TOI 700 d TESS's First Earth-sized Habitable-Zone Planet w/Dr. Joey Rodriguez/Dr. Andrew Vanderburg

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NASA's test spacecraft promises to reveal an entire host of new, close exoplanets.

And in fact, it already has.

My guests today have found an intriguing new exoplanet just about 100 light years away that looks a lot like Earth.

T-O-I-700 orbits a red dwarf within its habitable zone.

And maybe, just maybe, might host the conditions right for life.

Welcome to Event Horizon, with John Michael Godier.

In today's episode, John is joined by Dr. Andrew Vanderberg and Dr. Joey Rodriguez.

Dr. Vanderberg received his PhD in astronomy at Harvard University in 2017 and was a postdoctoral associate

at the Harvard Smithsonian Center for Astrophysics until he started his Sagan Fellowship at the University of Texas in Austin.

His research focuses on extracellar planet detection, in particular, studying the Kepler Space Telescope data.

Dr. Rodriguez works as a Smithsonian Astrophysical Observatory Astronomer at the Center for Astrophysics for Harvard Smithsonian.

His research focuses on understanding how planets form and evolve by studying circumstellar discs and exoplanets.

He received his PhD in physics from Vanderbilt University in 2016.

Andrew Vanderberg and Joey Rodriguez, welcome to the program.

Thanks for having us.

Thank you.

Now, gentlemen, you have found what appears to be a very Earth-like planet orbiting a red dwarf.

How Earth-like are we talking?

I mean, obviously this is not an analog of the Sun. It's a red dwarf.

But other than that, what does this planet look like in comparison to Earth from what you can tell so far?

So the planet is roughly about 20% larger than the Earth, and it receives about 85% of the flux that the Earth would receive from the Sun.

So right now all we know about it is that it's roughly the size of Earth, and it resides right inside the havel zone for this star.

Other than that, we don't know what the mass of the planet is, and that's actually going to be the focus of future studies.

Now this was discovered by Tess, which is our new planet-hunting transit searching instrument, replacing previous instruments like the Kepler Space Telescope.

So what exactly, how distant is this star system from us?

I know Tess tends to look closer in. How far away is this?

So it's about 100 light years, 31 of our sex. So it's relatively nearby, much closer than a lot of the planets that are found in previous missions.

And as you point out, that is really the focus of the Tess machine.

So it's within the habitable zone of this red dwarf, which being a red dwarf, that would imply that it's very close into the star.

There's been some question over the last few years that red dwarfs might not be ideal.

They might be bathing their habitable zones in ultraviolet light and things like that that might really preclude the possibility of water and even possibly life.

Is this the case with this planet? Is this a rambunctious young red dwarf, or is it something a little bit more quiescent?

So this star is a little bit older and therefore seems to be pretty quiet right now.

The Tess light curve doesn't show any evidence for variability due to star spots.

Although some ground based light curves show that there might be star spots and that the star might be rotating with an orbital period of about 55 days.

And we don't see any flares during the full year light curve that we have from Tess.

So this looks to be a quiet or star, then say for example, trapped one.

Now, still though this planet would likely be tightly locked, right?

That's correct.

So we're probably would be looking if habitable, it would be an eyeball world.

It would have a side that never sees the light of day, so to speak, and then a side that always sees it and then a twilight zone.

That's correct.

Where you might have something interesting, right?

Yeah, so that's the focus of the third paper in our series led by Gabby Engelman Suiza.

So she simulated the climate on this planet and found that indeed there are situations where you have a lot of clouds building up near the substellar point, the point closest to the star.

And then some temperature gradients, the temperature goes from very hot near that substellar point to cooler as you go away from the starlight.

And potentially, and there are some hospital places to live.

If this is a rocky world with a potentially habitable atmosphere.

Now your paper details that it does have some characteristics that it might be a rocky world.

What tells you that it might be that?

So most of what we know in the sense of assuming that it's likely rocky has to do with what we know from previous studies, which focused on planets around small red dwarfs like T-O-I-700 and found that planets that are the similar size to what we found for T-O-I-700-D are indeed rocky compositions.

