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hi everyone my name is chris visa

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program and i'm a fifth year phd student

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at caltech

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and today i'll be talking a little bit

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about our project studying the

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atmospheric evolution of three young

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planets in the v1298 tau system

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this is work that i've done with my

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advisor heather knudsen along with all

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of the collaborators you see on the

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screen

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first i'm going to be talking about the

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evolving atmospheres of young planets

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and why this topic might be

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astrobiologically relevant

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then i'm going to talk about how we

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actually make observations of

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atmospheric evolution in young planets

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and in particular i'll be discussing a

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technique called helium photometry

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and finally i'll discuss our recent

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results applying helium photometry to

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the planets in the v1298 tau system and

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i'll motivate some future work that i

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hope will get done over the next few

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years

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the fundamental question that we're

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trying to answer is how do planetary

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to situate ourselves the majority of

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extrasolar planets planets orbiting

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stars that are not our sun

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orbit quite close to their host stars

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within an orbital period of 100 days or

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so

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this means that these planets are

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getting constantly bombarded by

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high-energy radiation and that can

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really change how the planet looks to us

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and how the planet evolves over its

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lifetime

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consider a hydrogen atom living in an

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exoplanet atmosphere

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because the planet is so irradiated

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that hydrogen atom is going to see an

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extreme ultraviolet photon with

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energy exceeding 13.6 electron volts

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quite often

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and what that photon can do is it can

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ionize the hydrogen atom

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but if the photon energy exceeds 13.6 ev

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that excess energy has to go somewhere

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and it goes into heating

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the gas goes into the thermal energy of

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the planetary atmosphere

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and as this happens many many times

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because the planet is severely

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irradiated

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for many many hydrogen and helium atoms

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in the planetary atmosphere

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the net effect is the planetary

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atmosphere can heat up so much

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that it becomes gravitationally unbound

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that the planetary atmosphere is just

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and we can imagine that this has pretty

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important consequences for the planetary

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mass as the material is literally being

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lost from the system

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for the planetary radius as well as i'll

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show in a second

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and also for habitability the presence

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or absence of an atmosphere is known to

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be quite astrobiologically relevant and

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so understanding the primary factors

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that control whether a planet can retain

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or whether it loses an atmosphere is of

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utmost importance

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now to briefly mention one consequence

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of atmospheric evolution we can take a

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look at the histogram of observed

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exoplanet radii from bj fulton's work

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done in 2018.

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what's being shown here is the average

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number of planets per star as a function

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of planetary size and i've indicated the

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sizes of earth neptune and jupiter for

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reference

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and while you can see that the most

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common type of planet that's been

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discovered to date is something between

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the size of the earth and neptune

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we can see that the distribution is

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pretty bimodal

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and actually this has a lot to do with

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atmospheric evolution we think that

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planets on the large side of this radius

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gap called subneptunes

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at around 2.5 earth radii

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we think that these planets retained

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most of their atmosphere against the

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effects of atmospheric escape

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and we think planets on the small side

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of the radius gap

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at around 1.5 earth radii

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we think that they lost their

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atmospheres

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and so clearly atmospheric evolution has

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an outsized impact on how planets appear

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to us

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and how planets evolve over their

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lifetimes

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now to understand when this process is

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happening over a planet's lifetime it's

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necessary to look at the high energy

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radiation of the host star

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and what's being plotted here is the

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x-ray luminosity

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of three different stellar models with

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different

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rotation rates

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as a function of stellar age and you can

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see that for all three models the star

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is outputting much more x-ray luminosity

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over the first 100 million years or so

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of its life

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and what that leads us to conclude is

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that we should try to observe

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atmospheric escape when it's happening

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to planets that are less than around 100

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million years old or so

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now the problem is that most planets

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that have ever been discovered

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are about a gig a year or older most of

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these planets are positively ancient

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compared to the time scales on which

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atmospheric escape happens

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so the problem is we've never observed

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how the atmospheres of young planets

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evolved before

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and this really prevents us from

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answering the question how efficiently

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can young planets turn radiation into

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heat

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this is the primary control on whether a

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planet keeps or loses its atmosphere and

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so this quantity is of utmost

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astrobiological relevance

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to remedy this we looked at the young

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transiting system v1298 tau

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which was discovered by trevor david in

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2019

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now i'll clarify what transiting means

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in a second but basically it just means

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that the planets pass in front of the

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host star with respect to our line of

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sight

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now this system is 23 million years old

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so it's young enough for us to observe

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atmospheric escape occurring in action

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there are three known planets in the

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system planet c with orbital period 8

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days and a size about half that of

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jupiter

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planet d with orbital period 12 days and

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a little bit larger than planet c

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and then planet b with orbital period 24

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days and a size about that of jupiter

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so now that we've selected a target we

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can ask how do we observe atmospheric

