 to 16 – 23. A bar above 1 – 1.4 Planet Size rises to approximately 0.35 on the vertical axis. A bar above 1.4 – 2 Planet Size rises to approximately 0.27 on the vertical axis. A bar above 2 – 2.8 Planet Size rises to approximately 0.31 on the vertical axis. A bar above 2.8 – 4 Planet Size rises to approximately 0.14 on the vertical axis. A bar above 4 – 5.7 Planet Size rises to approximately 0.055 on the vertical axis. A bar above 5.7 – 8 Planet Size rises to approximately 0.048 on the vertical axis. A bar above 8 – 11 Planet Size rises to approximately 0.04 on the vertical axis. A bar above 11 – 16 Planet Size rises to approximately 0.01 on the vertical axis. A bar above 16 – 23 Planet Size rises to approximately 0.009 on the vertical axis. At the top of the graph planets in our solar system are shown above their representative size as labeled on the x-axis. A gap between 1.4 – 2 and 2 – 2.8 is labeled “Sizes not seen in our solar system”.
We show the average number of planets per star in each planet size range. (The average is less than one because some stars will have zero planets of that size range.) This distribution, corrected for biases in the Kepler data, shows that Earth-size planets may actually be the most common type of exoplanets. (credit: modification of work by NASA/Kepler mission)

We see that the most common planet sizes of are those with radii from 1 to 3 times that of Earth—what we have called “Earths” and “super-Earths.” Each group occurs in about one-third to one-quarter of stars. In other words, if we group these sizes together, we can conclude there is nearly one such planet per star! And remember, this census includes primarily planets with orbital periods less than 2 years. We do not yet know how many undiscovered planets might exist at larger distances from their star.

To estimate the number of Earth-size planets in our Galaxy, we need to remember that there are approximately 100 billion stars of spectral types F, G, and K. Therefore, we estimate that there are about 30 billion Earth-size planets in our Galaxy. If we include the super-Earths too, then there could be one hundred billion in the whole Galaxy. This idea—that planets of roughly Earth’s size are so numerous—is surely one of the most important discoveries of modern astronomy.

Planets with known densities

For several hundred exoplanets, we have been able to measure both the size of the planet from transit data and its mass from Doppler data, yielding an estimate of its density. Comparing the average density of exoplanets to the density of planets in our solar system helps us understand whether they are rocky or gaseous in nature. This has been particularly important for understanding the structure of the new categories of super-Earths and mini-Neptunes with masses between 3–10 times the mass of Earth. A key observation so far is that planets that are more than 10 times the mass of Earth have substantial gaseous envelopes (like Uranus and Neptune) whereas lower-mass planets are predominately rocky in nature (like the terrestrial planets).

Questions & Answers

which planet orbits the closest?
Alastair Reply
What is the angle between Earth's equator and the Celestial equator? In the drawing they seem pretty similar. Thank you for this study resource.
Chuck Reply
Describe the spectrum of each of the following: starlight reflected by dust, a star behind invisible interstellar gas, and an emission nebula
shakila Reply
If the Oort cloud contains 1012 comets, and ten new comets are discovered coming close to the Sun each year, what percentage of the comets have been “used up” since the beginning of the solar system?
Day Reply
what is spectral type of sun
Akshat Reply
what everyone asking here? and who answers for them?
Shashi Reply
highest frequency wavelengh
Kathy Reply
may I know which Kingdom shows largest diversity
Arpita Reply
or students should post tough likely questions
Adepitan Reply
for example questions on demand functions and etc
Adepitan Reply
are there no ways we can get tough questions to answer
Adepitan Reply
What do you mean?
Amman Reply
excellent book bro.... keep it up.... if I find any query i will ask u.... thanks
rao Reply
Hi! I'm a bit confused, what is this
Sizakele Reply
They hsve new set of questions every time they test us. i do revision with the tutorials they give us answer extra questions from their moodle site but every time i write exams there will be few not even 10% of the questions are they. most of the time i guess. They give us three lectures who do not
Phumza Reply
Practice Key Terms 2

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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