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[link] compares all the exoplanets that have both mass and radius measurements. The dependence of the radius on planet mass is also shown for a few illustrative cases—hypothetical planets made of pure iron, rock, water, or hydrogen.

Exoplanets with known densities.

Plot of Known Exoplanets. The vertical scale is labeled “Planet Radius (Earth Radii)”, and runs from zero at the bottom to 20 at the top in increments of one. The horizontal axis is labeled “Planet Mass (Earth Masses)”, and is a logarithmic scale going from 1 on the left to 1000 at right. Four curves are drawn showing the theoretical sizes of iron, rock, water, and hydrogen planets with increasing mass. The bottom curve is for iron planets, beginning with less than 1 Earth radius and mass, increasing to about 2 Earth radii at 1000 Earth masses. Next is rock, starting near one Earth radius and one Earth mass mass increasing to about 3 Earth radii at 1000 Earth masses. Water begins slightly above 1 Earth radius and 1 Earth mass and increases to over 5 Earth radii at 1000 Earth masses. Finally, hydrogen begins at 2.5 Earth radii and 1 Earth mass increasing to nearly 13 Earth radii at 1000 Earth masses. Over-plotted on the graph are data points for exoplanets with known masses and radii. Most of the points are clustered above the peak of the hydrogen curve, with most near 14 Earth radii at about 800 Earth masses. Another grouping is clustered around 3 Earth radii and 10 Earth masses. The planets of the Solar System are also shown, with Earth and Venus at 1 Earth radius and mass, Uranus and Neptune near 4 Earth radii and about 11 Earth masses, Saturn near 9 Earth radii and 100 Earth masses, and Jupiter near 11 Earth radii and 300 Earth masses. Mars is not plotted.
Exoplanets with known masses and radii (red circles) are plotted along with solid lines that show the theoretical size of pure iron, rock, water, and hydrogen planets with increasing mass. Masses are given in multiples of Earth’s mass. (For comparison, Jupiter contains enough mass to make 320 Earths.) The green triangles indicate planets in our solar system.

At lower masses, notice that as the mass of these hypothetical planets increases, the radius also increases. That makes sense—if you were building a model of a planet out of clay, your toy planet would increase in size as you added more clay. However, for the highest mass planets ( M> 1000 M Earth ) in [link] , notice that the radius stops increasing and the planets with greater mass are actually smaller. This occurs because increasing the mass also increases the gravity of the planet, so that compressible materials (even rock is compressible) will become more tightly packed, shrinking the size of the more massive planet.

In reality, planets are not pure compositions like the hypothetical water or iron planet. Earth is composed of a solid iron core, an outer liquid-iron core, a rocky mantle and crust, and a relatively thin atmospheric layer. Exoplanets are similarly likely to be differentiated into compositional layers. The theoretical lines in [link] are simply guides that suggest a range of possible compositions.

Astronomers who work on the complex modeling of the interiors of rocky planets make the simplifying assumption that the planet consists of two or three layers. This is not perfect, but it is a reasonable approximation and another good example of how science works. Often, the first step in understanding something new is to narrow down the range of possibilities. This sets the stage for refining and deepening our knowledge. In [link] , the two green triangles with roughly 1 M Earth and 1 R Earth represent Venus and Earth. Notice that these planets fall between the models for a pure iron and a pure rock planet, consistent with what we would expect for the known mixed-chemical composition of Venus and Earth.

In the case of gaseous planets, the situation is more complex. Hydrogen is the lightest element in the periodic table, yet many of the detected exoplanets in [link] with masses greater than 100 M Earth have radii that suggest they are lower in density than a pure hydrogen planet. Hydrogen is the lightest element, so what is happening here? Why do some gas giant planets have inflated radii that are larger than the fictitious pure hydrogen planet? Many of these planets reside in short-period orbits close to the host star where they intercept a significant amount of radiated energy. If this energy is trapped deep in the planet atmosphere, it can cause the planet to expand.

Planets that orbit close to their host stars in slightly eccentric orbits have another source of energy: the star will raise tides in these planets that tend to circularize the orbits. This process also results in tidal dissipation of energy that can inflate the atmosphere. It would be interesting to measure the size of gas giant planets in wider orbits where the planets should be cooler—the expectation is that unless they are very young, these cooler gas giant exoplanets (sometimes called “cold Jupiters”) should not be inflated. But we don’t yet have data on these more distant exoplanets.

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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