However, that study was only focused on planets that were very close to their star within their habitable zones.

And so we really, we don't actually know what we'll find for a planet in a tattle zone, whether that trend will continue.

But based on what we know from those studies, it's likely that this planet is rocky.

We have to extrapolate the results from these studies of hotter planets if we want to make any guesses about whether these cooler, potentially habitable planets are rocky or not.

And whenever you extrapolate, there's a possibility that you're just doing it totally wrong.

So that's the uncertainty of whether the planet is rocky or not.

How similar is this to some of the other things we found like the Trappist 1 system and Proxima B for that matter is this sort of in that class or is this something even more special than what those appear to be?

So Trappist 1 and Proxima are both lower mass stars than T-O-I-700.

Trappist 1 of course is extremely low mass. It has a mass about 8% that of the sun.

Proxima is a little bit higher mass. It has a mass about 15% that of the sun.

T-O-I-700 has a mass that's about 42% the mass of the sun.

So the star is much bigger and the planets are further away from the star.

Those other two stars Trappist 1 and Proxima are also more active right now than T-O-I-700 is.

So potentially this could be right now a more hospital place for life as we know it.

But on the other hand, it's possible that earlier on in its lifetime T-O-I-700 was also more active like these other lower mass stars.

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So this is the star itself.

It's a little bit more substantial than a lot of red dwarfs, right?

So it's sort of on the more hefty side of things.

That's right.

Now does that make...

Now you say it's a comm start.

Early in its history, would it have been a comm start?

Because I know the evolution of red dwarfs they tend to be a little bit dicey and then they sort of quiet down.

How old is this star system in this planet?

Do you have any ideas on that?

So that was actually part of the paper one that was led by Emily Gilbert from the University of Chicago.

And what they figured out was that the star has to be older than about one and a half giga years.

But that's all the constraint that they were able to put on at this point.

It's notoriously difficult to measure the ages of old stars.

If it's a young star, you can actually do pretty well younger than about a giga year.

But for the old stars, they just don't change very much at all over the many giga years of their lifetimes.

Which in the case of a red dwarf is a very, very long time.

Absolutely.

So 1.5 billion years at least, right?

That's correct.

That's interesting because if it's older than that, that's enough time to maybe have microbial life going on on this planet.

If there's liquid water there, what stands in the way of that?

Is there anything obvious that says, well, this is Earth-like?

But we shouldn't really speculate about life because it has this problem or that problem.

Is there anything that stands out that might say liquid water is sort of precluded from being here?

Or is it completely possible that it's there?

As far as we know, it's completely possible.

This is, of course, barring our earlier comments about the potentially more active history of the star in terms of flares and ultraviolet radiation.

And also just the fact that we really don't yet know that much about whether it is in fact rocky.

We think it could be, but we need to test that to be sure.

And we don't know what its atmosphere looks like.

So if we knew all the details, our answer might be totally different, but from where we are standing right now with our best knowledge.

As far as we can tell, this is a potential candidate to host life.

So we should look at it.

And when telescopes like James Webb come online, we should start looking for atmospheric biosignatures.

Now, to my understanding is some modeling of what the atmosphere might be like with this world has been done.

Give us some idea of what came out of that.

What could the atmosphere of this world be like so far as we know at this point?

So there's a lot of assumptions that have to go into this, obviously.

What that study really showed was that you could explore various different types of Earths.

So an Earth would say no water, similar to present day, but just no water.

You could have an early Mars-type planet.

But unfortunately in all of the cases, the detectability of the atmosphere signatures are just not feasible in the era of the James Webb Space Telescope.

And that was kind of one of the big conclusions out the paper.

Now, is there anything planned that might be better in the future?

So there are four mission concepts under study right now for the next generation of Space Telescopes

after the James Webb Telescope launches after the W first mission launches.

And one of those concepts is like a super James Webb.

It's called the origin space telescope and it also observes in the infrared.

And depending on what the specs of that telescope end up becoming and what the capabilities end up being,

it's plausible that that telescope could make progress even where James Webb could not.

