Astronomer Answers Exoplanet Questions

Hello, I'm Dr. Ruth Angus.

I'm an astronomer and an associate curator

and professor of astrophysics

at the American Museum of Natural History.

Let's answer some questions from the internet.

This is Exoplanet Support.

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22_Minutes asks,

Scientists believe there is a super-Earth

at the edge of the solar system,

but the way things are going around here,

even a slightly above-average Earth would be welcome.

Agreed.

A super-Earth is a cool name

for what is actually a slightly pedestrian thing,

which is just a planet

that is a little bit more massive than the Earth.

Do we have a super-Earth beyond Neptune?

Maybe.

So we talk about Planet Nine,

a potential ninth planet in our solar system.

If it exists,

we think that it might be tugging on other objects out there

right at the edge of the solar system.

If it's out there, it's extremely far away,

therefore, very faint,

because it doesn't reflect much sunlight.

And it's also moving extremely slowly.

Nobody's found it yet, but it's hypothesized.

realscientists asks, How do we find exoplanets?

Even though the idea

of exoplanets has been around for a long time,

they have only been confirmed in the last 30 years.

They are really hard to find. They are really hard to find.

So there are a couple of main ways that we detect them.

We can't directly image them most of the time.

They're just too faint compared to their host star.

And trying to photograph an exoplanet is like trying

to photograph a firefly next to a lighthouse.

Instead, what we have to do

is try to figure out the presence

of a planet based on the influence it has

over its host star.

One of the main techniques we use to find exoplanets

is called the transit method.

This is where a planet passes in front of its host star

and blocks out a little bit of light.

If we measure the brightness of a star

at repeating intervals,

we can actually see the star seem

to get a little bit more dim as the planet passes

in front of it, and then it brightens again

once the planet has passed all the way past it.

We've found more than 3,000 planets this way.

There's another technique that is also pretty successful,

and it's called the radial velocity method,

or sometimes the Doppler method,

or sometimes the wobble method.

This is where we actually see the star moving in space.

We can't see what's pulling on it,

but we can see that the star moves,

and we infer that there's a planet orbiting it,

dragging it around with its gravity.

What we can do is actually measure the Doppler shift

of the star's light.

When the star's moving towards us,

its light is blue-shifted,

and when it's moving away, its light is red-shifted.

So we infer that the star is moving

and it's being yanked around by a planet.

kaycie_hall asks, My toddler makes me read about space,

watch things about space, talk about space.

I'm just out here pretending I have any clue.

Sorry, but WTF is an exoplanet?

Well, I also have a toddler, so I can totally relate.

An exoplanet is just a planet outside

of our own solar system.

It's a planet that orbits another star.

And we found thousands of these exoplanets.

We're up to over 6,000. They're everywhere in our galaxy.

Probably every star has an exoplanet.

FinancialSun6933 asks, Is a planet orbiting two suns

at once actually possible IRL?

Yes, it is. And we have found planets just like this.

This is actually a pretty unlikely configuration,

and they're very difficult to find.

So these are called circumbinary planets.

The star itself, the double star, is called a binary.

Kepler-16b, for example, is a circumbinary planet.

It's one of the first ever discovered.

If you've seen Star Wars, like the original film,

on Tatooine, there's an image of two suns rising.

So Tatooine is actually a circumbinary planet.

It orbits both stars at the same time.

So binary stars,

stars that have another star for a companion,

are actually the most common type of star in the galaxy.

Most stars are binaries.

Our sun is actually slightly unusual,

in that we don't have another sun.

StarTalkRadio asks, What is the most extreme exoplanet?

We could talk about the hottest exoplanet,

which orbits its star extremely close in,

and it's basically as hot as a star is.

It's so hot, in fact, that on its day side,

molecular hydrogen, like H2 molecules,

are actually split apart.

We could also talk about planets

that are extremely eccentric.

Most exoplanets sort of orbit in a circular way,

like the Earth has a very circular orbit,

but some planets don't,

they orbit in a sort of oval shape, like an ellipse.

And these planets have extreme temperature swings.

There are some planets

that are actually kind of disintegrating as we watch.

So these are planets very close to their host star

that have a big, extended, puffy atmosphere,

which actually seems like it's kind of being blown away

from the planet.

A few different options for extreme planets there.

FluorescentLightbulb asks, Explain like I'm five,

what makes the Earth so gosh darn livable?

How is it different from the other planets

in our solar system, or viewable planets in our galaxy?

We have liquid water here

because Earth's in the habitable zone,

and also because it's small and rocky,

and it has a discernible surface.

But there are a lot of other qualities

that make Earth pretty perfect for habitability and life.

It has a pretty strong magnetic field,

and that magnetic field shields us from solar radiation.

