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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

why are the hyperlinks not working?
Gregory Reply
If the atmosphere blocks the view of Venus' surface, what am I seeing that looks like craters?
Michael Reply
clouds?
Michele
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters when view the planet?
Michael Reply
I don't Know! perhaps the mountains?
Michele
my question is if the surface of Venus is shining y not our earth
Tahir
when we look at venus we can't see any crater like things... check it once again
RIEM
and venus shines because of the clouds that are made up sulpher dioxide and sulphuric acid droplets. and the clouds are so dense. the case of earth is different
RIEM
also because of Venus's 70% albedo phenamenon
Shivam
and plus it reflects 70 percent of its light back into space earth don't shine because it's not a cloudy planet and its farther than the sun
americantuber
can I become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL Reply
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters? Are these atmospheric storms?
Michael
can a death of a massive star be the new big bang
Neeta Reply
No it might be a supernova. The big bang was much more massive.
Nick
It created the universe as we know it
Nick
the big bang is just a new beginning
Neeta
the death of a gigantic star makes it possible for a new begging
Neeta
Only in the local area of space
Nick
so many smaller universes are created and destroyed
Neeta
i think it created a supernova!
Michele
you still think? I am sure I created one supernova of my own
Neeta
I'm sure you did
Nick
neeta the death of a massive star could lead to an explosion that leads to a super nova and when the supernova explodes it becomes a nebula like messier 1
americantuber
a supernova remnant
americantuber
what is means by earthbound
Satyam Reply
you are earthbound, arnt we all earthbound? except the ISS etc.
Collins
I don't agree we may be earthbound because of our gross body/physical body...but there is also a subtle body which does not limit us to earthly existence
Neeta
good questions, as humans our habitat is earth, we are bound to this and have to alter ourselves to stay alive off land and outside the atmosphere, do you not agree? I'd guess you could also say the moon is earthbound. anything earthbound is restricted to earth. hope this helps, love your question
Collins
how do you describe astral projection
Neeta
Astral projection is spiritual. I'm assuming Satyam was talking about the physical world.
TheDirtyGhost
ah yes spiritually we are bound only to what we chose, like nothing, I like to think that way
Collins
anyone here also participating in seti@home?
dreamer
what is seti?
Mahesh
what has extraterrestrial entity got to do with this why @seti project..we are discussing human existence on earth and beyond earth
Neeta
please join me at the cosmic Diner where all your intergalactic dreams come true.
Alba
I was just thinking what if somebody ignorantly mistaken a cosmic microwave Background with a conventional microwave we be in a lot of trouble.
Alba
don't kno
dreamer
You mean the Restaurant at the end of the Universe...right we can come back and visit again but not sure of the mode of payment
Neeta
yeah I'm already heading there
dreamer
how to read stars
Amresh Reply
you don't
Max
astrology i guess
Uttam
astrologer is the study of star systems and constellations...but astronomy is the study of formation of universes multiverses birth and evolu of stars
Neeta
cool
Uttam
what is astronomy
babul Reply
study of formation evolution and the death of star star systems and galaxies
Neeta
astronomy is the study of universe
UNIVERSAL
astronomy and physics are basically the same except physics is the study of the motion and behavior of the universe
americantuber
What is 12 constellation of zodiac and why it is important to study astrology
MUHIBULLAH
And also what is main purpose of these 12 constellation of zodiac in astronomy
MUHIBULLAH
astronomy and astrology with two different things
Deja
it Is the study of universe and of life speaking generally! i think!
Michele
universe is born 13,7 miliardi of years with big Bang if i remember well! but there are also other theories for the universe, speaking generally! i remember so!
Michele
universe is actually about 13.7 billion years old
Nick
is astrology like astronomy
debjani Reply
Astrology is a lower dimension.. astronomy is much more vast and multidimensional
Neeta
yes.. astrology is about constellations only, and astronomy is about all stars, galaxies, gravity, dark matter, dark energy.. etc everything including astrology
Mahesh
can you become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL
I was reading the chapter on Cosmic Microwave Background. And, I can not seem to find it now. If anyone could help me find that portion of the Astronomy Textbook I would really appreciate it. AZ
Alba Reply
Where or which chapter discusses Cosmic Microwave Background?
Alba
29.4
Andrew
29 the big bang ch 29.4
Andrew
yes
pratham
interesting
Orlando
what's cold dark matter?
pratham Reply
How are Roche Worlds formed?
AlteredEdge Reply
In need to read some books about Astronomy so how can in get it actually leg live in Ethiopia can uh help me with that?
Mom Reply
I am sri Sharan .m .what is my best favourable numbers
Madhesh Reply
how can we know it
sruthi
😂😂
Rango
bahut hard
Rango
Why no object can travel with speed greater than light?
geometry Reply
Because the mass of light particles is zero
Asim
Can mass be negative?
geometry
No
Asim
anybody stated that the light is the only thing is speed
sruthi
geometry are you a korean or kpop fan
sruthi
in this universe anything can be faster than light because we simply can't get to an conclusion
sruthi
I am a an Army but lets not bring spam here.
geometry
But why mass can't be negative. What I mean is how can we claim an object made of strange matter has positive mass. In other words, theoretically can negative mass exist?
geometry
in our observable universe nothing is faster than the speed of light
Rango
Thanks
geometry
But theoretically, is it possible? Sorry I'm being very curious..
geometry
I don't know about dark matter but may be you should think about dark matter because dark matter are really strange
Rango
Exactly so theoretically, an dark matter lazer can be greater in speed. Also maybe quartz..
geometry
may be but i am not sure about quartz and btw techyons travel faster than light but techyons are hypothesized
Rango
there are many objects which are perceived to be faster than light
Ganeshan
The object have their own declination rate in speed,the air friction can develop higher temperature resulting the object to be disappear in molecules,not considering the Physical conditions in space.The object should be unaffected with the air friction and also speed should proportionate with fuel.
Near
the early universe expanded faster than the speed of light. Consider, at the speed of light in vacuum, time stops.
Robert
Does this mean something moving beyond the limit experiences negative time?
Robert
Under such conditions, would matter exist as wave-fronts?
Robert
When examining a cosmology it is possible to find quite unbelievable conditions. I like to imagine life at the tip of an asymptote, but to surpass it seems perilously remote.
Robert
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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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