Now, how hard is it to separate out the red dwarf from the planet?

You're looking at trying to look at the atmosphere and you're trying to say,

this is from that planet versus emissions separating it from the emissions and starts off.

How hard is that characterizing an atmosphere on a planet on a world like this?

So it's particularly difficult for a world like this where the signals are small.

So for giant planets, we've been able to do this over the past decade or so and doing quite well.

But for systems where the signal is quite small, it's actually quite difficult.

But the way that we do it is that we actually take spectra at various times throughout the planet orbit.

Specifically, we take it when the planet is in front of the host star and it's actually transiting.

So all the light that is going through is also passing through the atmosphere of the planet.

And then we also take a spectra when the planet is not in front of it when it's a different place in its orbit.

And we can actually take the difference between those two and that will actually give us the amount of the light that's actually going through the atmosphere.

And we can try to see those small features from the atmosphere of the planet.

But for this system, it's extremely challenging.

We're talking about measuring features on scales of one to ten parts per million,

which is orders of magnitude beyond what we can do right now.

And it's still beyond what James Webb will be able to do.

Now, this planet is transiting.

What can you tell from other things like radial velocity measurements?

Can you sort of constrain things down for this planet as far as mass to further observation or do you have it?

So that is the big thing that we want to do going forward.

So this target, well, it's not well suited for studies of its atmosphere at this time.

It's actually fairly well suited to try to go out and measure the mass of the planets using radial velocity measurements,

using the state-of-the-art facilities that are currently coming online or just recently came online.

But it will be a very difficult measurement.

We are talking about 80 centimeters per second is the actual amplitude of the orbit for this planet.

And many of the planets that we have gotten masses for have a few meter per second at the smallest amplitude,

with maybe a meter per second being the smallest.

So it's going to be a very challenging observation.

But of the habitable zone or size planets known to date,

it's actually the best one for this mass measurement.

And it's a really important measurement for us to make because you listen to us tell you

how we just don't know whether Earth-sized planets in their star's habitable zones are usually rocky.

That seems like something we should know, right?

If we want to be looking for these planets and we want to be looking for a lot,

we should really understand whether these planets are anything like the Earth at all.

So we're really excited to have the opportunity to make one of the first measurements of this type on this planet.

Yes, now, and to make that measurement, it's not just a measurement.

It's confirmed because you also looked at this with the Spitzer Space Telescope.

What did you look for with that?

I mean, what did that confirm?

Or did you learn anything more than you knew from TUS just from that confirmation?

So in the first paper, they were able to statistically validate the planetary nature of TOI-700D.

And so one of the things that that validation process does not take into account is systematics from the instrument itself.

And it kind of assumes that it has to be an astrophysical signal that you're seeing.

So we are still learning about the systematics in the telescope and the data itself.

And so what Spitzer, one of the big things that really allowed us to do was to independently confirm the transit

and completely rule out any systematics as being the origin of the transit and the test data.

The other thing it allows us to do is it actually really allowed us to refine the confidence in the parameters for TOI-700D.

Specifically, we get a tighter constraint on what the radio so the planet is and a tighter constraint on what the period of the planet is.

Our confidence in those two parameters.

Now, gut feelings, gentlemen, and I want to hear from both of you.

Is this the closest thing, the closest planet we have yet found to Earth?

That's a tough question.

Andrew, you first.

Can we say this is the closest we've gotten as far as spotting another Earth or something that recognizably similar to Earth?

In terms of the things we know, we can measure the temperature of the planet or estimate the temperature of the planet.

We can measure the size of the planet and we can study the star itself.

There are other planets that are probably more similar to Earth in one of those dimensions or the other.

For example, the trappist one planets are in general smaller and closer to exactly one Earth radius.

So you might say those are more Earth-like, but their star is a little bit more different from the Sun.

On the other hand, there are stars from the Kepler mission that have been found to have small planets in their habitable zones, like Kepler 62.

That star is higher mass than TOI-700 and closer and massed to the Sun.

So perhaps you might want to say that that planet is more Earth-like than TOI-700.