Earth also has an atmosphere,

which we think is essential for life.

Plate tectonics, we think, might be important for life,

because plate tectonics allow us

to actually almost moderate the amount of carbon

that's in the atmosphere.

And this kind of keeps the planet in equilibrium,

and it actually solves something

called the faint young Sun paradox.

Our sun used to be fainter and dimmer,

and yet the planet Earth

has sort of always maintained roughly

the same average temperature.

As the planet gets warmer, there's more rain,

more evaporation, more weathering of rock,

and rock weathering removes carbon.

Another thing that makes our planet special

is that we have a large moon,

which actually stabilizes Earth.

And without it, axial tilt might change,

and it would send the seasons sort of totally out of whack,

and the Earth itself

would experience massive climactic swings.

Overall, we're at a pretty steady state.

And we think that this is probably a prerequisite for life.

CourtneyWRocket asks,

Have we discovered any habitable planets yet,

because I'd really just like to start over.

Unfortunately, we haven't discovered anything

that we know for sure is habitable.

So what makes a planet habitable?

The main thing, we think,

is the existence of a liquid water on the surface.

This means a planet has to be about the right distance

from its star to be the right sort of temperature

for liquid water.

It also means

that the planet has to be relatively small like Earth,

because big planets like Jupiter, Saturn,

they tend not to have a discernible surface,

they're actually mostly made of gas,

but there are a lot of other criteria probably

that make a planet habitable.

For us on Earth, we have a stabilizing moon,

which stops our orbit from changing too much.

We have a magnetic field,

which prevents solar radiation

from irradiating the surface of Earth.

We have an atmosphere. We have plate tectonics.

Have we found planets in this habitable region,

the distance that makes liquid water possible?

Yes, we have.

Most of these planets, however,

orbit stars that are very different from our sun.

Most of these planets orbit little stars,

M-dwarfs or K-dwarfs.

K-dwarfs are a little smaller than the sun,

and M-dwarfs are a lot smaller than the sun.

The problem with that

is that M-dwarfs are very magnetically active.

Think solar flares, they have coronal mass ejections,

which are storms of harmful,

high-energy particles leaving their surfaces

and potentially blasting these exoplanets.

The James Webb Space Telescope has been looking

for signs of atmospheres

on habitable-zone exoplanets orbiting M-dwarfs,

and so far, it hasn't found any.

So we're now starting to wonder whether, actually,

maybe the star has to be just right as well

for a planet to be habitable.

Evan_Slayerz asks,

Which exoplanet do you think has the greatest chance

of harboring life?

#AskNASA.

One of the main targets in the search for life

is the TRAPPIST-1 planetary system.

The problem with TRAPIST-1

is that it is a very active M-dwarf star

with an extremely strong magnetic field.

And the planets orbiting TRAPIST-1 are blasted

with its radiation.

Now, TRAPIST-1 has three planets in the habitable zone,

and we don't yet know whether or not they have atmospheres,

but the James Webb Space Telescope is working on it.

I think a better candidate is a star called TOI-700.

It has two exoplanets in its habitable zone,

and it is also an M-dwarf,

so it's a small star with a strong magnetic field.

TOI-700 d and e are probably my bet

for the place most likely to have life.

TOI-700 is old, really old.

Old stars tend to be more stable, more peaceful,

more sanguine, more calm, a bit like our own sun.

And so TOI-700

doesn't have so much strong radiation blasting its planets.

A Reddit user asks, Explain like I'm five.

If the sun is getting hotter in billions of years,

does that mean that planets further away from us

may become suitable for life while Earth may not?

Yes, the sun is getting hotter,

and the habitable zone in our solar system

is moving outwards.

And it's actually been doing this for a really long time.

And in a billion years from now,

the Earth's oceans might evaporate,

and the Earth itself will not be habitable.

Humans have only been around for like 300,000 years,

so we've got a lot of time to figure it out.

It does mean that Mars is going to get warmer and warmer,

and it's gonna move into the habitable zone even more,

and it will stay in the habitable zone

for quite a long time.

jobmegens1 asks, Can't help but wonder

if an alien civilization would have detected Earth

the same way we detect exoplanets.

There's actually a zone

that's called the Earth Transit Zone, which is like a plane,

which is where an alien would have to be

to be able to look back at the sun

and see the Earth's transit.

Probably if there are aliens out there scanning the skies,

looking for Earth-sized exoplanets,

they might have spotted us

if they live in that Earth Transit Zone.

If an alien has seen us, why haven't they said hello yet?

It takes a really long time

for light to travel across space,

so it could be that distant aliens

haven't really noticed our presence yet

because human technology hasn't been around

for all that long in the grand scheme of things.

bl4ck4nti asks, How do we know what exoplanets look like?

