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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Dust (and biomarkers?) from impacts on exoplanets

Gianni Cataldi1,2, Alexis Brandeker1,2, Philippe Th´ ebault3

1Department of Astronomy, Stockholm University 2Stockholm University Astrobiology Centre 3LESIA-Observatoire de Paris, UPMC Univ. Paris 06, Univ. Paris-Diderot, France

Lund, 2015-05-06

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Outline

1 Motivation: Characterisation of exoplanets 2 Modelling the impact event and collisional evolution 3 Detectability

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Characterisation of Earth-sized exoplanets

• Earth-like planets around Sun-like stars appear to be

relatively common

• ultimate goal: find a second genesis of life
• large variety of planetary bodies in the solar system

(atmospheres, surface properties, composition,. . . )

• characterisation of detected exoplanets warranted

Image credit: NASA Ames/SETI Institute/JPL-Caltech 3 / 12

Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Techniques to characterise exoplanets

• radial velocity and transits (bulk properties)
• spectroscopy (atmosphere, surface)
• . . . impact event?

1.2 1.3 1.4 1.5 1.6 Wavelength (μm) 3,300 3,350 3,400 3,450 3,500 3,550 3,600 3,650 Best-ft model: 190× solar metallicity Pure water model: 10,000× solar metallicity Solar cloud-free model with low C/O Solar model with clouds at 80 mbar Solar cloud-free model

b

(Rp/Rs)2 (p.p.m.)

ʘ

Fraine et al. (2014) 4 / 12

Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Idea: dust from impact on exoplanet

• impact accelerates surface material (potentially including

biomarkers) to escape velocity

• escaping fragments form circumstellar belt
• mutual collisions produce dust (debris disk-type evolution),

resulting in much larger surface area

• is detection of planetary dust possible? Biomarkers?

Image credit: David Hardy 5 / 12

Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Modelling of the impact event

1 Calculate amount of escaping mass for given impact

scenario

• total escaping mass
• mass escaping without substantial shock damage

2 assess collisional evolution using simplified analytical

model (debris disk)

3 estimate fractional luminosity as a function of time

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

• 1. How much material is escaping?

For a 20 km diameter impactor:

parameter asteroid Earth Mars Moon M(> vesc) kg 6.7 × 1015 2.5 × 1016 8.4 × 1016 < 46 GPa % 9.9 17.2 20.3 parameter comet Earth Mars Moon M(> vesc) kg 5.8 × 1015 2.4 × 1016 8.0 × 1016 < 46 GPa % 11.2 11.8 12.6

reference values: Earth continental crust 1.5 × 1022 kg; asteroid belt 3 × 1021 kg; zodiacal dust 2 × 1016 kg

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

• 2. Collisional evolution
• relevant processes:
• atmospheric braking: lower size cutoff
• collisions (production of smaller fragments)
• radiation pressure (removal process)
• Poynting-Robertson (PR) drag (removal process)
• assess collisional evolution under various simplifying

assumptions (size distribution, collisions / PR drag,. . . )

Image credit: Kouji Kanba / ISAS / JAXA 8 / 12

Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

• 3. Fractional luminosity (as a function of time)

f = Ldust/L∗

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

fractional luminosities

parameter asteroid Earth Mars Moon tmin,ini Myr 1.8 0.7 0.3 tmax Myr 3.4 1.7 1.0 fmax,bl

• 5.8 × 10−9

1.7 × 10−8 4.7 × 10−8 fmax,PR

• 2.4 × 10−10

1.3 × 10−9 5.5 × 10−9 parameter comet Earth Mars Moon tmin,ini Myr 1.3 0.5 0.2 tmax Myr 2.1 0.9 0.5 fmax,bl

• 7.8 × 10−9

2.6 × 10−8 7.2 × 10−8 fmax,PR

• 3.9 × 10−10

2.5 × 10−9 1.1 × 10−8

reference values: HR 4796A 5 × 10−3; zodiacal dust 10−8 − 10−7; Kuiper belt dust ∼10−7

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Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

. . . so, can we detect the fragments?

• difficult with current instrumentation. . .
• . . . but in the reach of future instruments (e.g. Darwin,

ELT,. . . )

Roberge et al. 2012 11 / 12

Gianni Cataldi Motivation: Characterisa- tion of exoplanets Modelling Detectability

Summary

• for a given impact, we computed the escaping mass, its

collisional evolution and the fractional luminosity

• fdust ≈ fzodi
• dust from impacts on exoplanets is difficult to detect with

current instrumentation, but future instruments will be able to detect such dust

• background source: exozodiacal dust
• next step: study dust composition (biomarkers) with

spectroscopy?

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Gianni Cataldi

fragment size distribution

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Gianni Cataldi

timescale for (catastrophic) collisions

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Gianni Cataldi

Impact rate

Chapman 2004 3 / 3