JWST Transmission Spectroscopy of HD 209458b: A Supersolar Metallicity, a Very Low C/O, and No Evidence of CH4, HCN, or C2H2

<jats:p>We present the transmission spectrum of the original transiting hot Jupiter HD 209458b from 2.3 to 5.1 <jats:italic>μ</jats:italic>m as observed with the NIRCam instrument on the James Webb Space Telescope (JWST). Previous studies of HD 209458b’s atmosphere have given conflicting results on the abundance of H<jats:sub>2</jats:sub>O and the presence of carbon- and nitrogen-bearing species, which have significant ramifications on the inferences of the planet’s metallicity (M/H) and carbon-to-oxygen (C/O) ratio. We detect strong features of H<jats:sub>2</jats:sub>O and CO<jats:sub>2</jats:sub> in the JWST transmission spectrum, which when interpreted using a retrieval that assumes thermochemical equilibrium and fractional gray cloud opacity yields <jats:inline-formula> <jats:tex-math> <? CDATA ${3}_{-1}^{+4}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3. org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjlad2682ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>× solar metallicity and C/O = <jats:inline-formula> <jats:tex-math> <? CDATA ${0.11}_{-0.06}^{+0.12}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3. org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3. org/1999/xlink" xlink:href="apjlad2682ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. The derived metallicity is consistent with the atmospheric metallicity–planet mass trend observed in solar gas giants. The low C/O ratio suggests that this planet has undergone significant contamination by evaporating planetesimals while migrating inward. We are also able to place upper limits on the abundances of CH<jats:sub>4</jats:sub>, C<jats:sub>2</jats:sub>H<jats:sub>2</jats:sub>, and HCN of log(<jats:inline-formula> <jats:tex-math> <? CDATA ${\chi }_{{\mathrm{CH}}_{4}}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>CH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlad2682ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>) = −5.6, log(<jats:inline-formula> <jats:tex-math> <? CDATA ${\chi }_{{{\rm{C}}}_{2}{{\rm{H}}}_{2}}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlad2682ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>) = −5.7, and log(<jats:inline-formula> <jats:tex-math> <? CDATA ${\chi }_{\mathrm{HCN}}$? > </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>HCN</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlad2682ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>) = −5.1, which are in tension with the recent claim of a detection of these species using ground-based cross-correlation spectroscopy. We find that HD 209458b has a weaker CO<jats:sub>2</jats:sub> feature size than WASP-39b when comparing their scale-height-normalized transmission spectra. On the other hand, the size of HD 209458b’s H<jats:sub>2</jats:sub>O feature is stronger, thus reinforcing the low C/O inference. </jats:p>