We could know it's mass

because we can measure the radial velocity,

or Doppler shift, of the star as the planet orbits.

And if you have radius, you have size,

and you have mass,

then you know something about the density of your planet.

Maybe a planet's around the right density

to be entirely made of water,

or maybe it's around the right density

to be extremely heavy, like mostly made of metals and rock.

Mostly I've talked about exoplanets transiting their stars,

so passing in front of the star,

but planets can also pass around the back.

And if they're extremely hot,

and they glow brightly in infrared,

we can actually see when they pass behind the star,

because we lose their signal.

And we can even map the hotspots on the surface of a planet,

and we can figure out what the winds might be doing,

and understand, relatively speaking,

what they might look like.

So some of those exoplanets that we've discovered

are actually pretty bizarre-looking.

ProudReaction224 asks, Explain like I'm five

how scientists can figure out the composition

of exoplanet atmospheres.

Often, the way we do this

is by observing a planet as it transits its host star.

Most of the light coming from the star

is gonna pass directly through space into our telescopes,

but some of the light is gonna filter

through the planet's atmosphere

on its way to our telescopes.

And as it does that,

as it passes through a planet's atmosphere, it's changed.

Gas molecules in the atmospheres of planets absorb light

at certain distinct wavelengths.

Carbon dioxide, for example, removes certain colors,

oxygen removes certain colors.

And we can infer the presence of these molecules

in the atmosphere of an exoplanet

by seeing where those colors are missing

as a planet passes in front of its host star.

It's much easier to do this for a big, puffy planet,

because you get more light passing through the atmosphere.

For planets like Earth,

the atmosphere is actually just a teeny,

tiny layer on top of the rocky core,

and so you don't really get very much starlight passing

through the atmosphere.

It's really difficult to do this for Earth-like planets.

PrinceOfAtheism asks,

Just learned there's an exoplanet nearly the size

of our sun.

I wonder, are there planets bigger than stars?

The maximum size for an exoplanet

is about the size of Jupiter.

You could try adding mass to Jupiter,

but what you would actually do is just increase its density

and not its size.

The smallest stars, M-dwarfs, can be extremely small,

about as small as Jupiter.

So can you get a star that's smaller than a planet?

Actually, in certain circumstances, perhaps you could.

So some Jupiter-sized planets

that are very close to their host star

are actually irradiated so strongly that they are so hot

that they actually puff up a little bit.

So you can get a planet

that is a little bit bigger than Jupiter.

adhamaloka asks, What does it take for us to be able

to take images of exoplanets?

You need to have a planetary system

that is relatively nearby.

You have to block the light

from the star using something called a coronagraph.

When we're trying to get these really,

really high-resolution images from the ground,

the atmosphere makes it difficult.

You know, when you look at a star, you can see it twinkling?

That is because the light from that star passes

through a column of air that's turbulent,

winds, et cetera, right?

And as it lands on the telescope,

that twinkling messes up your observations basically.

You can't resolve really small distances in your images.

So what you can do is shine a laser through the atmosphere,

that allows you to predict the motion of the air,

and then the starlight falls on a mirror that you distort

to counteract the variations in the air.

And this is called adaptive optics,

and it allows you to focus light really extremely precisely

and get these precise images of exoplanets.

zeyad_shalaby asks,

So the Webb telescope already detected water

in the form of steam on an exoplanet already,

and I wonder if we will detect signs of life

like methane soon.

Water is a really common molecule in the galaxy,

it turns out, and a lot of exoplanets do have water.

You also asked about methane.

So methane is a molecule that's produced by life,

and it's something that we would call a biosignature gas.

The problem with methane

and a number of other single molecules

is that there are lots of ways to make methane

that actually doesn't have to involve life at all.

So methane on its own is not a perfect biosignature.

And now, the consensus

is that we actually probably wanna look

at a combination of two different molecules,

so, for example, methane and oxygen together,

because the two, methane and oxygen,

should not naturally coexist.

If they are seen together,

it's basically a sign that methane

and oxygen have to be constantly produced perhaps

by something like life.

PeterCorless asks, Which exoplanet fascinates you most?

One of my favorite exoplanets is called 55 Cancri Ae.

The planet is a super-Earth.

It's more massive than the Earth.

It orbits extremely close to its host star.

It whizzes around every few hours.

Its whole year is shorter than an Earth day.

And it's so hot that we think it's actually a lava world,

and yet, it's held on to an atmosphere.

That atmosphere is probably constantly being outgassed,

so it's being replenished.

Maybe it's being stripped away at the same time.

It's almost like a beacon of hope for me,

because it shows that planets can outgas new atmospheres

and recreate them,

even when their host star is doing everything it can

to strip those atmospheres away.