But I think it's fair to say that TOI-700, as far as we can tell, is pretty close to Earth in all of these dimensions, given our lack of knowledge about the mass of the planet, this actual composition, and its atmosphere so far.

Joey, your view?

And you really put it particularly well there.

There are certain aspects of TOI-700D that are very, very similar to the Earth, but then there are other known habitals on Earth-sized planets that have aspects that are much more similar to the Sun Earth system.

So I think the key point here is that this is the most accessible to get that mass measurement, but in terms of the similarity to the Earth Sun, I think there are enough differences that we just...

We just don't know at this stage, and I know it's probably not the greatest answer, but...

But we have that mass...

I'm sorry, Joey, talking over you.

When we have that mass measurement, we'll be able to say more.

Yeah, because that allows us to get the bulk composition and let us compare to what we know for, say, the threshold planets in our solar system.

I see. Now, my last question for you guys...

Tess, okay.

Almost brand new instrument in space that's already showing plenty of results.

What are the prospects for finding an Earth-like planet around a different type of star, say an orange dwarf or a Sun-like star?

Is it... is this... that we see with these red dwarfs, which also with the Trappist One and Proxima B.

But can we look closer? Is it possible to actually spot an Earth-like planet in the habitable zone of a type G star with that instrument?

I think that depends how long that lets us keep flying Tess.

So, right now, Tess has gone about 75% of its primary mission.

It will switch over into its first extended mission in July.

And we'll start observing the southern hemisphere again.

And that's really important for finding planets in the habitable zones of higher mass stars.

Because the higher mass your star, the longer the orbital period is for a planet in the habitable zone.

If you want to find a planet in the habitable zone of a Sun-like star, you have to wait a year between each transit.

And since we've only observed all of the stars in one part of the sky for a year at most,

there's no way we could find enough transits for us to actually detect such a planet yet.

But if we keep getting extended missions approved and continue collecting more and more data on these stars,

then I think it's entirely possible that we'll start finding planets in the habitable zones of higher mass stars.

And just to add to that, we typically want three transits before we declare this a candidate or a confirmation or anything.

And so that means we need at least three years of data to look for an Earth-sized planet around a Sun-like star.

So in essence, anyone that might be 100 light years out with a test spacecraft that doesn't observe for longer than a year is never going to see Earth.

They would have to get very, very, very, very long.

Unless they got very lucky, yes.

But it's very unlikely that they would have to, they would not happen.

It'll be challenging, too, though, because we'll have big gaps in our data.

Because tests will look at the southern hemisphere, then it is now looking at the northern hemisphere.

So we're not observing TOI 700 right now.

We're observing stars on the other side of the sky.

Right.

So when we go back to TOI 700 and at the stars in that region, we will have missed an entire year.

So it's possible we might have missed a transit of a planet with a period of one year.

So that's another wrinkly have to throw in.

Do you actually know the period well?

Could the period, if you find two transit separated by two years, is the period two years, or is the period one year?

So that's another wrinkly have to take care of and think about.

So test is a little bit tricky to do this.

But we think that there's a chance it could be done.

Now can you detect, say, test is looking in the wrong direction and you expect a transit coming.

Can you use ground-based telescopes to detect the transit?

For this, for TOI 700D, for the Earth's size, half of the zone planet, no.

The signal is just too shallow to really do from the ground.

But we were actually able to get ground-based transit follow-up of the middle planet in the system,

which is a more of a mini version of Neptune, most likely.

Now how many planets do you know of total in the system?

So we know of three planets, one's orbiting at 10 days and it's an Earth's size planet.

That's roughly 1.04 Earth radii.

We know of a sub-Neptune at, which is 2.6 Earth radii, that's about 16-day period.

And of course, we know about the Hattles Zone Earth's size planet to your wide 700D,

which is at 37.5 days.

Such short years for these planets so close into red dwarfs.

Well, gentlemen, we are out of time.

And it was a pleasure talking to you both, and I hope we learn more about this planet.

Thank you very much.

Thanks for having us.

Thanks for watching Event Horizon.

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