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escape in action

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one way to do this is with a technique

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called transit spectroscopy

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so in the transit method we're watching

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planets as they pass in front of their

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host stars and block out a little bit of

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the star's light

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and larger planets block out

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proportionally more light from the host

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star

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now we can visualize this diagram in

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this very not to scale cartoon shown

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here and again some light from the host

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star will be blocked by the planet and

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won't reach your telescope

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during a transit i should say but some

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of the light will reach your telescope

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and this is kind of the nominal case

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however when a planet's atmosphere is

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escaping some of that light is actually

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getting filtered through that low

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density escaping atmosphere

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and if we can choose

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to observe at a wavelength where that

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low density tenuous gas becomes opaque

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then suddenly the planet

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will absorb it'll block out much more

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light and the planet will appear much

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larger to us

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so now we'll quickly describe how we

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make these measurements in practice uh

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with helium photometry

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so our experimental experimental design

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is pretty simple

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we take light from the telescope we pass

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it through an optical element called an

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engineered diffuser which just helps us

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control systematics

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that light then goes through an ultra

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narrow bandpass filter that is centered

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on that 1080 1083 nanometer line

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uh and then it forms our image where we

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can take a picture much like you would

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take a picture with your iphone camera

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except that we take it in the infrared

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and we take many many many pictures over

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the course of a night

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and we track the star's brightness in

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all those pictures to form a light curve

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and based on how much light gets blocked

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out during the planetary transit we can

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tell whether or not helium absorption is

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going on in the planetary atmosphere and

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in so doing we can tell how quickly that

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so here's an example of a measurement we

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made when we were commissioning this

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observing mode of the planet wasp 69b

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now this planet when observed at other

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less special wavelengths

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looks like this in the blue

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what's being shown here is the star's

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brightness over time and you can see

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that the planet

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at the transit center is blocking out a

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little less than two percent of the

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star's light

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however when we measured it at 1083

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nanometers we got the data in gray

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which are binned in black and shown with

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our best fit model in red

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and clearly at 1083 nanometers the

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planet blocks out more than two percent

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of the star's light

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and so we use this measurement to

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confirm the presence of helium in the

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atmosphere of wasp 69b at 10.1 sigma

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confidence

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now

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this system is pretty old so it doesn't

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tell us about what's going on when

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atmospheric escape is most important for

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a planet's life but it helped us

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understand how our instrument worked and

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it helped us confirm that everything was

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so in this work we applied this

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technique to the planets in the v1298

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tau system and now we'll describe some

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of the results that we saw

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when we looked at the transit of v1298

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tau c we weren't able to detect anything

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at all

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our light curve is shown on the left

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with data in gray

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uh bend in black

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with a best fit model in the solid line

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and the model for the transit at

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non 1093 nanometer wavelengths in the

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dashed line and i've also shown a

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baseline model for

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just showing how the star and the

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weather evolve over the night

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uh in red

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and you can see that the model the solid

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line doesn't really indicate that we

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detected transit at any confidence

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this is kind of disappointing and it's

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probably due to poor weather which you

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know sometimes you get unlucky

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but at the same time we can definitely

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rule out a very large transit in the

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helium line which is an important result

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when we looked at planet b i've shown

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the same thing for planet b on the right

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we did detect the planet

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uh the transit of this planet

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but we don't think we detected any clear

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helium absorption

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our best fit model is shown

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on the right in the solid green line and

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the dash green line shows the best fit

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model at non 1083 nanometer wavelengths

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and you can see that the models kind of

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overly each other they have slightly

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different shapes but they're not really

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distinguishable in any statistically

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significant way

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so a larger transit in the helium line

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is still possible but we would

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definitely need more data to confirm

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that

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now when we looked at planet d we think

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we see a tentative detection

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now we took two knights of data on this

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planet which are shown with the circles

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and triangles in the light curve on the

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left

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and the signal appears consistent in

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amplitude across both knights of data

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collection which is really exciting

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however

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to stitch this light curve together we

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did require a transit timing offset

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which is the slight deviation from a

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keplerian orbit and that was enough to

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make us a bit hesitant about the result

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and so we're calling it a tentative

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detection rather than a slam dunk

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still it's a really interesting result

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and it means that this planet should be

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prioritized for future follow-up

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to summarize our work

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we are attempting to measure atmospheric

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escape from young planets for the first

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time

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and here's a graphic of the geometry of

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what we're actually measuring

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now we tentatively detect helium

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absorption for v1298 tau d

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which indicates that this might be the

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first known young planet with

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observationally accessible mass loss and

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in the future we'd of course like to get

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more data which uh we'll be hopefully

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gathering some more full transits in the

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winter weather permitting

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and we'd also like to measure the

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planetary masses to know the

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gravitational potentials from which

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these planets atmospheres may be

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escaping