SugarandBlotts asks, I have recently heard

that astronomers have found an exomoon.

I am quite scientifically illiterate.

So what exactly is an exomoon,

and could it affect us on Earth?

An exomoon is a moon in another solar system.

We have tons of moons in our solar system.

We're up to like basically 900,

or something like that, moons.

So if our solar system has so many moons,

it would be surprising

if other planetary systems didn't have any.

Moons also present another interesting possibility for life.

So in our own solar system,

we have icy moons, Europa and Enceladus,

and these moons are actually warmer than they should be.

They're outside the habitable zone,

but they have liquid oceans under an icy crust.

The reason for that is that they orbit massive planets

that have strong tidal forces.

So we think maybe there's life on our own icy moons,

in our own solar system.

And if icy moons could be a place for life to hang out,

then there's lots of opportunities for icy moons

to exist elsewhere in the galaxy

and another potential host for life.

heatherjs asks,

WTF, no one's concerned about rogue planets.

Rogue planets are these solitary planets

that don't have a host star, they probably once did,

but interactions maybe with other planets

in the planetary system has sent the planet outwards.

So we think that there are thousands and thousands,

maybe millions or billions,

of rogue planets out there in space.

johnalogue asks,

How does finding planets in habitable zones affect SETI?

Do we point instruments at these planets?

SETI is the search for extraterrestrial intelligence.

This is really more about technosignatures

rather than biosignatures.

It's searching for things like narrowband radio emission.

It's searching for potentially laser signals.

It's searching for also things called Dyson spheres.

A Dyson sphere is where the entire star is covered

in solar panels.

It would look like a very dim object

that's very bright in infrared.

There's a lot of space to cover.

There's a lot of stars to look at.

And so, in fact, SETI has really only covered

like a very small number of stars, but it's still searching.

MithandirsGhost asks, How do we know

that exoplanets aren't sunspots or starspots?

Spots on the surface of a star,

like our sunspots on our own sun,

are two-dimensional for the most part.

So as a star rotates,

you can see a spot coming into view,

and that spot actually makes

the star a little bit more dim overall,

just like a planet does.

A planet is a solid object,

and it passes in front of its star

without being foreshortened on the limb.

So variations in starlight caused by spots tend to be smooth

because as spots round the corner,

they almost disappear, they sort of flatten.

Whereas planets, as they transit,

they sharply block the star's light.

So transits tend to look like very sharp dips.

When we search for planets using the Doppler method,

the star itself actually has a Doppler signal,

and this is a massive problem.

Our Earth orbiting the sun actually makes the sun move

a little bit because both the Earth

and the sun orbit the center of mass

of the Earth-sun system.

The Earth is dragging it around as it orbits,

but it's doing that very slowly.

The sun itself has its own Doppler signal,

it's shifting and roiling and pulsating,

and that signal can be larger.

And a lot of the time,

when we find exoplanets using the Doppler method,

they turn out not to be real.

They turn out just to be interference from the spots.

If you've seen the movie Project Hail Mary,

40 Eridani, the planet that Rocky is from,

was thought to be a real planet

when Andy Weir was writing the book,

and it later turned out

that it was probably not a real planet.

It was probably just some spots on the surface of the star

that acted like a planet.

And unfortunately, Rocky's home world might not be real,

but it still lives on in our hearts and minds.

brrrrrrat asks, 'Interstellar,'

how is it possible for the waves to be so big

on the water planet if the gravity is stronger?

The water planet in Interstellar

is a planet that's orbiting a black hole,

and it has extremely strong tidal forces,

and it's tidally locked to the star.

Tidal locking happens a lot.

The moon is actually tidally locked to the Earth.

The moon's rotational period, how fast it spins,

is exactly the same as its orbital period,

how long it takes to go around the Earth.

So it always shows the same face to the Earth.

This water planet that orbits a black hole is the same.

It has these big tidal bulges on each side,

on the side facing the black hole

and on the side away from the black hole,

because the planet's distorted

into this football or rugby-ball shape,

and the waves on the planet are created

by these tidal forces.

The tidal bulges actually should stay put.

They shouldn't move around.

So you'd have a shallow part of the planet,

and you'd have a deep part of the planet,

and you wouldn't necessarily have waves like traveling

and rippling across the planet like you do in the film.

If the planet wasn't quite tidally locked,

if it was moving around just a little bit,

perhaps you could get some sort of sloshing back and forth

of the ocean,

and maybe you could make these kinds of exotic waves.

Maybe that is what Kip Thorne was going for in the science.

I'm not totally sure.

Those are all the questions for today.

Thank you for watching Exoplanet Support.

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