Discovering planets beyond our Solar System

In 2017, NASA announced the discovery of seven earth-sized exoplanets orbiting a single star called, Trappist-One. Now, data has revealed new clues about the composition of these exoplanets and their potential to support life. Astronomy and astrophysics professor at the CUNY College of Staten Island, Emily Rice, joins Hari Sreenivasan to discuss.

TRANSCRIPT

In 2017, NASA announced the discovery of seven Earth-sized exoplanets orbiting a single star called TRAPPIST-1.

Now, data has revealed new clues about the composition of these exoplanets and their potential to support life.

Astronomy and astrophysics professor at the CUNY College of Staten Island Emily Rice joins us to discuss.

How did we find these?

That's a good question.

We found these exoplanets the way that we've actually found most exoplanets so far.

It's called the transit technique.

So what we do is, just monitor the brightness of a star, and when we see the brightness dip by a little bit, and then that repeats several times at least -- So we want to see at least three dips kind of with the same amount of time in between them.

Then we need to rule out some other things, but we can determine that that dip is caused by a planet passing in front of a star and actually blocking the light from that star for just a little bit of time.

So if you were watching from a different solar system, you would basically see, 'Oh, this is when the Earth gets between us and the Sun.'

Yeah.

And this is when Mercury gets between us and the Sun, and so it would create some -- So from that, how -- And then we start to focus more attention to that area.

Mm-hmm.

But then how do we know what the composition of the planets are like, whether it's inhabitable or not?

Yeah.

It's still -- From that really tiny dip, it's still a long way to finding out all of these different properties about the planets because really, again, all we're measuring is the brightness of the star and seeing these regular dips.

From the dips, though, we can get the relative size of the planet to the star.

So by assuming the size of the star, which is also kind of problematic in itself -- We have to figure out what the actual, physical size of the star -- It's not always easy.

Mm-hmm.

Then we figure out the size of the planets relative to the star.

And that might give us a little bit of a hint as to what type of planet they are because we think that depending on the sizes of planets, that tells us a little bit about what they're made out of.

Like, Jupiter and gas planets are big.

Rocky, earth planets are small.

We don't think we can have a rocky planet the size of Jupiter.

But really, to know more about the composition, we also need to know the masses of the planets.

We can't get the masses of the planets directly from this transiting technique, but we can get it if we do a little bit of follow-up observations.

So if we are lucky enough that there's multiple planets in the system, which we are for the TRAPPIST-1 system, and then if those transits actually are not perfectly regular but change a little bit -- The technique is called a TTV or a transit timing variation.

Okay.

So if the planets kind of tug on one another enough, that's caused by gravity.

And from that, we can determine the masses of the planets.

So we figure out the kind of gravity that a planet must be having to be able to pull or push another planet, and that means it has a core.

It is rocky.

Yeah.

That information, the mass information, combined with the radius information, with the size information, can give us what we call an overall bulk density of the planet.

And from that, we then try to infer composition because that's a little bit more constrained than just the size of the planet.

And then is it just about the sort of Goldilocks of it all?

Are you far enough from the sun?

Are you close enough?

But then, how do we know if there's water?

Yeah.

So, it's still a couple more steps to finding water.

So, in order to figure out kind of this habitable zone, also called the Goldilocks Zone, we have to know how far away the planet is from the star.

We can get that from just the transit itself, but then it also depends on the size of the star, the type of star that the planets are orbiting.

So a Sun-like star, we might think we know fairly well because the Earth is nicely in the habitable zone.

Mars and Venus are outside of the habitable zone.

But the interesting thing, especially about the TRAPPIST-1 system, is that there's lots and lots of exoplanets around stars that are not anything like the Sun.

And this TRAPPIST-1 planetary system is actually around a star that's much, much smaller than the Sun.

And so we can kind of -- I don't want to say guess, because it's a very, very educated guess.

Yeah.

But we can kind of estimate the habitable zone and how far away it is from that particular star, kind of understanding the stars as well as we do.

Okay.

It's important to point out that this is not a trip that we're going to take any time soon.

How close is this system?

Yeah.

This system is still about 40 light-years away.

That means traveling at the speed light...

Yeah.

It would take us 40 years.

...which we can't do yet.

Nope.

It would take us 40 years to get there.

Yeah.

So why continue to pursue this, because this is a system that we have studied a lot and know a fair amount about, compared to a lot of other things in space.

This particular system is just intense.

We should send everything that we have at this system.

This system, I just love it.

It is one of the closest exoplanetary systems out there.

In particular, it's really exciting for me because of the star that these planets are around.

It's a very low-mass star.

It's only about a tenth the mass of the Sun, and the cool thing about these stars is that they're everywhere.

There's way more of these stars in our galaxy than there are Sun-like stars.

And so the fact that we can find so many of these Earth-sized planets around this very low-mass star, it just means that these Earth-sized planets are probably everywhere in the galaxy, and we happen to have one really close by.

It's edge-on, so we can do these transit measurements.

We can do the transit timing measurements.

We can also start to do transmission spectroscopy to actually look at the atmospheres of these planets.

It's very kind of preliminary right now, but upcoming instruments like the James Webb Space Telescope will be able to break open this field.

And so the fact that we have this super exciting system so close by is just a fantastic gold mine for us to explore.

Even if we were trying to send messages there, it would take 40 years?

Yeah.

Yeah.

Right, to blink, 'SOS.

Hi.

Just saying hello.'

Yeah.

It's a long wait.

'In the neighborhood.'

So is there -- What are we learning?

You know, a lot of times, the tools that we build in studying something like this end up having other kind of ripple effects.

I mean, they maybe improve our existing telescope technology or optics for our phones, who knows?

But what are some of the other things, as you pursue something like this, that we, as a society, benefit from?

I think this one kind of tells us to keep looking.

Like, this one, you know, even if this particular system, if these planets end up not having dense atmospheres, end up not being habitable, just the fact that we've found them and that we can study them so well around this low-mass star that's really everywhere, that kind of gives us a little bit of more information to answering the bigger-picture question of, 'Are we alone in the universe?'

I think a planetary system like this makes it much less likely that we are actually alone because there's so many more of these small Earth-like planets out there around these small stars, we can assume.

So does this mean that we will start looking around a lot more of these small stars, maybe much, much closer to us, and see if there are exoplanets circling them?

Yeah, absolutely.

In fact, the telescopes that made these discoveries about these exoplanets in the TRAPPIST-1 system are specifically designed to study these low-mass stars.

And so it's very exciting that very, very quickly, like, the instruments and the telescopes aren't even fully online yet.

The capabilities aren't even as great as they will be in a few short years, and so the fact that they did find this really exciting system relatively quickly, it bodes very well for the future of exploring planets around these small stars.

Emily Rice, thanks so much for joining us.

Thanks for having me.